Figure 6.

Determination of the type of inhibition of cathepsin B by dipeptide nitrile 1a. (A) Initial rates of substrate turnover as a function of substrate concentration (x axis) and inhibitor concentration (legend). (B) Double-reciprocal plot of 1/v_i vs 1/[S] for different inhibitor concentrations (transformed data set identical to that of panel A). The lines intersect in the fourth quadrant, which allows the characterization of 1a as a noncompetitive inhibitor with an α value of >1. Measurements were performed as duplicate determinations in assay buffer (pH 6.0) containing 200 μM Z-RR-AMC, 25 ng/mL cathepsin B, and 1.5% DMSO. Measured values ± SEM are shown.