title: Cholesterol side-chain cleavage by mitochondria from the human placenta. Studies using hydroxycholesterols as substrates.
abstract: The side-chain cleavage of cholesterol by cytochrome P-450scc in mitochondria from the human placenta was studied using hydroxycholesterol substrates and intermediates of the reaction. 25-Hydroxycholesterol inhibited 3 beta-hydroxy-5-pregnen-20-one (pregnenolone) production by placental mitochondria. It was converted to pregnenolone at a maximum velocity of only 19% of that for cholesterol. Addition of 20 alpha-hydroxycholesterol or 22R-hydroxycholesterol to placental mitochondria caused a lag in pregnenolone synthesis which was concentration dependent. Measurement of the concentration of 20 alpha,22R-dihydroxycholesterol during incubation of placental mitochondria with 22R-hydroxycholesterol revealed that the lag in pregnenolone production was caused by accumulation of 20 alpha,22R-dihydroxycholesterol. This intermediate of the reaction dissociated from the active site of cytochrome P-450scc. Only after its concentration had increased, presumably to a level where it could compete with 22R-hydroxycholesterol for binding to cytochrome P-450scc, was it converted to pregnenolone. These results indicate a lack of kinetic stabilization of the cytochrome P-450scc-20 alpha,22R-dihydroxycholesterol complex with dissociation occurring more rapidly than the final hydroxylation. Similar measurements of side-chain cleavage of 22R-hydroxycholesterol by mitochondria from the bovine adrenal cortex showed that kinetic stabilization of the cytochrome P-450scc-20 alpha,22R-dihydroxycholesterol complex does not occur in that tissue either. The relative hydroxylation rates of 20 alpha-hydroxycholesterol, 22R-hydroxycholesterol and 20 alpha,22R-dihydroxycholesterol indicate that all three hydroxylations catalysed by human cytochrome P-450scc occur at approximately the same rate.
authors.affiliation: Department of Biochemistry, University of Western Australia, Nedlands.
authors.forename: R C
authors.initials: RC
authors.lastname: Tuckey
mesh_terms: D000818:Animals; D002417:Cattle; D002786:Cholesterol Side-Chain Cleavage Enzyme / Q000378:metabolism*; D005260:Female; D006801:Humans; D006888:Hydroxycholesterols / Q000378:metabolism*; D006900:Hydroxylation; D008928:Mitochondria / Q000378:metabolism*; D010920:Placenta / Q000378:metabolism*; D011284:Pregnenolone / Q000096:biosynthesis; D051381:Rats
chemical_list: D006888:Hydroxycholesterols; C003585:22-hydroxycholesterol; C025813:20-hydroxycholesterol; D011284:Pregnenolone; C007997:25-hydroxycholesterol; D002786:Cholesterol Side-Chain Cleavage Enzyme