In vivo and in vitro inhibition of rat liver vitamin D3-25-hydroxylase activity by 19-hydroxy-10(S),19-dihydrovitamin D3.

Rats treated with varying amounts of 19-hydroxy-10(S),19-dihydrovitamin D3 prior to administration of physiologic doses of vitamin D3 exhibit normal intestinal calcium transport but are unable to mobilize bone calcium. In contrast, 19-hydroxy-10(R),19-dihydrovitamin D3 had no inhibitory activity. Circulating serum levels of 25-hydroxy[3H]vitamin D3 and 1 alpha, 25-dihydroxy[3H]vitamin D3 are markedly suppressed but not totally eliminated in animals predosed with 19-hydroxy-10(S),19-dihydrovitamin D3 before [3H]vitamin D3. Hepatic 25-hydroxy[3H]vitamin D3 levels were approximately equal in both 19-hydroxy-10(S),19-dihydroviotamin D3 treated and untreated rats. However, the rate of conversion of [3H]vitamin D3 to 25-hydroxyvitamin D3 in vivo is greatly reduced in the treated rats. The inhibitory vitamin analogue was also show to block hepatic microsomal 25-hydroxylation in vitro. These results indicate that 19-hydroxy-10(S),19-dihydrovitamin D3 is a specific inhibitor for a hepatic microsomal vitamin D3-25-hydroxylase system.