The in vitro modification of phosphorylated pyruvate kinase by a Ca2+-activated protease from rat liver

A Ca/+-activated protease from rat liver cell sap was prepared. It was shown to act on rat liver pyruvate kinase that had been phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase, the activity being optimum at neutral pH. The modified pyruvate kinase had the same Vmax as the phosphoenzyme but showed a lower affinity for the substrate phosphoenolpyruvate. The possibility that this proteolytic attack is the step that initiates further degradation in the cell is discussed.