Activation of cathepsin D by polyanionic compounds

We examined the nature of the activation of cathepsin D by polyanionic compounds. Tripolyphosphate, a model compound for polyanions, decreased the Km value of porcine cathepsin D for bovine serum albumin without affecting VMAX. Half-maximal activation was achieved at 0.2 mM free tripolyphosphate. Electrophoretic mobility of cathepsin D decreased as the tripolyphosphate concentration was increased, and the enzyme had no net charge at 50 mM triP. The concentration for the half-maximal mobility change of cathepsin D (0.18 mM) was similar to that for half-maximal activation. These results suggest that tripolyphosphate increased affinity of the enzyme for its substrate by cancelling positive charges on cathepsin D and thus decreasing the electrostatic repulsion.