Isotope Discrimination in Steroid Hydroxylations

The possible occurrence of isotope effects in steroid hydroxylations by rat and guinea‐pig liver was studied in vitro and in vivo . In vitro , the 7α‐hydroxylation of [24‐ 14 C + 7α− 3 H]taurodexycholic acid was found to involve a moderate isotope effect, whereas 6β−hydroxylation of [4− 14 C + 6β− 3 H]taurochenodeoxycholic acid and 7α−hydroxylation of [4− 14 C + 6− 3 H, 7α− 2 H]cholesterol and [4− 14 C + 6− 3 H, 7α− 2 H]‐cholestanol did not involve significant isotope effects. An isotope effect was observed in the autoxidative 7α‐hydroxylation of [4− 14 C + 6− 3 H, 7α− 2 H]cholesterol. In vivo , [4− 14 C + 7α− 3 H]cholesterol was converted into chenodeoxycholic acid and cholic acid and [4− 14 C + 6β− 3 H]chenodeoxycholic acid into α‐ and β−muricholic acid without the occurrence of isotope effects, whereas [24− 14 + 7α− 3 H]deoxycholic acid was converted into cholic acid with the occurrence of a moderate isotope effect. The results show that breaking of the C‐H bound is not rate limiting in the 7α−hydroxylation of cholesterol and cholestanol or in the 6β−hydroxylation of taurochenodeoxycholic acid but may be rate limiting in the 7α−hydroxylation of taurodeoxycholic acid.