25-hydroxylation of vitamin D3 by a reconstituted system from rat liver microsomes

Abstract Using isotope dilution—mass fragmentography as assay technique, it was shown that highly purified preparations of cytochrome P-450 from rat liver microsomes catalyzed 25-hydroxylation of vitamin D 3 when combined with NADPH-cytochrome P-450 reductase and a phospholipid. The rate of conversion was approximately linear with the amount of cytochrome P-450, and was considerably higher than the rate of conversion obtained with crude liver microsomes. The possibility is discussed that the microsomal fraction contains inhibitors of 25-hydroxylase activity, which may be of regulatory importance in vitamin D 3 metabolism.