Effects of age and diseases on human serum 25-hydroxycholecalciferol determined by competitive protein-binding assay

Competitive protein-binding assay of 25-OH-D was developed by the use of specific vitamin D-binding proteins from vitamin D-deficient rat serum. Ether extract of serum sample which was dried and dissolved in ethanol, or standard solution of 25-OH-D3, was incubated with 3H-25-OH-D3 and vitamin D-binding protein for 2 hours at 4degreesC. Free and bound 3H-25-OH-D3 were separated through dextran-coated charcoal. The sensitivity of the assay system was 0.22 ng/tube. Percent cross reaction in the assay was 2.18% in vitamin D3, 0.70% in 1, 25-(OH)2-D3, less than 0.28% in 1alpha-OH-D3, and less than 0.06% in dihydrotachysterol, cholesterol and cortisol. Human serum 25-OH-D is 28.9+/-2.9 ng/ml in 19 normal subjects. Serum 25-OH-D in the old-age group (50-70 years of age) was significantly decreased, compared with that in the young-age group (20-40 years of age). Serum 25-OH-D was significantly decreased in gastrectomized and osteoporotic patients as well as in the patients with liver cirrhosis, in comparison with their age-controls.