A 21-carbon steroid, derived from CHOLESTEROL and found in steroid hormone-producing tissues. Pregnenolone is the precursor to GONADAL STEROID HORMONES and the adrenal CORTICOSTEROIDS. Pregnenolone is a derivative of cholesterol, the product of Cytochrome P450 side-chain cleavage (EC, CYP11A1); this reaction consists of three consecutive monooxygenations; a 22-hydroxylation, 20-hydroxylation and the cleavage of the C20-C22 bond, yielding pregnenolone. Pregnenolone is the precursor to gonadal steroid hormones and the adrenal corticosteroids. This reaction occurs in steroid hormone-producing tissues such as the adrenal cortex, corpus luteum and placenta. The most notable difference between the placenta and other steroidogenic tissues is that electron supply to CYP11A1 limits the rate at which cholesterol is converted to pregnenolone in the placenta. The limiting component for electron delivery to CYP11A1 is the concentration of adrenodoxin reductase in the mitochondrial matrix which is insufficient to maintain the adrenodoxin pool in a fully reduced state. Pregnenolone is also a neurosteroid, and is produced in the spinal cord; CYP11A1 is the key enzyme catalyzing the conversion of cholesterol into pregnenolone, the rate-limiting step in the biosynthesis of all classes of steroids, and has been localized in sensory networks of the spinal cord dorsal horn. In the adrenal glomerulosa cell angiotensin II, one of the major physiological regulators of mineralocorticoid synthesis, appears to affect most of the cholesterol transfer to the mitochondrial outer membrane and transport to the inner membrane steps and thus to exerts a powerful control over the use of cholesterol for aldosterone production. (PMID: 17222962, 15823613, 16632873, 15134809) [HMDB]

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