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Proton accumulation and ATPase activity in Golgi apparatus–enriched vesicles from rat liver

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Abstract

We have studied the mechanism by which liver Golgi apparatus maintains the acidity of its contents, using a subcellular fraction from rat liver highly enriched in Golgi marker enzymes. Proton accumulation (measured by quenching of acridine-orange fluorescence) and anion-dependent ATPase were characterized and compared. Maximal ATPase and proton accumulation required ATP; GTP and other nucleotides gave 10% to 30% of maximal activity. Among anions, Cl and Br approximately doubled the activities; others were much less effective. Half-maximal increase of ATPase and H uptake required 55 mmol/L and 27 mmol/L Cl, respectively. In predominantly chloride media, SCN and NO3 markedly inhibited H+ uptake. Nitrate competitively inhibited both the chloride-dependent ATPase (apparent Ki 6 mmol/L) and proton uptake (apparent Ki 2 mmol/L). Nitrate and SCN also inhabited uptake of 36Cl. Replacing K+ with Na+ had no effect on the initial rate of proton uptake but somewhat reduced the steady state attained. Replacement of K+ with NH4+ and choline reduced proton uptake without affecting ATPase. The ATPase and H+ uptake were supported equally well by Mg2+ or Mn2+. The ATPase was competitively inhibited by 4-acetamido-4′-isothiocyano-stilbene-2,2′-disulfonic acid (apparent Ki 39 μmol/L). Other agents inhibiting both H+ uptake and ATPase were N-ethylmaleimide, N,N′-dicyclohexylcarbodiimide, chlorpromazine, diethylstilbestrol, Zn2+, Co2+ and Cu2+. In the Cl medium, accumulated protons were released by ionophores at the relative rates, monensin = nigericin > valinomycin > carbonyl cyanide mchlorophenylhydrazone; the last of these also reduced ATPase activity. In the absence of Cl, monensin and valinomycin both stimulated the ATPase. These results show a close association between ATPase activity and acidification of liver Golgi vesicles. They support a role for Cl that depends on its uptake as a counter ion for H+ and suggest that it may also stimulate proton transport by a more direct effect on a component of the transport system. (HEPATOLOGY 1991;13:523–533.)

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