A time-dependent inactivation of aromatase by 19-oxygenated androst-4-ene-3,6,17-triones.
19-Hydroxyandrost-4-ene-3,6,17-trione (19-OHAT), its 19-oxo derivative (19-oxo AT) and 4 beta, 5 beta-epoxyandrostane-3,6,17-trione (5) were synthesized as possible intermediates involved in a mechanism-based inactivation of aromatase caused by androst-4-ene-3,6,17-trione (AT). These compounds, inhibited the enzyme in a competitive manner with Ki's of 0.61, 7.5 and 5.1 microM for 19-OHAT, 19-oxo AT, and compound 5. The two 19-oxygenated steroids showed a time-dependent, pseudo-first order rate of inactivation of aromatase with kinact's of 0.222 and 0.076 min-1 for 19-OHAT and 19-oxo AT, respectively, while compound 5 did not. NADPH and oxygen were required for the inactivation. Androstenedione blocked the inactivation, while L-cysteine partially prevented that of 19-OHAT and almost completely that of 19-oxo AT. When the 19-oxygenated steroids were separately subjected to reaction with N-acetyl-L-cysteine, these rapidly disappeared from the reaction mixture with t1/2 of 25 min (19-OHAT) and 20 s (19-oxo AT). This finding indicates that L-cysteine prevents inactivation by a chemical dependent elimination of the inhibitors from the incubate. These results suggest that the 19-oxygenation rather than the 4,5-epoxidation may be involved in the time-dependent inactivation by AT.
Numazawa
M
M
Tohoku College of Pharmacy, Sendai, Japan.
Mutsumi
A
A
Hoshi
K
K
Kigawa
H
H
Oshibe
M
M
D000737:Androstenols / Q000737:chemistry / Q000494:pharmacology*; D001141:Aromatase / Q000378:metabolism; D047072:Aromatase Inhibitors; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D010920:Placenta / Q000201:enzymology
D000737:Androstenols; D047072:Aromatase Inhibitors; D001141:Aromatase