Isotope Discrimination in Steroid Hydroxylations

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Abstract
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 [24‐14C + 7α−3H]taurodexycholic acid was found to involve a moderate isotope effect, whereas 6β−hydroxylation of [4−14C + 6β−3H]taurochenodeoxycholic acid and 7α−hydroxylation of [4−14C + 6−3H, 7α−2H]cholesterol and [4−14C + 6−3H, 7α−2H]‐cholestanol did not involve significant isotope effects. An isotope effect was observed in the autoxidative 7α‐hydroxylation of [4−14C + 6−3H, 7α−2H]cholesterol. In vivo, [4−14C + 7α−3H]cholesterol was converted into chenodeoxycholic acid and cholic acid and [4−14C + 6β−3H]chenodeoxycholic acid into α‐ and β−muricholic acid without the occurrence of isotope effects, whereas [24−14+ 7α−3H]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.

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