MECHANISM AND STEREOCHEMISTRY OF ENZYMATIC CONVERSION OF A DELTA4-3-OXOSTEROID INTO A 3-OXO-5ALPHA-STEROID

The conversion of 7α‐hydroxycholest‐4‐en‐3‐one into 7α‐hydroxy‐5α‐cholestan‐3‐one and 5α‐cholestane‐3β,7α‐diol has been studied in a microsomal system. When [4‐14C]7α‐hydroxy‐cholest‐4‐en‐3‐one was incubated with female rat liver microsomes in the presence of NADPH labeled with 3H in the 4A‐ or the 4B‐position of the nicotinamide ring, 3H was incorporated into the 5α‐position of 7α‐hydroxy‐5α‐cholestan‐3‐one. The incorporation was five times greater with [4B‐3H]NADPH than with [4A‐3H]NADPH. In the presence of microsomal fraction fortified with NADPH and NADH [4‐3H,4‐14C]7α‐hydroxycholest‐4‐en‐3‐one was converted into 5α‐cholestane‐3β,7α‐diol with retention of the main part of the 3H. The [4‐3H,4‐14C]5α‐cholestane‐3β,7α‐diol formed was converted chemically into 3β‐acetoxycholest‐5‐en‐7‐one and 3β‐benzoyl‐oxycholest‐5‐en‐7‐one. These two compounds were transformed into cholest‐3,5‐dien‐7‐one by respectively ionic trans elimination and pyrolytic cis elimination of the 4‐hydrogen. On the basis of the content of 3H in the cholest‐3,5‐dien‐7‐one prepared by the two methods it was concluded‐that at least half of the original 3H in the enzymatically formed [4‐3H,4‐14C]5α‐cholestane‐3β,7α‐diol was located in the 4β‐position. Thus, during the conversion of 7α‐hydroxycholest‐4‐en‐3‐one into 7α‐hydroxy‐5α‐cholestan‐3‐one at least half of the hydrogen entering the 4‐position is introduced into the 4α‐position. The results indicate that saturation of the double bond involves either a cis addition of hydrogens or a non‐stereospecific addition rather than a trans addition. Copyright © 1969, Wiley Blackwell. All rights reserved