Effect of ionophores on intralysosomal pH.

1. The effect of ionophores on the intralysosomal pH (as estimated from the distribution of a weak acid or base), on the distribution of 42K+ across the lysosomal membrane, and on the intralysosomal degradation of 125I-labelled bovine serum albumin has been studied. 2. Nigericin and X537A equilibrate both 42K+ and H+ across the lysosomal membrane. Gramicidin equilibrates H+ across the lysosomal membrane, this equilibration being more effective in a NaCl than in a KCl medium. Thus all three ionophores exhibit the same ion specificity as in other membranes. 3. The effect of the exchange-diffusion ionophores cannot be imitated by the combination of valinomycin with an uncoupler. Valinomycin by itself also has no effect. 4. X537A and nigericin inhibit the intralysosomal degradation of 125I-labelled albumin only when potassium is present. In a sucrose-containing medium no effect is found. Similar results were obtained with gramicidin. 5. These data suggest that the lysosomal membrane is impermeable to monovalent cations at 25 or 37 degrees C, and that the transport of protons is organised in such a way that electroneutrality is maintained.