Objective: In order to reliably predict in vivo pharmacokinetic parameters from in vitro data, we must thoroughly understand the systems we currently use to determine enzyme kinetic parameters. There have been a number of reports of atypical Michaelis-Menten kinetics for cytochrome- (CYP) P4503A mediated metabolism in vitro but little discussion of its clinical relevance. In this manuscript, we examined the scope of CYP autoactivation and confirmed that CYP1A2 demonstrates atypical Michaelis-Menten kinetics in vitro.
Materials: Human liver microsomes, baculovirus-expressed CYP1A2, CYP1A2 in the RECO format, and E. coli expressed CYP1A2 were utilized.
Methods: Enzyme kinetics were performed using the various human CYP1A2 sources and ethoxyresorufin O-deethylation as a prototypical biotransformation. The data were fit to various models of enzyme kinetics. In some cases the data best fit the Hill equation, which was used to empirically model allosteric-type autoactivation kinetics.
Results: RECO CYP1A2 and E. coli expressed CYPIA2 both demonstrated autoactivation kinetics for ethoxyresorufin O-deethylation. When the data were fit to the Hill equation, n (the slope factor) was found to be 1.4 and 1.8 for RECO and E. coli expressed CYP1A2, respectively. Human liver microsomal and insect expressed sources of CYP1A2 illustrated classical Michaelis-Menten kinetics for the O-deethylation of ethoxyresorufin.
Conclusion: Data generated in the current study and previous work suggest many CYPs, not only CYP3A, appear to behave as allosteric enzymes. We would argue that this is not necessarily a classical allosteric mechanism because n is frequently a non-integer. This autoactivation appears to be a function of several factors including substrate physicochemical characteristics, specific interactions of the substrates (activators) with the enzyme active site, and presence of other enzyme modulators. These factors interact to increase the catalytic activity of CYP and thus the complexity of predicting enzyme kinetic parameters or drug interactions.