Clinical Chemistry
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Clinical Chemistry 40: 435-441, 1994;
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Clinical Chemistry, Vol 40, 435-441, Copyright © 1994 by American Association for Clinical Chemistry

Simple method for determining specific binding capacity of vitamin D- binding protein and its use to calculate the concentration of "free" 1,25-dihydroxyvitamin D

R Vieth
Department of Clinical Biochemistry, University of Toronto, Canada.

A quantitative method to measure the specific binding capacity for 25- hydroxyvitamin D (25D-binding capacity) is described that resembles the qualitative "T3-uptake" assay. Patient's serum or standard (10 microL) is mixed with 0.5 mL of reagent containing 0.5 mumol/L 25- hydroxyvitamin D [25(OH)D3] plus 3000 counts/min [3H]25(OH)D3. After 0.5 h at 37 degrees C, the samples are treated with dextran/charcoal on ice for 1 h and centrifuged. The radioactivity of the bound tracer in the supernate is counted. Calibration is linear to approximately 10 mumol/L. 25D-binding capacity in reference-group serum samples was 4.33 (0.58 SD) mumol/L. The relationship between the inverse of 25D-binding capacity and the free fraction of [3H]1,25-dihydroxyvitamin D3 [1,25(OH)2D3] measured by ultrafiltration isodialysis was essentially linear (r = 0.934, P < 0.0001). Given this relationship, the calculated free fraction of 1,25(OH)2D3 equals 4.88 x 10(-3)/25D-binding capacity. The 25D-binding capacity was significantly lower in newborn babies and in adults with liver disease, and was increased during pregnancy (P < 0.01 for each). This method is applicable to situations where the biologically available concentration of 1,25(OH)2D is of interest.


The following articles in journals at HighWire Press have cited this article:


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R. Vieth
Critique of the Considerations for Establishing the Tolerable Upper Intake Level for Vitamin D: Critical Need for Revision Upwards
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[Abstract] [Full Text] [PDF]

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Z. H. Al-oanzi, S. P. Tuck, N. Raj, J. S. Harrop, G. D. Summers, D. B. Cook, R. M. Francis, and H. K. Datta
Assessment of Vitamin D Status in Male Osteoporosis
Clin. Chem., February 1, 2006; 52(2): 248 - 254.
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R. Vieth, S. Milojevic, and V. Peltekova
Improved Cholecalciferol Nutrition in Rats Is Noncalcemic, Suppresses Parathyroid Hormone and Increases Responsiveness to 1,25-Dihydroxycholecalciferol
J. Nutr., March 1, 2000; 130(3): 578 - 584.
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S. Puri, D. D. Bansal, M. R. Uskokovic, and R. R. MacGregor
Induction of tissue plasminogen activator secretion from rat heart microvascular cells by fM 1,25(OH)2D3
Am J Physiol Endocrinol Metab, February 1, 2000; 278(2): E293 - E301.
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Copyright © 1994 by the American Association for Clinical Chemistry.