Inactivation of contraceptive steroid hormones by human intestinal clostridia.

Steroid hormones reduced in ring-A are devoid of hormonal activity. In metabolic experiments we found that human fecal flora reduced the delta 4-3-keto structure of natural progestins to 3 alpha-hydroxy, 5 beta-steroid metabolites (3 alpha,5 beta) and of synthetic progestins to a mixture of 3 alpha,5 beta and 3 beta,5 beta compounds. 3 alpha,5 beta-Reductase was synthesized by Clostridium paraputrificum and had a strong affinity for natural progestins such as progesterone. 3 beta,5 beta-Reductase was synthesized by Clostridium innoculin and had a stronger affinity for synthetic progestins. A third enzyme, 3 beta,5 alpha-reductase, was synthesized by St. Luke's strain 209 (Clostridium species "J-1") but was only observed when pure cultures were used. Ring-A reduction of synthetic progestins was 3 to 10 times slower than that of natural progestins, thus explaining the pharmacological superiority of synthetic progestins over naturally occurring analogs.