Michael Waterman
Faculty Member
Last active: 2/12/2015

Progesterone 16 alpha-hydroxylase activity is catalyzed by human cytochrome P450 17 alpha-hydroxylase.

Swart P, Swart AC, Waterman MR, Estabrook RW, Mason JI
J Clin Endocrinol Metab. 1993 77 (1): 98-102

PMID: 8325965 · DOI:10.1210/jcem.77.1.8325965

Progesterone and pregnenolone are metabolized to 17 alpha-hydroxysteroids by a cytochrome P450-dependent 17 alpha-hydroxylase (P450c17). The same enzyme can also catalyze the removal of the side-chain of these 17 alpha-hydroxylated steroids to yield androstenedione and dehydroepiandrosterone, respectively. We investigated the metabolism of progesterone by monkey kidney tumor (COS 1) cells transfected with a plasmid vector containing the cDNA encoding the complete amino acid sequence for human cytochrome P450c17. Transfected COS 1 cells converted progesterone to 17 alpha-hydroxyprogesterone as well as 16 alpha-hydroxyprogesterone, but no detectable androstenedione was produced. However, pregnenolone was converted to 17 alpha-hydroxypregnenolone and, ultimately, dehydroepiandrosterone. No 16 alpha-hydroxypregnenolone was produced. The kinetics of progesterone metabolism by the enzyme expressed in COS 1 cells indicated that both 17 alpha- and 16 alpha-hydroxylated products were products were produced from a common active site. Microsomes prepared from fetal adrenal and adult testis converted progesterone to 17 alpha-hydroxyprogesterone as well as 16 alpha-hydroxyprogesterone. No detectable androstenedione was produced by these preparations. Antibodies raised against porcine cytochrome P450c17 inhibited the 17 alpha- and 16 alpha-hydroxylation of progesterone to the same extent when using fetal adrenal microsomes, whereas no inhibition of 21-hydroxylation of progesterone was observed. Similar results were obtained with the imidazole antimycotic agent ketoconazole, which is a preferential cytochrome P450c17 inhibitor. From these results we conclude that human cytochrome P450c17 exhibits marked progesterone 16 alpha-hydroxylase activity in addition to its 17 alpha-hydroxylase function when expressed not only in a heterologous cell expression system but also, importantly, in human steroidogenic cells. Furthermore, the human enzyme has extremely low C-17,20-lyase activity toward progesterone, 17 alpha-hydroxyprogesterone, and 16 alpha-hydroxyprogesterone and fails to convert these to corresponding C19 steroids.

MeSH Terms (24)

17-alpha-Hydroxyprogesterone Adrenal Glands Animals Aryl Hydrocarbon Hydroxylases Cell Line, Transformed Chlorocebus aethiops Cytochrome P-450 CYP2C8 Cytochrome P-450 CYP2C9 DNA Humans Hydroxyprogesterones Ketoconazole Kidney Kinetics Male Microsomes NADP Pregnenolone Progesterone Steroid 16-alpha-Hydroxylase Steroid 17-alpha-Hydroxylase Steroid Hydroxylases Testis Transfection

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