Formation of steroids by the pregnant mare. V. Metabolism of 14C-isopentenylpyrophosphate and 3H-dehydroisoandrosterone injected into the fetus.

A mixture of 1-14C-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, 17alpha-estradiol, equilin and equilenin were isolated. Only estrone and 17alpha-estradiol contained both 3H and 14C, while the ring B unsaturated estrogens contained only 14C. From the neutral fraction 14C-labeled 3beta-hydroxy-5alpha-pregnan-20-one, 5alpha-pregnane-3beta,20beta-diol and 5alpha-pregnan-3beta, 20alpha-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.