4.7.6 Summary of Effects of Various Inhibition Types
of Kinetic Parameters
A summary of the effects onKmandVmaxresulting from the various types of
inhibition is presented in Table 4.1, for reference.
4.7.7 Meanings of IC 50 andKiParameters
The parameters of IC 50 andKiare frequently used to describe the inhibitory
potential of a compound. However, it should be realized that the two
parameters are not interchangeable and differ in the information provided by
their estimation. Estimation of the IC 50 provides the concentration of drug
that results in 50 % inhibition of substrate metabolism. Yet, the IC 50
parameter is dependent on the concentration of both substrate and enzyme
studied. An estimation of IC 50 for a given inhibitor is only comparable to the
IC 50 estimate of another inhibitor when studied at the same substrate and
enzyme concentrations. As long as the incubation conditions are consistent,
IC 50 values can be compared for inhibition potency differences. The IC 50 does
have the advantage of being relatively easy to obtain as it requires the use of a
single substrate concentration and fewer data points than determinations ofKi
(see below). In contrast,Kivalues represent the dissociation constant of the
enzyme–inhibitor complex (not the concentration producing 50 % inhibition
as is sometimes mistakenly believed). To properly estimate the Ki for a
given inhibitor one typically uses three to four substrate concentrations and
four to five inhibitor concentrations (including zero) in the incubations.
However, the Kihas the advantage of being both enzyme and substrate
concentration independent. Thus,Kivalues can be compared across inhibitors,
regardless of the conditions studied (substrate concentrations and enzyme
concentrations).
4.8 Inhibition Kinetics Graphical Plots
As with estimation of enzyme kinetic parameters, the most accurate method for
determining inhibition kinetic parameters is through nonlinear regression
fitting of the data set. However, it may be beneficial to plot the data graphically
TABLE 4.1 Alterations in kinetic parameters caused
by various types of inhibition.
Type of inhibition Vmaxapp Vappmax=Kappm Vmapp
Competitive $""
Mixed #l"
Pure noncompetitive ##$
Uncompetitive #$#106 ENZYME KINETICS