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Clinical Chemistry 49: 155-161, 2003; 10.1373/49.1.155
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(Clinical Chemistry. 2003;49:155-161.)
© 2003 American Association for Clinical Chemistry, Inc.

Plasma Vitamin B6 Vitamers before and after Oral Vitamin B6 Treatment: A Randomized Placebo-controlled Study

Mustafa Vakur Bor1a, Helga Refsum2, Marianne R. Bisp1, Øyvind Bleie3, Jorn Schneede4, Jan Erik Nordrehaug3, Per Magne Ueland4, Ottar Kjell Nygard3,4 and Ebba Nexø1

1 Department of Clinical Biochemistry AKH, Aarhus University Hospital, Nørrebrogade 44, DK-8000 Aarhus C, Denmark.

2 Department of Pharmacology,
3 Institute of Medicine, and
4 Locus for Homocysteine and related Vitamins, University of Bergen, N-5021 Bergen, Norway.

aAuthor for correspondence. Fax 45-89493060; e-mail vakbor{at}hotmail.com.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Background: Vitamin B6 has attracted renewed interest because of its role in homocysteine metabolism and its possible relation to cardiovascular risk. We examined the plasma B6 vitamers, pyridoxal 5'-phosphate (PLP), pyridoxal (PL), pyridoxine (PN), and 4-pyridoxic acid (4-PA) before and after vitamin B6 supplementation.

Methods: Patients (n = 90; age range, 38–80 years) undergoing coronary angiography (part of the homocysteine-lowering Western Norway B-Vitamin Intervention Trial) were allocated to the following daily oral treatment groups: (A), vitamin B12 (0.4 mg), folic acid (0.8 mg), and vitamin B6 (40 mg); (B), vitamin B12 and folic acid; (C), vitamin B6; or (D), placebo. EDTA blood was obtained before treatment and 3, 14, 28, and 84 days thereafter.

Results: Before treatment, PLP (range, 5–111 nmol/L) and 4-PA (6–93 nmol/L) were the predominant B6 vitamers identified in plasma. During the 84-day study period, the intraindividual variation (CV) in patients not treated with vitamin B6 (groups B and D) was 45% for PLP and 67% for 4-PA. Three days after the start of treatment, the increases in concentration were ~10-, 50-, and 100-fold for PLP, 4-PA, and PL, respectively. No significant additional increase was observed at the later time points. The PLP concentration correlated to the concentrations of 4-PA and PL before treatment, but not after treatment. The PL concentration correlated with 4-PA before and after treatment.

Conclusions: Vitamin B6 treatment has an immediate effect on the concentrations and the forms of B6 vitamers present in plasma, and the changes remain the same during prolonged treatment. Our results suggest that the B6 vitamers in plasma reflect vitamin B6 intake.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Vitamin B6 functions as a coenzyme in >100 enzymatic reactions involved in the metabolism of amino acids, carbohydrates, neurotransmitters, and lipids (1). Pyridoxine (PN),1 pyridoxal (PL), pyridoxamine (PM), their phosphorylated derivatives [pyridoxal 5'-phosphate (PLP) and pyridoxamine 5'-phosphate (PMP)], and the end product of vitamin B6 metabolism, 4-pyridoxic acid (4-PA), are the major forms of vitamin B6 found in mammalian tissues and body fluids (1).

In the diet, vitamin B6 is predominantly present in three forms, PN, PM, and PL. After passive intestinal absorption, the major part of the vitamin is delivered to the liver and converted to PLP (2)(3)(4). PLP is available for other cells only after being hydrolyzed to PL by alkaline phosphatase, but most cells have pyridoxal kinase activity and are therefore able to rephosphorylate PL to PLP (4)(5). Most of the PL in excess of tissue requirements is oxidized to 4-PA by the liver and excreted in the urine (4).

The development of HPLC methods for the analysis of vitamin B6 vitamers in plasma (6) has enabled analysis of vitamers in the low concentrations present in samples from healthy individuals. However, the data are sparse, and increased knowledge of physiologic concentrations may lead to a more complete understanding of the connection between plasma concentrations and nutritional status for this vitamin.

Primary dietary deficiency of vitamin B6 is considered rare in developed countries because the vitamin is present in a wide variety of foods (7). Recently, low vitamin B6 concentrations have been reported in elderly populations (8)(9), in individuals with traumatic femoral fractures (10), and in alcoholics (11)(12). Such low concentrations have been related to increased risk of cardiovascular diseases (9)(13)(14)(15)(16)(17)(18). It is, however, uncertain whether the increased risk is mediated by increased homocysteine or whether low B6 is an independent risk factor (19)(20).

PN therapy is frequently recommended for various conditions, including genetic disorders, such as cystathioninuria and homocystinuria, and a range of other disorders, such as premenstrual tension, seizures, carpal tunnel syndrome, and the nausea and vomiting that can occur during pregnancy (1)(3)(21)(22)(23). Because it is considered a safe compound, very high doses of B6 [>2000-fold higher than the recommended dietary allowance (1.4–2.1 mg/day, depending on sex and age)] are used in some circumstances (7)(24). Such doses may be associated with peripheral neuropathy, but only sparse data are available on the mechanism and which B6 vitamers are responsible (25). Indeed, little is known about the effect of PN supplementation on plasma vitamin B6 concentrations and long-term response to this treatment.

Currently, several clinical trials on B-vitamin treatment for cardiovascular disease are underway, many of them including vitamin B6 (26)(27)(28). Hence, there is increased interest in this vitamin.

We recently developed a HPLC method for measuring the B6 vitamers PLP, PL, PMP, PN, and PM and the degradation product 4-PA in plasma (29). In the current study, we used this method to assess plasma B6 vitamer concentrations before and after 3, 14, 28, and 84 days of treatment with PN (40 mg/day) in patients undergoing coronary angiography. In addition, we calculated the inter- and intraindividual variation in B6 vitamers from serial measurements of the analytes in the group not treated with PN.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
patients and blood samples
This investigation is a substudy of the ongoing Western Norway B-Vitamin Intervention Trial (WENBIT), a prospective randomized double-blind study on the effects of homocysteine-lowering therapy on mortality and cardiovascular events. The participants of this substudy have recently been described (28). In brief, adult patients undergoing coronary angiography for suspected coronary artery disease or aortic valvular stenosis are eligible for WENBIT, independent of subsequent therapy. Exclusion criteria are malignant disease, alcohol abuse, mental illness, lack of availability for long-term follow-up, and participation in other trials.

A total of 90 consecutive patients (age range, 38–80 years; 21 females and 69 males) with suspected coronary artery disease recruited to the WENBIT study at Haukeland University Hospital were eligible for this study of plasma vitamin B6 vitamers. Informed consent was obtained from all patients, and the regional ethics committee approved the study protocol.

In WENBIT, the possibility that vitamin B6 has an independent effect on cardiovascular disease (30) has been taken into account by use of a factorial design where one of the groups includes vitamin B6 alone. Hence, recruited patients were randomized in a 2 x 2 factorial design for daily oral treatment according the following protocol: A (n = 22), vitamin B12 (0.4 mg), folic acid (0.8 mg), and vitamin B6 (40 mg); B (n = 23), vitamin B12 and folic acid; C (n = 21), vitamin B6; or D (n = 24), placebo. For the first 2 weeks, the folic acid groups (A and B) received an additional loading dose of folic acid (5 mg/day). Packages of study tablets were prepared in random order and given serial numbers by Alpharma A/S (Copenhagen, Denmark).

In the present analyses, patients were grouped according to treatment with vitamin B6 (A and C, treatment group; n = 43) or without vitamin B6 (B and D, control group; n = 47). Nonfasting (basal) EDTA samples were collected before and, 3, 14, 28, and 84 days after the start of vitamin treatment. The blood samples were immediately placed on ice, centrifuged within 30 min, and stored at -80 °C until further analyses. Plasma vitamin B6 vitamers were analyzed for the whole group in samples collected at days 0 and 84 and for approximately one-half of the participants on day 3 (n = 23), day 14 (n = 23), and day 28 (n = 24). The sets of samples analyzed on the intervening days were chosen randomly.

biochemical analysis
Vitamin B6 vitamers were analyzed by ion-pair reversed-phase chromatography as described previously (29). The HPLC system was calibrated by injecting known quantities of vitamin B6 vitamers and 4-PA and measuring the resulting peak areas. The working calibration solution was prepared daily and diluted to obtain 10 calibrators with the following concentrations: 4000, 2000, 1333, 667, 333, 167, 83, 42, 21, and 10 nmol/L (Sigma). All calibration curves within this concentration range showed a linear response (data not shown).

As assessed from control samples (n = 49) assayed over 3 months, the CVs (mean measured concentrations) were 13% (74 nmol/L) for PLP, 8% (40 nmol/L) for PA, 46% (11 nmol/L) for PL, and 16% (22 nmol/L) for PN.

statistical analysis
The distributions of values for vitamin B6 vitamers were normalized by logarithmic (base 10) transformation. Nonparametric testing was used for comparison of the control group with the B6-treated group and for comparison of values obtained on the same individuals. To compare relationships between variables, nonparametric testing (Spearman correlation) was used. Alterations in B6 vitamers as a function of time were analyzed by comparing the control group (day 0) with the vitamin B6-treated group for the day in question. Data were analyzed with use of QuatroPro and GraphPad (Prism2) software.

One participant not treated with vitamin B6 had a plasma PN concentration at day 14 that was dramatically higher than at the other serial measurements. Thus, this outlying value (444 nmol/L) was omitted from the presentation of data in Fig. 1 .



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  		Figure 1. Changes in plasma concentrations of PLP, PL, 4-PA, and PN in patients receiving vitamin supplementation with (•) or without ({circ}) 40 mg/day vitamin B6.

For the series without vitamin B6, the numbers of participants included were 47 (day 0), 23 (day 3), 23 (day 14), 24 (day 28), and 42 (day 84). The corresponding numbers for the series with vitamin B6 were 43, 21, 18, 28, and 39, respectively. Note that the y axis is logarithmic scale. Data are presented as the mean ± SE (error bars). Because PN was not detected on day 0 in any of the patients receiving the vitamin supplement, the data point at zero time could not be shown on the logarithmic scale.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Before intervention with vitamin or placebo, plasma concentrations of B6 vitamers were similar in the four groups, A–D (data not shown). The vitamin B6 vitamer concentrations (medians and interquartile ranges) for the whole study group before intervention are listed in Table 1 . PLP and 4-PA were detected in all samples analyzed, whereas PL was present in 65% and PN and PM in only 3% of the control samples. PMP was not detected in any of the samples.


View this table:
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  		Table 1. Plasma concentrations and intraindividual and analytical variation of vitamin B6 vitamers measured in patients (n = 90) participating in the WENBIT study.1

The age of participants ranged from 38 to 80 years. We found no significant correlation with age for PLP, PL, or 4-PA, but there was a tendency for a decrease in the PLP concentration and an increase in the 4-PA concentration (data not shown). There were no differences according to gender before treatment, whereas PL (P = 0.011) and 4-PA (P = 0.009), but not PLP (P = 0.072), were significantly higher in women than in men after 84 days of treatment with PN. When the concentrations of PL, 4-PA, and PLP were corrected for body weight (plasma concentration times body weight), the differences between males and females were the same after 84 days of treatment with PN [PL (P = 0.036), 4-PA (P = 0.036), PLP (P = 0.45)].

We found a significant correlation between plasma creatinine and plasma 4-PA (r = 0.38; P = 0.001), but not PL and PLP, before treatment. Plasma PLP was significantly correlated with plasma albumin both before (r = 0.32; P = 0.002) and after treatment (r = 0.44; P = 0.006). In contrast, PLP was not associated with C-reactive protein, suggesting that the association between PLP and albumin is not explained by concomitant changes in these two factors as the result of an acute-phase reaction.

The intraindividual variations as calculated from the serial measurement of the analytes in the control group were 45% for plasma PLP, 67% for 4-PA, and 92% for PL (Table 1Up ).

After 3 days of supplementation with oral vitamin B6, both qualitative and quantitative changes were observed in plasma B6 vitamers (Fig. 1Up ). The mean concentration increased >10-fold for PLP, ~50-fold for 4-PA, and >100-fold for PL and PN. These increases remained almost the same at 14, 28, and 84 days of treatment (Fig. 1Up ). PM was detectable in only five patients, whereas PMP was not detected in any of the samples after PN treatment. The changes in plasma B6 vitamers were not different between the two groups (A and C) receiving vitamin B6, indicating no effect of vitamer B6 distribution from folic acid or cobalamin.

The correlations of PLP to PL, PLP to 4-PA, and PL to 4-PA were evaluated at day 0 for all persons studied and at day 84 for the treatment group. PLP was correlated to PL (r = 0.51; P <0.0001) and 4-PA (r = 0.525; P <0.0001), but only before treatment. The correlation between PL and 4-PA was significant before treatment (day 0; r = 0.52; P <0.0001), and this correlation increased considerably after treatment (day 84; r = 0.91; P <0.0001).


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
In this report, we have presented data on vitamin B6 vitamers before and after treatment with a daily oral dose of PN (40 mg) for a period of 12 weeks in a trial including patients with confirmed coronary artery disease. Although vitamin B6 has been included in this and several similar trials (26) with the intention to lower homocysteine, some studies suggest that low vitamin B6 status is independently associated with cardiovascular risk (18)(30)(31). Notably, prospective studies indicate that increased homocysteine may be only a weak risk factor, if a risk factor at all (32)(33). Therefore, these trials should focus not only on increased homocysteine as the possible cause of disease, but should also include detailed investigations on all relevant vitamins, including vitamin B6.

In human plasma, vitamin B6 vitamer concentrations are low unless the person takes supplements containing vitamin B6 (6). Therefore, it is essential to have a method that can detect low concentrations of vitamin B6 compounds in plasma. We recently developed a HPLC-based method that separates and detects nanomolar quantities of five B6 vitamers and the degradation product 4-PA (29). In the present study, we were able to identify PLP, 4-PA, and to a lesser degree PL, as well as occasionally PN and PM in plasma of patients not treated with PN. PMP was not detected in any of the samples either before or after PN treatment. This finding is in agreement with most published studies (24)(34)(35)(36)(37). Previously, PLP has been shown to comprise 70–90% of the total vitamin B6, whereas PL was reported to account for ~8–30% of the total plasma vitamin B6 (38). It is important to note that there is wide variability in the measurement of PL in the literature. The concentration of PL has been reported to vary between 1.1 and 90 nmol/L in the general population (35)(36)(37)(39)(40)(41)(42)(43)(44)(45)(46). Although the plasma PL concentrations found in the current study are comparable to these values, we cannot rule out the possibility that our method may underestimate PL. At present, there are no commercial samples with assigned values or external quality assessment schemes available; it thus remains difficult to establish generally accepted reference values for vitamin B6 vitamers and 4-PA. Therefore, there is a pressing need for biological quality-control samples with B6 values assigned by reference procedures to clarify the accuracy of the existing methods.

Protein binding plays a dominant role in the distribution and elimination of B6 metabolites. PLP is almost completely bound to protein in plasma (47), whereas PL is only partly bound and PN and the end product 4-PA are completely free (47). In humans, 4-PA represents >90% of the B6 compounds that are excreted in the urine (48).

In the present study, the B6 vitamers PLP, PL, and PN and the metabolite 4-PA were measured before treatment and after 3, 14, 28, and 84 days of treatment. We thus obtained serial measurements (two to five measurements per patient) of the analytes. This allowed us to calculate the intraindividual variation in B6 vitamers based on the findings in the group not treated with PN. Very little is known concerning the intraindividual variation for blood tests reflecting vitamin B6 status. Previous studies have focused only on circadian variations. In these studies, the plasma concentration profiles of PLP and PL were constant within the observation period of 24 h (49)(50), whereas urinary excretion of 4-PA was lower during the night compared with the time after lunch (24). These results suggest that the timing of phlebotomy is not critical, at least for the PLP and PL concentrations, and that circadian variations may not interfere with assessment of intraindividual variation. Our data show that over a 3-month period, the intraindividual variation is considerable: 45% for plasma PLP, 67% for 4-PA, and ~92% for PL. We do not have any clear explanation for the observed high intraindividual variation in the present study. Because the concentrations of PLP, PL, and 4-PA are related to dietary variables (51), variation in dietary intake may account for part of the variability; thus, the intraindividual variations observed reflect what one would expect in the routine setting. Notably, the intraindividual variation of PLP is lower than the values observed for the other B6 vitamers, and this may be related to the marked protein binding of this species. We speculate that protein binding may antagonize degradation by alkaline phosphatase and cellular uptake and thereby ensure a more stable PLP concentration in the circulation.

We found no significant correlation between age and PLP or 4-PA, but there was a tendency for a decrease in PLP and an increase in 4-PA with increasing age. Previous studies have shown that plasma PLP concentrations tend to decrease with age and increasing frailty, whereas plasma 4-PA increases with age (8)(52)(53)(54). The suggested explanations for this age effect are lower vitamin B6 intake in older individuals, less efficient retention of the vitamin, increased B6 catabolism, and variations in kidney function (8)(52)(53).

Plasma 4-PA, but not PL and PLP, was significantly correlated with plasma creatinine. This finding is in agreement with previous studies (8)(51) and indicates that 4-PA is more sensitive to variations in kidney function than other vitamin B6 compounds. Supporting this, Coburn et al. (55) recently reported high plasma concentrations of 4-PA during renal insufficiency.

We found no differences according to gender before treatment, whereas PL and 4-PA, but not PLP, were significantly higher in women than in men after 84 days of treatment with PN. The larger increment in women could partly be explained by a smaller body dilution pool. We reached this conclusion because the difference remained after the concentrations of the vitamin B6 vitamers were adjusted according to body weight. Data from animal studies have suggested pronounced sex differences in tissue concentrations of enzymes of vitamin B6 metabolism (56). However, controlled clinical studies on sex differences in human vitamin B6 metabolism have not been performed.

After supplementation with a relatively high dose of PN (40 mg/day), a new steady state was reached by 3 days. At this time, PLP had increased ~10-fold, whereas the increases in both PL and 4-PA were considerably higher. Our results confirm (35)(42)(48)(57)(58) and expand earlier studies on the pharmacokinetics of vitamin B6.

Speitling et al. (57) investigated the metabolism of 600 mg of PN given orally to 10 healthy young males. The maximum plasma concentration of PN was reached after 1.3 h, and it took ~9 h until PN was totally eliminated from the plasma. Plasma PL and 4-PA showed essentially the same time courses. Compared with the other B6 compounds, PLP increased only moderately. Our study extends these observations and shows that the chronic administration of oral PN produces steady-state concentrations of the B6 vitamers after 3 days and that further increases in plasma PLP, PL, and 4-PA are not obtained by prolonged vitamin B6 treatment.

The concentration of PLP in plasma does not increase in a dose-dependent manner after intake of supraphysiologic PN doses. This was shown by Edwards et al. (59) in healthy volunteers who received PN doses of 10–800 mg/day for 1 week. The concentrations of PL, PN, and 4-PA in plasma, on the other hand, increased as a function of the PN dose administered. Similar results were obtained by Ubbink et al. (60) for healthy females who received doses of 10–100 mg of PN. In the present study, the mean PLP concentration increased ~10-fold, whereas 4-PA increased ~50-fold and PL and PN increased >100-fold after vitamin B6 treatment. The increases that we observed are comparable to those observed by Edwards et al. (59). We speculate that only a limited amount of PLP can be bound by the albumin in plasma and that surplus PLP is metabolized by alkaline phosphatase to PL and eventually 4-PA. In accordance with this view, we found that PLP correlated with albumin.

Currently, there is no agreement concerning which vitamin B6 vitamer should be used for routine assessment of vitamin B6 status. Furthermore, there is not always a good correlation between different analytes, and it is difficult at this stage to recommend a definitive single status measurement (61). In the present study, we demonstrated the lack of correlation between different vitamin B6 vitamers except between PL and 4-PA. The correlation of these two vitamers became more pronounced after treatment with vitamin B6, possibly because of an increase in the metabolism/excretion rate after supplementation. Our results warrant further studies to assess whether measurement of one or a few of the vitamin B6 vitamers will suffice for the assessment of vitamin B6 status.

In conclusion, we found that PLP, 4-PA, and to a lesser degree PL were the predominant B6 metabolites in plasma. After treatment with vitamin B6, PN was also detectable, and PN and PL showed the largest increases in concentration after treatment. Vitamin B6 treatment had an immediate effect on the concentrations and the forms of B6 vitamers in plasma, whereas no further changes were observed during a more prolonged treatment.


   Acknowledgments
 
This study was part of the EU BIOMED Project (BMH4-CT98-3549). Alpharma A/S (Copenhagen, Denmark) provided the study medication free of charge. The project was also financed with the aid of extra funds from Alpharma A/S; The Norwegian Foundation for Health and Rehabilitation; The Norwegian Heart and Lung Association (LHL); the Advanced Research Programe, Norway; and the Regional Committee of Health Region West, Norway. We gratefully acknowledge the excellent technical assistance of Anna-Lisa Christensen.


   Footnotes
 
1 Nonstandard abbreviations: PN, pyridoxine; PL, pyridoxal; PM, pyridoxamine; PLP, pyridoxal 5'-phosphate; PMP, pyridoxamine 5'-phosphate; 4-PA, 4-pyridoxic acid; and WENBIT, Western Norway B-Vitamin Intervention Trial.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Leklem JE. Vitamin B6. Ziegler EE Filer LJ eds. Present knowledge in nutrition, 7th ed 1996:174-183 ILSI Press Washington. .
  2. Wozenski JR, Leklem JE, Miller LT. The metabolism of small doses of vitamin B-6 in men. J Nutr 1980;110:275-285.
  3. Leklem JE. Vitamin B6. Reservoirs, receptors, and red-cell reactions. Ann N Y Acad Sci 1992;669:34-41.[ISI][Medline] [Order article via Infotrieve]
  4. Merrill AH, Jr, Henderson JM. Vitamin B6 metabolism by human liver. Ann N Y Acad Sci 1990;585:110-117.[Abstract]
  5. Lumeng L, Brashear RE, Li TK. Pyridoxal 5'-phosphate in plasma: source, protein-binding, and cellular transport. J Lab Clin Med 1974;84:334-343.[ISI][Medline] [Order article via Infotrieve]
  6. Gregory JF. Methods for determination of vitamin B6 in foods and other biological materials: a critical review. J Food Composition Anal 1988;1:105-123.
  7. Bender DA. Vitamin B6 requirements and recommendations. Eur J Clin Nutr 1989;43:289-309.[ISI][Medline] [Order article via Infotrieve]
  8. Bates CJ, Pentieva KD, Prentice A, Mansoor MA, Finch S. Plasma pyridoxal phosphate and pyridoxic acid and their relationship to plasma homocysteine in a representative sample of British men and women aged 65 years and over. Br J Nutr 1999;81:191-201.[ISI][Medline] [Order article via Infotrieve]
  9. Selhub J, Jacques PF, Wilson PW, Rush D, Rosenberg IH. Vitamin status and intake as primary determinants of homocysteinemia in an elderly population. JAMA 1993;270:2693-2698.[Abstract]
  10. Reynolds TM, Marshall PD, Brain AM. Hip fracture patients may be vitamin B6 deficient. Controlled study of serum pyridoxal-5'-phosphate. Acta Orthop Scand 1992;63:635-638.[ISI][Medline] [Order article via Infotrieve]
  11. Cravo ML, Gloria LM, Selhub J, Nadeau MR, Camilo ME, Resende MP, et al. Hyperhomocysteinemia in chronic alcoholism: correlation with folate, vitamin B-12, and vitamin B-6 status. Am J Clin Nutr 1996;63:220-224.[Abstract/Free Full Text]
  12. Fonda ML. Vitamin B6 metabolism and binding to proteins in the blood of alcoholic and nonalcoholic men. Alcohol Clin Exp Res 1993;17:1171-1178.[ISI][Medline] [Order article via Infotrieve]
  13. Vermeulen EG, Stehouwer CD, Twisk JW, van den Berg M, de Jong SC, Mackaay AJ, et al. Effect of homocysteine-lowering treatment with folic acid plus vitamin B6 on progression of subclinical atherosclerosis: a randomised, placebo-controlled trial. Lancet 2000;355:517-522.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  14. Verhoef P, Stampfer MJ, Buring JE, Gaziano JM, Allen RH, Stabler SP, et al. Homocysteine metabolism and risk of myocardial infarction: relation with vitamins B6, B12, and folate. Am J Epidemiol 1996;143:845-859.[Abstract/Free Full Text]
  15. Dalery K, Lussier-Cacan S, Selhub J, Davignon J, Latour Y, Genest J, Jr. Homocysteine and coronary artery disease in French Canadian subjects: relation with vitamins B12, B6, pyridoxal phosphate, and folate. Am J Cardiol 1995;75:1107-1111.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  16. Ubbink JB, Vermaak WJ, van der Merwe A, Becker PJ. Vitamin B-12, vitamin B-6, and folate nutritional status in men with hyperhomocysteinemia. Am J Clin Nutr 1993;57:47-53.[Abstract/Free Full Text]
  17. Brattstrom L, Stavenow L, Galvard H, Nilsson-Ehle P, Berntorp E, et al. Pyridoxine reduces cholesterol and low-density lipoprotein and increases antithrombin III activity in 80-year-old men with low plasma pyridoxal 5-phosphate. Scand J Clin Lab Invest 1990;50:873-877.[ISI][Medline] [Order article via Infotrieve]
  18. Rimm EB, Willett WC, Hu FB, Sampson L, Colditz GA, Manson JE, et al. Folate and vitamin B6 from diet and supplements in relation to risk of coronary heart disease among women. JAMA 1998;279:359-364.[Abstract/Free Full Text]
  19. Friso S, Jacques PF, Wilson PW, Rosenberg IH, Selhub J. Low circulating vitamin B(6) is associated with elevation of the inflammation marker C-reactive protein independently of plasma homocysteine levels. Circulation 2001;103:2788-2791.[Abstract/Free Full Text]
  20. Folsom AR, Nieto FJ, McGovern PG, Tsai MY, Malinow MR, Eckfeldt JH, et al. Prospective study of coronary heart disease incidence in relation to fasting total homocysteine, related genetic polymorphisms, and B vitamins: the Atherosclerosis Risk in Communities (ARIC) study. Circulation 1998;98:204-210.[Abstract/Free Full Text]
  21. Merrill AH, Jr, Henderson JM. Diseases associated with defects in vitamin B6 metabolism or utilization. Annu Rev Nutr 1987;7:137-156137–56.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  22. Bassler KH. Megavitamin therapy with pyridoxine. Int J Vitam Nutr Res 1988;58:105-118.[ISI][Medline] [Order article via Infotrieve]
  23. Niebyl JR, Goodwin TM. Overview of nausea and vomiting of pregnancy with an emphasis on vitamins and ginger. Am J Obstet Gynecol 2002;185:S253-S255.
  24. Zempleni J. Pharmacokinetics of vitamin B6 supplements in humans. J Am Coll Nutr 1995;14:579-586.[Abstract]
  25. Bender DA. Non-nutritional uses of vitamin B6. Br J Nutr 1999;81:7-20.[ISI][Medline] [Order article via Infotrieve]
  26. Clarke R, Armitage J. Vitamin supplements and cardiovascular risk: review of the randomized trials of homocysteine-lowering vitamin supplements. Semin Thromb Hemost 2000;26:341-348.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  27. MacMahon M, Kirkpatrick C, Cummings CE, Clayton A, Robinson PJ, Tomiak RH, et al. A pilot study with simvastatin and folic acid/vitamin B12 in preparation for the Study of the Effectiveness of Additional Reductions in Cholesterol and Homocysteine (SEARCH). Nutr Metab Cardiovasc Dis 2000;10:195-203.[ISI][Medline] [Order article via Infotrieve]
  28. Nexø E, Hvas AM, Bleie Ø, Refsum H, Fedosov SN, Vollset SE, et al. Holo-transcobalamin is an early marker of changes in cobalamin homeostasis. A randomized placebo controlled study. Clin Chem 2002;48:1768-1771.[Abstract/Free Full Text]
  29. Bisp MR, Bor MV, Heinsvig EM, Kall MA, Nexø E. Determination of vitamin B6 vitamers and pyridoxic acid in plasma: development and evaluation of a high performance liquid chromatographic assay. Anal Biochem 2002;305:82-89.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  30. Robinson K, Arheart K, Refsum H, Brattstrom L, Boers G, Ueland P, et al. Low circulating folate and vitamin B6 concentrations: risk factors for stroke, peripheral vascular disease, and coronary artery disease. European COMAC Group. Circulation 1998;97:437-443.[Abstract/Free Full Text]
  31. Miner SE, Cole DE, Evrovski J, Forrest Q, Hutchison S, Holmes K, et al. Pyridoxine improves endothelial function in cardiac transplant recipients. J Heart Lung Transplant 2002;20:964-969.
  32. Eikelboom JW, Lonn E, Genest J, Jr, Hankey G, Yusuf S. Homocyst(e)ine and cardiovascular disease: a critical review of the epidemiologic evidence. Ann Intern Med 1999;131:363-375.[Abstract/Free Full Text]
  33. Christen WG, Ajani UA, Glynn RJ, Hennekens CH. Blood levels of homocysteine and increased risks of cardiovascular disease: causal or casual?. Arch Intern Med 2000;160:422-434.[Abstract/Free Full Text]
  34. Edwards P, Liu PK, Rose GA. A simple liquid-chromatographic method for measuring vitamin B6 compounds in plasma. Clin Chem 1989;35:241-245.[Abstract/Free Full Text]
  35. Lui A, Lumeng L, Li TK. The measurement of plasma vitamin B6 compounds: comparison of a cation-exchange HPLC method with the open-column chromatographic method and the L-tyrosine apodecarboxylase assay. Am J Clin Nutr 1985;41:1236-1243.[Abstract/Free Full Text]
  36. Coburn SP, Mahuren JD. A versatile cation-exchange procedure of measuring the seven major forms of vitamin B6 in biological samples. Anal Biochem 1983;129:310-317.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  37. Hollins B, Henderson JM. Analysis of B6 vitamers in plasma by reversed-phase column liquid chromatography. J Chromatogr 1986;380:67-75.[ISI][Medline] [Order article via Infotrieve]
  38. Leklem JE. Vitamin B-6: a status report. J Nutr 1990;120(Suppl 11):1503-1507.
  39. Shephard GS, Van der Westhuizen L, Labadarios D. Analysis of vitamin B6 vitamers in human tissue by cation-exchange high-performance liquid chromatography. J Chromatogr 1989;491:226-234.[ISI][Medline] [Order article via Infotrieve]
  40. Lumeng L, Lui A, Li TK. Plasma content of B6 vitamers and its relationship to hepatic vitamin B6 metabolism. J Clin Invest 1980;66:688-695.
  41. Sampson D, O’Connor D. Analysis of B-6 vitamers and pyridoxic acid in plasma, tissues and urine using high performance liquid chromatography. Nutr Res 1989;9:259-272.[CrossRef]
  42. Kurioka S, Ishioka N, Sato J, Nakamura J, Ohkubo T, Matsuda M. Assay of vitamin B6 in human plasma with graphitic carbon column. Biomed Chromatogr 1993;7:162-165.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  43. Sharma SK, Dakshinamurti K. Determination of vitamin B6 vitamers and pyridoxic acid in biological samples. J Chromatogr 1992;578:45-51.[ISI][Medline] [Order article via Infotrieve]
  44. Kimura M, Kanehira K, Yokoi K. Highly sensitive and simple liquid chromatographic determination in plasma of B6 vitamers, especially pyridoxal 5'-phosphate. J Chromatogr A 1996;722:296-301.[Medline] [Order article via Infotrieve]
  45. Driskell JA, Chrisley BM. Plasma B-6 vitamer and plasma and urinary 4-pyridoxic acid concentrations in young women as determined using high performance liquid chromatography. Biomed Chromatogr 1991;5:198-201.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  46. Chrisley BM, McNair HM, Driskell JA. Separation and quantification of the B6 vitamers in plasma and 4-pyridoxic acid in urine of adolescent girls by reversed-phase high-performance liquid chromatography. J Chromatogr 1991;563:369-378.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  47. Anderson BB, Newmark PA, Rawlins M, Green R. Plasma binding of vitamin B6 compounds. Nature 1974;250:502-504.[CrossRef][Medline] [Order article via Infotrieve]
  48. Rose RC, McCorrmick DB, Li TK, Lumeng L, Haddad JG, Jr, Spector R. Transport and metabolism of vitamins. Fed Proc 1986;45:30-39.[ISI][Medline] [Order article via Infotrieve]
  49. Leinert J, Simon I, Hotzel D. Methoden und deren wertung zur bestimmung des vitamin B6 versorgungszustandes beim menschen. 6. Mitteilung. S-PLP. Aussagefahigkeit des parameters. Int J Vitam Nutr Res 1983;53:166-178.
  50. Mascher H. Determination of total pyridoxal in human plasma following oral administration of vitamin B6 by high-performance liquid chromatography with post-column derivatization. J Pharm Sci 1993;82:972-974.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  51. Brussaard JH, Lowik MR, van den Berg H, Brants HA, Bemelmans W. Dietary and other determinants of vitamin B6 parameters. Eur J Clin Nutr 1997;51(Suppl 3):S39-S45.
  52. Lowik MR, van den Berg H, Westenbrink S, Wedel M, Schrijver J, Ockhuizen T. Dose-response relationships regarding vitamin B-6 in elderly people: a nationwide nutritional survey (Dutch Nutritional Surveillance System). Am J Clin Nutr 1989;50:391-399.[Abstract/Free Full Text]
  53. Kant AK, Moser-Veillon PB, Reynolds RD. Effect of age on changes in plasma, erythrocyte, and urinary B-6 vitamers after an oral vitamin B-6 load. Am J Clin Nutr 1988;48:1284-1290.[Abstract/Free Full Text]
  54. van den Berg H, Bode W, Mocking JA, Lowik MR. Effect of aging on vitamin B6 status and metabolism. Ann N Y Acad Sci 1990;585:96-105.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  55. Coburn SP, Reynolds RD, Mahuren JD, Schaltenbrand WE, Wang Y, Ericson KL, et al. Elevated plasma 4-pyridoxic acid in renal insufficiency. Am J Clin Nutr 2002;75:57-64.[Abstract/Free Full Text]
  56. Bode W, Mocking JA, van den Berg H. Influence of age and sex on vitamin B-6 vitamer distribution and on vitamin B-6 metabolizing enzymes in Wistar rats. J Nutr 1991;121:318-329.
  57. Speitling A, Heseker H, Kubler W. Pharmacokinetic properties of the plasma B6 vitamers after single and chronic oral pyridoxine mega doses. Ann N Y Acad Sci 1990;585:557-559.[CrossRef]
  58. Mansoor MA, Kristensen O, Hervig T, Bates CJ, Pentieva K, Vefring H, et al. Plasma total homocysteine response to oral doses of folic acid and pyridoxine hydrochloride (vitamin B6) in healthy individuals. Oral doses of vitamin B6 reduce concentrations of serum folate. Scand J Clin Lab Invest 1999;59:139-146.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  59. Edwards P, Liu PK, Rose GA. Liquid chromatographic studies of vitamin B6 metabolism in man. Clin Chim Acta 1990;190:67-80.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  60. Ubbink JB, Serfontein WJ, Becker PJ, de Villiers LS. Effect of different levels of oral pyridoxine supplementation on plasma pyridoxal-5'-phosphate and pyridoxal levels and urinary vitamin B-6 excretion. Am J Clin Nutr 1987;46:78-85.[Abstract/Free Full Text]
  61. Bates CJ. Vitamin analysis. Ann Clin Biochem 1997;34:599-626.



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