| Biosynthesis of the steroid hormones
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| Conversion of cholesterol into steroid hormones occurs in only three organs: the adrenal cortex, the testis in men and the ovary in women.
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| It is normal practice to consider the corticosteroids as the products of the adrenal cortex, the androgens as the products of the testis and the estrogens as the products of the ovary. A simplified pathway of the steroid synthesis is shown in Figure 16.7 (see also Chapter 37). The relative activity of the steroidogenic enzymes in each of the three organs determines the major secreted product; however, this is not absolute and all three organs are capable of secreting small amounts of steroids from other groups. In pathologic situations, such as a defect in steroidogenesis or a steroid-secreting tumor, a very abnormal pattern of steroid secretion may be observed.
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| Cytochrome P450 mono-oxygenase enzymes control steroidogenesis
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| Most of the enzymes involved in converting cholesterol into steroid hormones are cytochrome P450 proteins that require oxygen and NADPH. In its simplest form, this enzyme complex catalyzes the replacement of a carbon-hydrogen bond with a carbon-hydroxyl bond; hence, the collective term mono-oxygenase. Hydroxylation of adjacent carbon atoms is the forerunner to cleavage of the carbon-carbon bond. Comparison of the structure of cholesterol (see Fig. 16.1) with those of the steroid hormones (see Fig. 16.7) demonstrates that the biosynthetic pathway is largely made up of cleavage of carbon-carbon bonds and hydroxylation reactions. These enzymes have their own nomenclature in which the symbol CYP is followed by a specific suffix. Thus, CYP21A2 refers to the enzyme that hydroxylates carbon atom 21. (See also Chapter 28, p. 408.)
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