pH gradient across the lysosomal membrane generated by selective cation permeability and Donnan equilibrium.

The pH within isolated Triton WR 1339-filled rat liver lysosomes was determined by measuring the distribution of [14C]methylamine between the intra- and extralysosomal space. The intralysosomal pH was found to be approximately one pH unit lower than that of the surrounding medium. Increasing the extralysosomal cation concentration lowered the pH gradient by a cation exchange indicating the presence of a Donnan equilibrium. The lysosomal membrane was found to be significantly more permeable to protons than to other cations. The relative mobility of cations through the lysosomal membrane is H+ greater than Cs+ greater than Rb+ greater than K greater than Na+ greater than Li+ greater than Mg2+, Ca2+. The presented data suggest that the acidity within isolated Triton WR 1339-filled lysosomes is maintained by: (1) a Donnan equilibrium resulting from the intralysosomal accumulation of nondiffusible anions and (2) a selective permeability of the lysosomal membrane to cations.