plot suffers less from the error issues discussed above regarding the double-
reciprocal plot and thus, may be the most preferred graphical plot for
estimating kinetic values.
Finally, investigators also have used the Eadie–Hofstee plot to estimate
enzyme kinetic parameters (Fig. 4.4). In this last case,nis plotted along the
y-axis andn/[S] along thex-axis. The slope of the best-fit line is equal toKm,
the y-intercept =Vmax and the x-intercept =Vmax/Km. As opposed to the
double-reciprocal plot, the Eadie–Hofstee plot can make good data look worse
(Dowd and Riggs, 1965). Interestingly, another use of the Eadie–Hofstee plot
is to diagnose atypical kinetic profiles based on the shape of the data-fit
obtained. This will be discussed in detail later in the chapter.
4.5 ATYPICAL KINETICSALLOSTERIC EFFECTS
4.5.1 Overview of Atypical Kinetic Phenomena
For the purposes of this discussion, the term ‘‘atypical kinetics’’ is used to
describe an in vitro drug metabolism kinetic profile that does not fit the
standard hyperbolic function when velocity is plotted versus substrate
concentration. Though examples of atypical kinetic phenomena have existed
for many years, it has only been in the past 10 years that these types of kinetic
profiles in drug metabolism enzyme kinetics have been extensively documented
and studied in a systematic manner. Though the primary focus of these studies
has been on the cytochrome P450 enzymes, increasing numbers of examples of
atypical phenomena in glucuronosyl transferase enzymes, transporters, and
other drug metabolizing and disposition enzymes are becoming evident. The
FIGURE 4.4 Eadie–Hofstee plot of a reaction that follows Michaelis–Menten kinetics.
Kinetic parameters are as defined previously.
94 ENZYME KINETICS