An adenosine triphosphate-dependent stabilization of proteolytic activity in heterolysosomes. Evidence for a proton pump.

1. Homogenization of mouse kidneys or livers in 0.25m-sucrose buffered with tris-acetate, pH7.3, resulted in a decreased rate of proteolysis within isolated heterolysosomes containing injected (125)I-labelled albumin when these particles were incubated at 35 degrees C. Proteolysis in mouse kidney or liver heterolysosomes isolated from homogenates made in 0.25m-sucrose buffered at pH7.3 was stimulated by pH5 buffer or by additions of ATP. 2. A greater inhibition of proteolysis was produced by including bicarbonate or pH8 borate buffers in the incubation media, and this inhibition was also reversed by ATP. 3. Other nucleoside triphosphates were not as effective as ATP, but GTP and ITP were more effective than CTP or UTP. ADP, AMP, or adenosine 3':5'-cyclic monophosphate were completely without effect. 4. Although ATP prevented some heterolysosome breakage in media containing bicarbonate, the primary effect appeared to be to promote proteolytic activity. 5. These observations are consistent with the presence of a proton pump in the heterolysosome membrane, which functions to maintain intralysosomal pH in alkaline media.