Involvement of a non-proton pump factor (possibly Donnan-type equilibrium) in maintenance of an acidic pH in lysosomes

Change of the internal pH of isolated lysosomes was measured with fluorescein isothiocyanate-dextran. In buffer of pH 7.0, isolated lysosomes had an acidic pH of about 5.5, which decreased to pH 5.2 on addition of ATP. Addition of bafilomycin inhibited the acidification by H⁺-ATPase and resulted in an increase of the internal pH to 5.5 due to passive diffusion of protons across the lysosomal membrane. However, no further alkalization was observed. The acidic pH (pH 5.5) of isolated lysosomes could be maintained for at least 48 h in the absence of ATP, but increased gradually to pH 5.9-6.4 upon incubation with monovalent cations (K⁺ or Na⁺), amines, or ionophores. These results suggest that a non-proton pump factor (possibly Donnan equilibrium) is involved in maintaining the acidic pH of isolated lysosomes.