CHARACTERISTICS OF THE RAT-LIVER MICROSOMAL-ENZYME SYSTEM CONVERTING CHOLECALCIFEROL INTO 25-HYDROXYCHOLECALCIFEROL - EVIDENCE FOR THE PARTICIPATION OF CYTOCHROME-P-450

Properties of the rat hepatic cholecalciferol 25-hydroxylase have been studied. An assay system has been developed in which 25-hydroxycholecalciferol production is linear for at least 2 h in both homogenates and microsomal fraction. Furthermore, the initial reaction velocity is linearly related to the amount of liver tissue or microsomal fraction. This enzyme system also metabolizes an analogue of cholecalciferol, namely dihydrotachysterol3, into 25-hydroxydihydrotachysterol3. The 25-hydroxylase is in the microsomal fraction and not in mitochondria. It has a Km of 44nM for cholecalciferol and 360nM for dihydrotachysterol. Its activity is not altered by dietary concentrations of calcium and phosphorus. Vitamin D-deficient rats have higher activities of the hepatic 25-hydroxylase than those receiving 25 ng of cholecalciferol daily. The 25-hydroxylase is inhibited by metyrapone. An atmosphere of CO/O2 (9:1, v/v) inhibits the reaction by 87%. This inhibition is partially reversed by white light. Additionally, cholecalciferol and 25-hydroxycholecalciferol competitively inhibit aminopyrine demethylase. These results support the idea that the cholecalciferol 25-hydroxylase is a cytochrome P-450-dependent mono-oxygenase.