Canadian Medical Guide > Chemicals and Drugs > Enzymes and Coenzymes > Cytochromes > Cytochrome P-450 Enzyme System > Steroid Hydroxylases Terms and Definitions

Steroid Hydroxylases

Medical Definition:	Cytochrome P-450 monooxygenases (MIXED FUNCTION OXYGENASES) that are important in steroid biosynthesis and metabolism.
Guide Notes:	GEN; prefer specifics
CAS Number:	EC 1.14.-
Previously Indexed:	Hydroxylases (1966-1972)

Steroid Hydroxylases Categories.

	25-Hydroxyvitamin D3 1-alpha-Hydroxylase - A mitochondrial cytochrome P450 enzyme that catalyzes the 1-alpha-hydroxylation of 25-hydroxyvitamin D3 (also known as 25-hydroxycholecalciferol) in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP27B1 gene, converts 25-hydroxyvitamin D3 to 1-alpha,25-dihydroxyvitamin D3 which is the active form of VITAMIN D in regulating bone growth and calcium metabolism. This enzyme is also active on plant 25-hydroxyvitamin D2 (ergocalciferol).
	Aldosterone Synthase - A mitochondrial cytochrome P450 enzyme that catalyzes the 18-hydroxylation of steroids in the presence of molecular oxygen and NADPH-specific flavoprotein. This enzyme, encoded by CYP11B2 gene, is important in the conversion of CORTICOSTERONE to 18-hydroxycorticosterone and the subsequent conversion to ALDOSTERONE.
	Aromatase - An enzyme that catalyzes the desaturation (aromatization) of the ring A of C19 androgens and converts them to C18 estrogens. In this process, the 19-methyl is removed. This enzyme is membrane-bound, located in the endoplasmic reticulum of estrogen-producing cells of ovaries, placenta, testes, adipose, and brain tissues. Aromatase is encoded by the CYP19 gene, and functions in complex with NADPH-FERRIHEMOPROTEIN REDUCTASE in the cytochrome P-450 system.
	Cholesterol 7-alpha-Hydroxylase - A membrane-bound cytochrome P450 enzyme that catalyzes the 7-alpha-hydroxylation of CHOLESTEROL in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP7, converts cholesterol to 7-alpha-hydroxycholesterol which is the first and rate-limiting step in the synthesis of BILE ACIDS.
	Cholesterol Side-Chain Cleavage Enzyme - A mitochondrial cytochrome P450 enzyme that catalyzes the side-chain cleavage of C27 CHOLESTEROL to C21 PREGNENOLONE in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP11A1 gene, catalyzes the breakage between C20 and C22 which is the initial and rate-limiting step in the biosynthesis of various gonadal and adrenal steroid hormones.
	Steroid 11-beta-Hydroxylase - A mitochondrial cytochrome P450 enzyme that catalyzes the 11-beta-hydroxylation of steroids in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP11B1 gene, is important in the synthesis of CORTICOSTERONE and HYDROCORTISONE. Defects in CYP11B1 cause congenital adrenal hyperplasia (ADRENAL HYPERPLASIA, CONGENITAL).
	Steroid 12-alpha-Hydroxylase - A liver microsomal cytochrome P450 enzyme that catalyzes the 12-alpha-hydroxylation of a broad spectrum of sterols in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP8B1gene, converts 7-alpha-hydroxy-4-cholesten-3-one to 7-alpha-12-alpha-dihydroxy-4-cholesten-3-one and is required in the synthesis of BILE ACIDS from cholesterol.
	Steroid 16-alpha-Hydroxylase - A liver microsomal cytochrome P450 enzyme that catalyzes the 16-alpha-hydroxylation of a broad spectrum of steroids, fatty acids, and xenobiotics in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme is encoded by a number of genes from several CYP2 subfamilies.
	Steroid 17-alpha-Hydroxylase - A microsomal cytochrome P450 enzyme that catalyzes the 17-alpha-hydroxylation of progesterone or pregnenolone and subsequent cleavage of the residual two carbons at C17 in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP17 gene, generates precursors for glucocorticoid, androgen, and estrogen synthesis. Defects in CYP17 gene cause congenital adrenal hyperplasia (ADRENAL HYPERPLASIA, CONGENITAL) and abnormal sexual differentiation.
	Steroid 21-Hydroxylase - An adrenal microsomal cytochrome P450 enzyme that catalyzes the 21-hydroxylation of steroids in the presence of molecular oxygen and NADPH-FERRIHEMOPROTEIN REDUCTASE. This enzyme, encoded by CYP21 gene, converts progesterones to precursors of adrenal steroid hormones (CORTICOSTERONE; HYDROCORTISONE). Defects in CYP21 cause congenital adrenal hyperplasia (ADRENAL HYPERPLASIA, CONGENITAL).

Steroid Hydroxylases Medical Definitions and Terms

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