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Automation and Analytical Techniques |
1 Academic Medical Center, University of Amsterdam, Laboratory Genetic Metabolic Diseases, Emma Childrens Hospital and Department of Clinical Chemistry, Amsterdam, The Netherlands.
2 Erasmus Medical Center, Department of Clinical Genetics, Rotterdam, The Netherlands.
aAddress correspondence to this author at: Laboratory Genetic Metabolic Diseases, F0-224, PO Box 22700, 1100 DE Amsterdam, The Netherlands. Fax 31-20-6962596; e-mail w.kulik{at}amc.uva.nl.
Background: Sialic acid storage diseases (SSDs) are severe autosomal recessive neurodegenerative disorders caused by a transport defect across the lysosomal membrane, which leads to accumulation of sialic acid in tissues, fibroblasts, and urine. Defective free sialic acid transport can be established by quantification of free sialic acid in urine.
Methods: Urine sample size was adjusted to the equivalent of 100 nmol of creatinine. After addition of 2-keto-3-deoxy-D-glycero-D-galactonononic acid as internal standard, samples were diluted with water to an end volume of 250 µL. We used 10 µL for HPLCtandem mass spectrometric analysis in the negative electrospray ionization mode, monitoring transitions m/z 308.3
m/z 86.9 (sialic acid) and m/z 267.2
m/z 86.9 (internal standard). The overall method was validated and studied for ion suppression, interfering compounds, and pH effects. Samples from controls (n = 72) and SSD patients (n = 3) were analyzed.
Results: The limit of detection was 3 µmol/L. Intraassay imprecision (CV; n = 10) was 6%, 3%, and 2% at 30, 130, and 1000 mmol/mol creatinine, respectively; corresponding interassay CV (n = 10) were 5%, 5%, and 2%. Recovery was 109% (1001000 mmol/mol creatinine). The mean (SD) [range] excretion rates (mmol/mol creatinine) were 31.3 (16.6) [0.756.9] at 01 year (n = 20), 21.2 (9.8) [6.338.3] at 13 years (n = 15), 14.4 (8.2) [1.732.9] at 310 years (n = 25), and 4.6 (2.6) [09.8] above age 10 years (n = 12). SSD patients 1.2, 3.9, and 12 years of age had concentrations of 111.5, 54.2, and 36.1 mmol/mol creatinine, respectively.
Conclusions: The HPLC-tandem MS method for free sialic acid in urine is more rapid, accurate, sensitive, selective, and robust than earlier methods and may serve as a candidate reference method for free sialic acid in diagnosis of SSD.
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