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The effect of RMI 12,936, a synthetic antiprogestational steroid, on ovarian steroidogenesis in the rat

Summary. A series of physiochemical investigations confirmed that the product of chemical and enzymatic isomerization of RMI 12,936 was 7-methyltestosterone. The total activity per pair of ovaries of ⁵ 3-ketosteroid isomerase in vitro was unchanged by RMI 12,936 pretreatment or by advancing pregnancy, significant changes in ovarian weight being accompanied by reciprocal changes in enzyme activity/mg tissue. The initial linear rate of isomerization of RMI 12,936 was approximately five times greater than the corresponding rate of ⁵-progesterone isomerization at equal substrate concentrations. It is concluded that RMI 12,936 does not inhibit progesterone biosynthesis by alteration of ⁵ 3-Ketosteroid isomerase activity, but that it may do so by acting as an alternative substrate for this enzyme.