Activation of mitochondrial ATP-dependent protease by peptides and proteins

We examined the effect of peptides or protein on the proteolytic and ATPase activities of mitochondrial ATP-dependent LON protease purified from bovine adrenal cortex. Peptides/proteins including angiotensin I which stimulated ATPase activity without hydrolysis of any peptide bonds stimulated proteolysis of 125I-labeled substrates at low concentrations; whereas at high concentrations they competitively inhibited proteolysis, thus displaying a biphasic activity profile. All peptides and proteins thus examined stimulated degradation of 125I-labeled substrates. When an ATP analog was substituted for ATP, only inhibition; i.e., no stimulation, of proteolysis by unlabeled peptides was observed. Without activator peptides, degradation of [125I] peptides was higher in the presence of an ATP analog than that in the presence of ATP. ADP, a product of the ATPase reaction, inhibited the proteolytic activity in the absence of an activator peptide but not in its presence. From analogy to E. coli ATP-dependent protease La (LON), we suggest that the activator peptides stimulated the proteolysis by releasing enzyme-bound ADP.