Having published "An Introduction to Enzyme Kinetics" in this Journal (1), I turned quickly to Raymond S. Ochs's recent paper entitled "Understanding Enzyme Inhibition" (2). While there was much of interest to me in Ochs's paper, I found myself in disagreement with him at several points.

In contrast to Ochs, I believe inhibition is best characterized by the linear double reciprocal plots, since the three possible effects on a straight line correspond exactly to the three possible modes of inhibition: a slope effect (competitive inhibition), an intercept effect (uncompetitive inhibition), and both slope and intercept effects (noncompetitive, combined, or "mixed" inhibition). It is clear that there can be no other possibilities.

Furthermore, whereas Ochs presents the initial velocity equations as eqs 5, 6, and 7, I believe that an intuitive understanding of the initial velocity equations is more easily obtained from equations in the forms shown in eqs 6 and 8 and the unnumbered equation at the bottom of the left column of page 385 in my paper. The reason I believe this is that the terms in the denominators of the fractions in the equations in my paper correspond exactly to the forms in which the enzyme can be present, and the kinetics are completely determined by the relative amounts of the forms.

Also, when the inhibitor can bind to both E and ES (noncompetitive, combined, or "mixed" inhibition), the K_m in the presence of the inhibitor can be either greater or smaller than in its absence. Ochs's assertion that "the fact that K_m is unchanged in the mixed case" can be true only if the affinity of the inhibitor for E and for ES is exactly the same--an unlikely possibility.

Additionally, while it may be surprising or even counterintuitive that K_m is smaller in the presence of a purely uncompetitive inhibitor than in its absence, further analysis provides the needed insight, as I explained in my paper starting with the last paragraph on page 384 (1).

Finally, I do not believe that the concern for data analysis is relevant. A series of experiments will contain a certain amount of information, and a mathematically and statistically valid analysis of either a "direct" plot or a "double reciprocal" plot will extract the same information.

Literature Cited

1. Ault, Addison. J. Chem. Educ. 1974, 51, 381-386.
2. Ochs, Raymond S. J. Chem. Educ. 2000, 77, 1453-1456.

See author's reply.