RABBIT LIVER CONTAINS ONE MAJOR STEROL 12-ALPHA-HYDROXYLASE WITH BROAD SUBSTRATE-SPECIFICITY

Conversion of cholesterol into cholic acid in mammalian liver requires a 12 alpha-hydroxylation step. Results have been presented suggesting that two different enzymes are involved in this hydroxylation with di fferent activities towards the two steroids believed to be the physiol ogical substrates for the enzyme, 7 alpha-hydroxy-4-cholesten-3-one an d 5 beta-cholestane-3 alpha,7 alpha-diol. It is shown here that rabbit Liver microsomes and partly purified sterol 12 alpha-hydroxylase as w ell as COS cells transfected with a cDNA coding for this enzyme are ab le to catalyze 12 alpha-hydroxylation of the two substrates at similar relative rates. Also 7 alpha-hydroxycholesterol and 3 alpha,7 alpha-d ihydroxy-5 beta-cholestanoic acid are 12 alpha-hydroxylated by the thr ee systems. It is concluded that rabbit liver contains one major stero l 12 alpha-hydroxylase with a broad substrate specificity. (C) 1998 El sevier Science B.V.