Nonhyperbolic kinetics

It should be noted that the mechanism depicted in Scheme 1 is the simplest that is consistent with mechanism-based inhibition. The mechanism for a given inhibitor and enzyme may be considerably more complex due to (a) multiple intermediates [e.g., MIC formation often involves four or more intermediates (29)], (b) detectable metabolite that may be produced from more than one intermediate, and (c) the fact that enzyme-inhibitor complex may produce a metabolite that is mechanistically unrelated to the inactivation pathway. Events such as these will necessitate alternate definitions for Z inact, Kh and r in terms of the microrate constants of the appropriate model. The hyperbolic relationship between rate of inactivation and inhibitor concentration will, however, remain, unless nonhyperbolic kinetics characterize this interaction. Silverman discussed this possibility from the perspective of an allosteric interaction between inhibitor and enzyme (5). Nonhyperbolic kinetics has been observed for the interaction of several drugs with members of the CYPs (30). 

Nonhyperbolic kinetics have been reported for some P450 2B6-catalyzed reactions but these have not been extensively characterized. ... 

A second type of nonhyperbolic saturation kinetics became apparent during studies on the metabolism of naproxen to desmethylnaproxen (32). Studies with human liver microsomes showed that naproxen metabolism has biphasic kinetics and is activated by dapsone (T. Tracy, unpublished results). The unactivated data shows what appears to be a typical concentration profile for metabolism by at least two different enzymes. However, a similar biphasic profile was obtained with expressed enzyme (25). This biphasic kinetic profile is observed with the two-substrate model when V/rn2 > Eml and Kml Km2. The appropriate equation for the two-site model when [S] < Kml is... 

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