The Measurement of the Vitamin D-Binding Protein in Human Serum

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Abstract
The concentration of the vitamin D-binding protein was measured in human serum by single radial immunodiffusion. Normal serum concentrations were slightly higher in normal women than normal men. No race-related difference was found between white people from Belgium and black people from Zaire. Lower concentrations were found in cord serum and in patients with cirrhosis of the liver. Increased serum levels were observed during pregnancy or during the intake of estro-progestogens. The serum level of the vitamin D-binding protein was not altered in various diseases of Ca metabolism (primary osteoporosis, primary and secondary hyperparathyroidism, rickets, osteomalacia or vitamin D intoxication). No correlation was found between serum levels of 25-hydroxyvitamin D and those of its binding protein. The serum concentration of the vitamin D-binding protein (about 6.10-6 M) largely exceeds the normal serum concentration of 25-hydroxyvitamin D (about 4.10-8 M), so that this protein is normally < 1% saturated. Normal serum levels of the vitamin D-binding protein were observed in several diseases of calcium metabolism and the free concentration of 25-hydroxyvitamin D is not regulated at a constant level.