Inactivation of delta5-3-ketosteroid isomerase(s) from beef adrenal cortex by beta, gamma-acetylenic ketosteroids.
The beta, gamma-acetylenic ketosteroids, 5-10-seco-19-norpregn-5-yne-3,-10,20-trione approximately 1 and 5,10-secoestr-5-yne-3,10,17-trione approximately 2 irreversibly inactivate both the C19-and the C21-delta 5-3-ketosteroid isomerase activities of beef adrenal cortex microsomes. At saturating concentrations of inhibitor half-lives of these enzyme activities vary from 45 to 240 s. It is uncertain whether the enzyme generates its own alkylating agent by isomerizing compounds approximately 3 and approximately 4 to the corresponding allenic ketones, namely (4R)-5,10-seco-19-norpregn-4,5-diene-3,10,20-trione approximately 3 and (4R)-5,10-secoestra-4,5-diene-3,10,17-trione approximately 4 since these are formed spontaneously in the buffer used to stabilize enzyme activity. In the presence of catalytic quantities of adrenal enzyme compound approximately 4 is a powerful competitive inhibitor for both 5-androstene-3,17-dione (Ki 8.0 microM) and 5-pregnene-3,20-dione (Ki 3.5 microM) indicating that the eventual alkylating event is active site-directed. The differences in Ki values and half-lives for inactivation support the view that the C19- and C21-delta 5-3-ketosteroid isomerase activities do not reside at the same catalytic site in beef adrenal cortex microsomes.
D000302:Adrenal Cortex / Q000201:enzymology*; D000478:Alkylation; D000818:Animals; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D004963:Estrenes / Q000494:pharmacology*; D006207:Half-Life; D007535:Isomerases / Q000037:antagonists & inhibitors*; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D009652:Norpregnenes / Q000494:pharmacology*; D012632:Secosteroids / Q000494:pharmacology*; D013251:Steroid Isomerases / Q000037:antagonists & inhibitors*; D013997:Time Factors
D004963:Estrenes; D009652:Norpregnenes; D012632:Secosteroids; C024783:5,10-secoestr-5-yne-3,10,17-trione; C030828:5,10-seco-19-norpregn-5-yne-3,10,20-trione; D007535:Isomerases; D013251:Steroid Isomerases