Rabbit liver contains one major sterol 12alpha-hydroxylase with broad substrate specificity.

Conversion of cholesterol into cholic acid in mammalian liver requires a 12alpha-hydroxylation step. Results have been presented suggesting that two different enzymes are involved in this hydroxylation with different activities towards the two steroids believed to be the physiological substrates for the enzyme, 7alpha-hydroxy-4-cholesten-3-one and 5beta-cholestane-3alpha,7alpha-diol. It is shown here that rabbit liver microsomes and partly purified sterol 12alpha-hydroxylase as well as COS cells transfected with a cDNA coding for this enzyme are able to catalyze 12alpha-hydroxylation of the two substrates at similar relative rates. Also 7alpha-hydroxycholesterol and 3alpha,7alpha-dihydroxy-5beta-cholestanoic acid are 12alpha-hydroxylated by the three systems. It is concluded that rabbit liver contains one major sterol 12alpha-hydroxylase with a broad substrate specificity.