7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid by cytochrome P-450 in pig liver microsomes.

Pig liver microsomes were found to catalyze the 7 alpha-hydroxylation of several potential bile acid precursors besides cholesterol. 26-Hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid were all efficiently converted into the 7 alpha-hydroxylated products. Two cytochrome P-450 fractions showing 7 alpha-hydroxylase activity could be isolated. One fraction catalyzed 7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid but was inactive towards cholesterol. The other fraction catalyzed 7 alpha-hydroxylation of cholesterol in addition to the other substrates. 26-Hydroxycholesterol in equimolar concentration did not inhibit the cholesterol 7 alpha-hydroxylase activity of this fraction. It is concluded that liver microsomes contain a cytochrome P-450 catalyzing 7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid. The results indicate that this cytochrome P-450 is different from that catalyzing 7 alpha-hydroxylation of cholesterol.