Biosynthesis Of Steroids

A. Introduction

The principal tissues of synthesis of the five classical classes of steroid hormones (estrogens, androgens, progestins, glucocorticoids, and mineralocorticoids) are the adrenal cortex, ovaries, and testes. Also, during pregnancy, the fetal-placental unit can serve as a source of estrogen and some other hormones. The sites of synthesis of the sixth class of steroid hormones, the prohormone vitamin D3 and its metabolites, are the skin, liver, and kidney (see Section IV.F). The bile acids, which are the seventh important structural class of mammalian steroids, have no known hormonal activity; they are principally synthesized in the liver (see Section IV.G).

Figure 2-18 outlines the general framework of the metabolic pathways that are used to convert cholesterol into steroid hormones; the major steps are listed, as are the tissues where the transformations occur.

Table 2-5 summarizes the tissue-specific expression of the cytochrome P450 enzymes involved with the production of the classical steroid hormones. A total of six P450 enzymes is associated with the production of the classical steroid hormones produced by the adrenal cortex, ovaries, and testes.

Figure 2-19 summarizes the peptide hormones that stimulate the biosynthesis of the six steroid hormones.

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