Steroidogenesis in the Mammalian Testis

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Abstract
THE MAMMALIAN testis has long been known to synthesize androgens, primarily testosterone, from several steroidal and nonsteroidal precursors. Some of the precursors that are converted to testosterone include acetate (1), cholesterol (2), pregnenolone1 (3, 4), and progesterone (5). The primary site for the synthesis of testosterone within the mammalian testis is the interstitial cells of Leydig, which are specialized cells located in the angular spaces between seminiferous tubules (6). Histochemical procedures indicate that androgens are also synthesized and secreted to some degree by the Sertoli cells and specific germinal cells within the seminiferous tubules, with the Leydig cells being the most efficient testicular components for the synthesis of androgens (6). The Leydig cell contains an extensive smooth endoplasmic reticulum, prominent Golgi complex, lipid droplets, and numerous lysosomes, all indicative of an actively synthesizing cell (6). The synthesis of androgens involves a complex process in which two-carbon acetate precursors are linked together. This sequence of events initially involves the formation of cholesterol, a 27-carbon intermediate sterol, with mevalonic acid, farnesyl-pyrophosphate, squalene, and lanosterol identified as precursors of cholesterol (Fig. 1). The conversion of cholesterol to pregnenolone is accomplished by cholesterol esterase, an enzyme that is under the regulation of luteinizing hormone (LH) from the pituitary. This conversion involves the removal of a six-carbon side chain from the C-17 of cholesterol, converting it to pregnenolone.

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