In comparison with the reversible enzyme inhibition, the MBI can be
characterized to be (1) dose-dependent; (2) preincubation time-dependent; (3)
bioactivation required for inactivation of the target enzyme; (4)de novoprotein
synthesis to return metabolic capacity; and (5) slow onset of the effects but
more profound than the reversible inhibition.
16.3.1 Kinetic Model for Mechanism-Based Inhibition
Kinetic model for mechanism-based inhibition is proposed in Scheme 16.3
(Waley, 1980; Walsh et al., 1978). Inactivation of the enzyme is an irreversible
process over the time scale of the experiment. At the given concentrations of
inhibitor and enzyme, the reactions indicated in Scheme 16.3 are governed by
the first-order rate constantsk 1 ,k 1 ,k 2 ,k 3 andk 4 , respectively. The rate of
enzyme inactivation can be introduced by Equation 16.3 (Jung and Metcalf,
1975; Kitz and Wilson, 1962).
ln½E
½Etotð%activity remainingÞ¼kinact½It
KIþ½Ið 16 : 3 ÞFIGURE 16.5 Celecoxib inhibition of CYP2C19-catalyzed (S)-mephenytoin 4^0 -
hydroxylation in human liver microsomes: Vmax= 0.400.02 nmol/min/mg,
Km= 13.81.9mM,Ki= 3.20.4mM, RSS = 0.002, andR^2 = 0.981, respectively.
All kinetic values were determined by multiple nonlinear regression (3-dimension) using
the velocity equation of competitive inhibition in Table 16.3.
528 ANALYSIS OFIN VITROCYTOCHROME P450 INHIBITION