Both lysosomes and endosomes are acidified by an electrogenic proton pump, although studies in intact cells indicate that the steady-state internal pH (pHi) of lysosomes is more acid than that of endosomes. We undertook the present study to examine in detail the acidification mechanism of purified rat liver secondary lysosomes and to compare it with that of a population of early endosomes. Both endosomes and lysosomes exhibited ATP-dependent acidification, but proton influx rates were 2.4- to 2.7-fold greater for endosomes than for lysosomes because of differences in both buffering capacity and acidification rates, suggesting that endosomes exhibited greater numbers or rates of proton pumps. Lysosomes, however, exhibited a more acidic steady-state pHi due in part to a slower proton leak rate. Changes in medium Cl- increased acidification rates of endosomes more than lysosomes, and the lysosome ATP-dependent interior-positive membrane potential was only partially eliminated by high-Cl- medium. Permeability studies suggested that lysosomes were less permeable to Na+, Li+, and Cl- and more permeable to K+ and PO4(2-) than endosomes. Na-K-adenosine-triphosphatase did not appear to regulate acidification of either vesicle type. Endosome and lysosome acidification displayed similar inhibition profiles to N-ethylmaleimide, dicyclohexyl-carbodiimide, and vanadate, although lysosomes were somewhat more sensitive [concentration producing 50% maximal inhibition (IC50) 1 nM] to bafilomycin A1 than endosomes (IC50 7.6 nM). Oligomycin (1.5-3 microM) stimulated lysosome acidification due to shunting of membrane potential. Overall, acidification of endosomes and lysosomes was qualitatively similar but quantitatively somewhat different, possibly related to differences in the density or rate of proton pumps as well as vesicle permeability to protons, anions, and other cations.
The preendosomal compartment comprises distinct coated and noncoated endocytic vesicle populations.