25-Hydroxycholecalciferol: High affinity substrate for hepatic cytochrome P-450

Abstract

Summary Addition of 25-hydroxycholecalciferol (25-OH-D 3 ) to rat liver microsomes revealed a type I spectral change with a spectral dissociation constant (K S ) of approximately 80nM; this value remained unchanged in phenobarbital-treated rats, although the maximum binding value was tripled. The inhibition constant, K i , of 25-OH-D 3 for aminopyrine N-demethylation was 59nM, in good agreement with the K S value. Stopped-flow studieds showed a two-fold increase in microsomal NADPH cytochrome P-450 reductase activity in the presence of 25-OH-D 3 . These findings suggest that the P-450 system plays a role in the blotransformation of 25-OH-D 3 .