The in vitro Modification of Phosphorylated Pyruvate Kinase by a Ca(2+)-activated Protease from Rat Liver.

Abstract
A Ca2+-activated protease from rat liver cell sap was prepared. It acted on rat liver pyruvate kinase that had been phoshorylated by the catalytic subunit of cAMP-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.