Formation of Steroids by the Pregnant Mare.VI. Metabolism of [14C]Farnesyl Pyrophosphate and [3H]Dehydroepiandrosterone Injected into the Fetus*

Abstract

A mixture of [4,8,12-14C]farnesyl pyrophosphate and [3H]dehydroepiandrosterone was injected into a horse fetus im during laparotomy, after which maternal urine was collected for 6 days. Steroid conjugates in the urine were extracted with Amberlite XAD-2 resin, hydrolyzed, and separated into phenolic and neutral fractions. Estrone, 17 alpha-estradiol, equilin [3-hydroxy-1,3,5(10),7-estratetraen-17-one], and 17 alpha-dihydroequilin [1,3,4(10),7-estratetraene-3,17 alpha-diol] were isolated from the phenolic fraction and their radiochemical purities were established. Only estrone and 17 alpha-estradiol contained both 3H and 14C, while the B ring unsaturated estrogens, equilin and 17 alpha-dihydroequilin, contained only 14C. From the neutral fraction, 14C-labeled 3 beta-hydroxy-5 alpha-pregnan-20-one, 5 alpha-pregnane-3 beta-20 beta-diol, and 5 alpha-pregnane-3 beta, 20 alpha-diol were isolated. These results together with our previous findings demonstrate that the route of biosynthesis of both the ring B saturated and unsaturated estrogens is the same up to the stage of farnesyl pyrophosphate. Thus, the bifurcation in the classical pathway of steroid biosynthesis is occurring at a point after the formation of farnesyl pyrophosphate and before the formation of squalene and cholesterol.