title: Side-chain cleavage of C21 steroids by testicular microsomal cytochrome P-450 (17 alpha-hydroxylase/lyase): involvement of heme.
abstract: The two steps in the side-chain cleavage of C21 steroids to give C19 steroids (i.e. 17 alpha-hydroxylation and C17,20 lyase activity) were examined using a highly purified cytochrome P-450 from microsomes of neonatal pig testis to determine the photochemical action spectra for the two reactions. Photochemical action spectra, using either 4-ene (progesterone) or 5-ene (pregnenolone) substrates, showed maximal reversal of inhibition by CO with light of 451 nm. Evidently the heme of cytochrome P-450 is involved in both 17 alpha-hydroxylation and in C17,20-lyase activity as in the case of the side-chain cleavage of cholesterol. Mechanisms proposed to account for enzymatic cleavage of the alpha-ketol side-chain of C21 steroids (C17,20 lyase activity) must be consistent with these findings.
mesh_terms: D000446:Aldehyde-Lyases / Q000378:metabolism*; D000818:Animals; D002248:Carbon Monoxide / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D008297:Male; D008861:Microsomes / Q000201:enzymology*; D013254:Steroid 17-alpha-Hydroxylase / Q000378:metabolism*; D013250:Steroid Hydroxylases / Q000378:metabolism*; D013256:Steroids / Q000378:metabolism*; D013379:Substrate Specificity; D013737:Testis / Q000201:enzymology*
chemical_list: D013256:Steroids; D006418:Heme; D002248:Carbon Monoxide; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D013254:Steroid 17-alpha-Hydroxylase; D000446:Aldehyde-Lyases