Pyroglutamyl peptidase II (EC 3.4.19.-), a highly specific membrane-bound TRH-degrading enzyme, is inactivated in Y-79 human retinoblastoma cells by exposure to 12-O-tetradecanoyl phorbol-13-acetate (TPA) in a biphasic manner. We have previously demonstrated a rapid decrease in pyroglutamyl peptidase II activity to 10% of the control level within 15 min, which returns to 70% of the control level by 1 h. This decrease results from enzyme phosphorylation by TPA-activated protein kinase-C. We now report a second phase of inactivation after longer exposure of cells to TPA. After 1 h, enzymatic activity slowly and progressively declined. By 7 h, only 15% of control activity remained. Cotreatment of cells with H-7, a protein kinase-C inhibitor, prevented this second phase of inactivation. Immunoblot experiments demonstrated a reduction in the amount of pyroglutamyl peptidase II in Y-79 membranes after long term exposure to TPA. Y-79 cells were labeled with [35S]methionine, and pyroglutamyl peptidase II was immunoprecipitated. A decreased incorporation of [35S]methionine paralleled the decrease in enzyme activity. These studies demonstrate that the second phase of inactivation after exposure to TPA is due to an inhibition of enzyme synthesis.
Download Full PDF Version (Non-Commercial Use)