CHOLECALCIFEROL 25-HYDROXYLATION IS SIMILAR IN LIVER-MICROSOMES FROM MALE AND FEMALE RATS WHEN CHOLECALCIFEROL CONCENTRATION IS LOW

We compared cholecalciferol 25-hydroxylation in liver microsomes of ma le and female rats. The rate of production of 25-hydroxycholecalcifero l was similar in liver microsomes from female rats and those from male rats when cholecalciferol concentration ranged from 50 to 200 nmol/L. The liver cytosolic fraction stimulated the 25-hydroxylase activity o f the microsomes up to 100% in both male and female rats at 44 nmol/L cholecalciferol. Cytosol metabolized cholecalciferol to a currently un identified metabolite. At 300 nmol/L cholecalciferol, synthesis of the cytosolic metabolite was 100% greater than at 100 nmol/L and coincide d with 32% lower synthesis of 25-hydroxycholecalciferol. These results suggest similar 25-hydroxy-lase activity in liver microsomes from mal e and female rats and similar ability of liver cytosol from these rats to stimulate 25-hydroxylation at low nanomolar concentrations of chol ecalciferol, whereas inhibitory effects of cytosol at higher concentra tions of cholecalciferol were shown.