activity in 30 min (first order kinetics) is measured and apparent rate constant
(kobs) can be obtained by an initial slope of the linear regression lines of
semilogarithmic plots (Fig. 16.6a). A comprehensive evaluation of compounds
on MBI is perfomred to generate its kinetic parameters (KI and kinact)
(Fig. 16.6b and c). As reported in literature,KIandkinactvalues for most
FIGURE 16.6 Methods to determine KIand kinact of Zileuton in human liver
microsomes. (a) Aliquots were removed from the primary reaction mixture at the
indicated time points and were assayed for residual marker activity (phenacetin O-
deethylation). Percent activity remaining (related to time zero in the presence of solvent
alone) was plotted in the logarithmic scale determined from a single experiment. The
slopes in linear ranges at various inhibitor concentrations were defined askobs.(b)
Determination ofkinact(0.045 min^1 ) andKI(131.7mM) using nonlinear regression
(Eq. 16.5). (c) Determination ofkinact(0.035 min^1 ) andKI(117mM) using Linewever–
Burk plot (Eq. 16.6).
530 ANALYSIS OFIN VITROCYTOCHROME P450 INHIBITION