Steroid Biosynthesis

Although the adrenal cortex is primarily involved in the synthesis and secretion of corticosteroids, it is also capable of producing and secreting such steroid intermediates as progesterone, androgens, and estrogens. The adrenal gland synthesizes steroids from cholesterol, which is derived from plasma lipoproteins via the low-and high-density lipoprotein pathways. Additionally, cholesterol is enzymatically released extramitochondri-ally from cholesterol esters catalyzed by a cholesterol ester hydrolase. The corticotrophin-dependent stimulation of cholesterol ester hydrolase activity provides an additional source of cholesterol for steroidogenesis.

Cholesterol is transported into the mitochondria of steroidogenic tissue, where side chain cleavage is carried out. In common with other mixed-function oxidase systems, the cholesterol side chain cleavage requires reduced nicotinamide-adenine dinucleotide phosphate

(NADPH), oxygen, and a specific cytochrome P450. The rate-limiting step in steroid biosynthesis is the conversion of cholesterol to pregnenolone (Fig. 60.1).

Pregnenolone leaves the mitochondria to become the obligatory precursor of corticosteroids and adrenal androgens. The biosynthetic pathway next branches into two separate routes. One route passes through progesterone and corticosterone to aldosterone, and the other

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C=O c=o ch2oh ch2oh

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