Ontogeny and Effect of Vitamin D Deprivation on Rat Serum 25-Hydroxyvitamin D Binding Protein

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Abstract
Specific serum binding of 25-hydroxycholecalciferol (25-OHD3) was measured by saturation analysis in rats of various ages, and during vitamin D deprivation. The serum binding capacity for 25-OHD3 was observed to increase until age 6-8 wk, then decline and remain stable thereafter at 3.7 .times. 10-6 M. The 25-hydroxyvitamin D (25-OHD) concentration decreased in serum from rats fed vitamin D-free diet (t1/2 [half life] = 7 days). Rats fed 2 IU of vitamin D3/g of diet maintained stable serum levels of 25-OHD at 10-12 ng/ml. Serum binding capacity and affinity for 25-OHD3 was not affected by vitamin D deprivation or hypocalcemia. In addition, the binding affinity did not differ as a function of age (Kd = 3.3 .times. 10-9 M). Since normal serum concentrations of 25-OHD in the rat are 2-5 .times. 10-8 M, only 1-2% of the serum binding sites for this sterol are occupied under physiological conditions.

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