ATP stimulates amino acid accumulation by lysosomes incubated with amino acid methyl esters. Evidence for a lysosomal proton pump.

Lysosomes incubated with leucine methyl ester accumulate leucine against a steep concentration gradient. In the presence of 140 mM KCl, the cation exchange ionophore nigericin inhibits lysosomal leucine accumulation by 40-60%; in the presence of 250 mM sucrose, however, the ionophore stimulates leucine accumulation by 30-40%. The effects of nigericin suggest that leucine accumulation is inhibited by the alkalinization of, and is stimulated by the acidification of, the lysosomal interior. Mg.ATP stimulates leucine accumulation by more than 40% and partially protects against the inhibitory effects of nigericin in the presence of 10-40 mM KCl. In 140 KCl, nigericin abolishes the effects of Mg.ATP on leucine accumulation. The stimulatory effect of Mg.ATP is not exerted immediately but requires several minutes for maximal development; conversely, when the cofactor Mg2+ is removed after a period of incubation with Mg.ATP, the stimulating effect of Mg.ATP is not lost immediately but dissipates over a time course of several minutes. Protonophores, such as 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone, block the effect of Mg.ATP, although they do not alter the extent of leucine accumulation in the absence of Mg.ATP. The stimulation of leucine accumulation by Mg.ATP appears to be due to intralysosomal acidification. The results thus provide support for the presence of an ATP-dependent proton pump in the lysosomal membrane.