250HD, A CIRCULATING VITAMIN-D METABOLITE IN FISH

Serum and hepatic 25-hydroxyvitamin D (25OHD), and serum calcium, phosphate, 25OHD3 binding capacity and binding affinity were measured in male and female trout. Both serum and hepatic 25OHD levels are decreased in female trout with elevations in protein bound calcium and phosphate. Whereas the apparent dissociation constant (Kd) for serum binding of 25OHD3 of 1.0-2.0 × 10-9M is similar in males and females, the 25OHD3 binding capacity of hypercalcemic spawning trout (1.39 × 10-7M) is significantly less than that of male fish (1.88 × 10-7M). At circulating serum concentrations of 25OHD which average 9.5 × 10-9M only 5-7% of trout serum 25OHD binding sites are occupied. © 1979.