Decreased vacuolar acidification capacity in drug-resistant rat liver preneoplastic nodules

Rat liver nodules produced by intermittent 2-acetylaminofluorene feeding exhibit alterations in cell surface receptors reminiscent of impairment of vacuolar acidification. In this report, vacuolar acidification activity, measured as the ATP-dependent quenching of acridine orange, was characterized in liver and nodular membrane fractions using various ion-transport inhibitors and with respect to nucleotide specificity and divalent cation dependence. Based on these criteria and on the comparison of vacuolar acidification activity with mitochondrial, lysosomal, and plasma membrane marker enzymes in different subcellular fractions, it was concluded that the assay measures the proton pump associated with exocytic and/or endocytic vacuolar compartments. When the vacuolar acidification activity was compared in liver and nodular subcellular membrane fractions, it was found that the vacuolar acidification was most strongly reduced in nodular low-density membrane fractions enriched in Golgi-derived membranes and endocytic vesicles. The data indicate that vacuolar, i.e., exocytic and/or endocytic, prelysosomal intracellular compartments in rat liver nodules are markedly deficient in acidification capacity, possibly providing an explanation to various metabolic aberrations, such as diminished iron accumulation and reduced protein degradation, observed in rat liver nodular cells.