(A,B) Initial reaction velocities determined at pH 5.0 (A) and 6.5 (B) in the presence of 8 nM recombinant human LDH-A tetramer (32 nM subunits), 125 µM β-NADH, and 100 µM pyruvate (control, gray bars). The LDH-A activities observed using reaction mixtures supplemented with the indicated compounds at 2.5 mM are also shown. The reported values of initial reaction velocity are the average of three samples. The error bars represent standard deviation (n=3). Experimental mean values were compared by Student’s t test (***, ** and * indicate P<0.001, P<0.01, P<0.05, respectively). (C,D) Initial reaction velocities determined at pH 5.0 (C) and 6.5 (D) in the presence of 0.3 nM natural human LDH-A tetramer (1.2 nM subunits), 125 µM β-NADH, and 100 µM pyruvate (control, gray bars). The LDH-A activities observed using reaction mixtures supplemented with the indicated compounds at 2.5 mM are also shown. The reported values of initial reaction velocity are the average of three samples. The error bars represent standard deviation (n=3). Experimental mean values were compared by Student’s t test (***, ** and * indicate P<0.001, P<0.01, P<0.05, respectively).