Formation of Steroids by the Pregnant Mare. V. Metabolism of14C-Isopentenylpyrophosphate and3H-Dehydroisoandrosterone Injected into the Fetus1

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Abstract
A mixture of l-14C-isopentenylpyrophosphate and 3H-dehydroisoandrosterone was injected into a horse fetus intramuscularly during laparotomy, after which maternal urine was collected for 4 days. Steroid conjugates in the urine were extracted with Amberlite XAD-2 resin, hydrolysed and separated into phenolic and neutral fractions. From the phenolic fraction estrone, 17α-estradiol, equilin and equilenin were isolated. Only estrone and 17α-estradiol contained both 3H and 14C, while the ring B unsaturated estrogens contained only 14C. From the neutral fraction 14C-labeled 3β-ydroxy- 5α-pregnan-20-one, 5α-pregnane-3β,20β-diol and 5α-pregnan-3β,20α-diol were isolated. These results demonstrate that the route of biosynthesis of both the ring B saturated and unsaturated estrogens is the same up to the stage of isopentenylpyrophosphate. Thus, the bifurcation in the classical pathway of steroid biosynthesis reported previously by us is occurring at a point after the formation of isopentenylpyrophosphate and prior to the formation of squalene. (Endocrinology96: 1009, 1975)

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