Mechanism-Based Inactivation of Cytochrome P450 2B1 by 7-Ethynylcoumarin:  Verification of Apo-P450 Adduction by Electrospray Ion Trap Mass Spectrometry

7-Ethynylcoumarin was synthesized as a potential mechanism-based inhibitor, and it was found to be an effective inactivator of 7-ethoxy-4-(trifluoromethyl)coumarin (7EFC) O-deethylation catalyzed by purified, reconstituted P450 2B1. In contrast, 7-ethynylcoumarin demonstrated minimal inactivation of P450 2A6-mediated 7-hydroxycoumarin formation. The inactivation of P450 2B1 demonstrated pseudo-first-order kinetics and was NADPH- and inhibitor-dependent. The maximal rate constant for the inactivation of 2B1 was 0.39 min-1 at 30 °C, and thus, the time required to inactivate 50% of the P450 2B1 that was present (t 1/2) was 1.8 min. The estimated concentration which led to half-maximal inactivation (K I) was 25 μM. No protection from inactivation was seen in the presence of nucleophiles (glutathione and sodium cyanide), an iron chelator (deferroxamine), or superoxide dismutase and catalase. Addition of the substrate (7EFC) protected P450 2B1 from inactivation, in a concentration-dependent manner. The partition ratio for P450 2B1 was 25; i.e., the number of metabolic events was 25-fold higher than the number of inactivating events. Incubations of 7-ethynylcoumarin with P450 2B1 for 10 min resulted in an 80% loss in enzymatic activity, while 90% of the ability to form a reduced-CO complex remained. This activity loss was not recovered following dialysis, indicative of irreversible inactivation. Covalent attachment of the entire inhibitor and oxygen to apo-P450 2B1, in a 1:1 ratio, was shown via electrospray ion trap mass spectrometry. This method also verified the absence of modification to the heme or the cytochrome P450 reductase. Taken together, the characterization of the inhibition seen with P450 2B1 and 7-ethynylcoumarin was consistent with all of the criteria required to distinguish a mechanism-based inactivator. In addition, electrospray ion trap mass spectrometry has the potential to be applied to protein adducts above and beyond those associated with the mechanism-based inactivation of cytochrome P450s.