RT Journal Article SR Electronic T1 Inactivation of cytochrome P4502B1 by the monoamine oxidase inhibitors R-(-)-deprenyl and clorgyline. JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 669 OP 675 VO 24 IS 6 A1 Sharma, U A1 Roberts, E S A1 Hollenberg, P F YR 1996 UL http://dmd.aspetjournals.org/content/24/6/669.abstract AB The monoamine oxidase inhibitors R-(-)-deprenyl (deprenyl) and clorgyline inactivated the 7-ethoxy-4-trifluoromethylcoumarin O-deethylase activity of purified cytochrome P4502B1 (P4502B1) in a reconstituted system containing P4502B1, NADPH-cytochrome P450 oxidoreductase, and L-alpha-phosphatidylcholine dilauroyl as the lipid. The inactivation was time- and concentration-dependent. The inactivation required NADPH, demonstrated saturation kinetics, and no lag time for inactivation was observed. The inactivation was not affected by the presence of an excess exogenous nucleophile, such as glutathione. Deprenyl exhibited a KI (concentration of inactivator required for half-maximal inactivation) of 1.05 microM, and clorgyline had a KI of 1.5 microM. The maximum rate of inactivation of the enzyme at saturating levels of inactivator (kinactivation) was 0.23 min-1 for deprenyl and 0.28 min-1 for clorgyline. The partition ratio for deprenyl and clorgyline was between 2 and 3. Studies on the isozyme specificity of these compounds in microsomes from rats pretreated with specific inducers demonstrate that the inactivation was relatively specific for the major phenobarbital-inducible isozyme, P4502B1, because significant inactivation of P4501A1 and P4502E1 was not observed. When the purified P4502B1 in the reconstituted system was incubated with either inactivator in the presence of NADPH, there was approximately a 60-70% loss in spectrally detectable P450 over 5 min at a 1:21 ratio of P450 to inactivator. However, at this ratio of enzyme to inactivator, there was > 90% loss of the O-deethylase activity. This loss in enzyme activity was irreversible, and the protein did not regain activity after dialysis or extensive washing to remove excess inactivator.