(A–D) Initial reaction velocities were assayed over a pH interval ranging from 5.0 to 8.0, in the presence of 2.45 nM LDH-A tetramer (9.8 nM subunits), 125 µM β-NADH, and a universal buffer (containing Mes, Mops, and Tris, 25 mM each). The dependence of initial reaction velocities on pyruvate concentration at pH 5.0 (A, empty circles), pH 5.5 and 6.0 (B, filled and empty circles, respectively), pH 6.5 and 7.0 (C, filled and empty circles, respectively), pH 7.5 and 8.0 (D, filled and empty circles, respectively) are shown. The continuous lines represent the best fit of the Hill (pH 5.0–7.0) or the Michaelis–Menten (pH 7.5–8.0) equation to the experimental observations.