Charge and acidity compensation during proton-sugar symport in Chlorella: The H+-ATPase does not fully compensate for the sugar-coupled proton influx

Abstract

This study was undertaken in order to demonstrate the extent to which the activity of the plasmalemma H(+)-ATPase compensates for the charge and acidity flow caused by the sugar-proton symport in cells of chlorella vulgaris Beij.. Detailed analysis of H(+) and K(+) fluxes from and into the medium together with measurements of respiration, cytoplasmic pH, and cellular ATP-levels indicate three consecutive phases after the onset of H(+) symport. Phase 1 occurred immediately after addition of sugar, with an uptake of H(+) by the hexoseproton symport and charge compensation by K(+) loss from the cells and, to a smaller degree, by loss of another ion, probably a divalent cation. This phase coincided with strong membrane depolarization. Phase 2 started approximately 5 s after addition of sugar, when the acceleration of the H(+)-ATPase caused a slow-down of the K(+) efflux, a decrease in the cellular ATP level and an increase in respiration. The increased respiration was most probably responsible for a pronounced net acidification of the medium. This phase was inhibited in deuterium oxide. In phase 3, finally, a slow rate of net H(+) uptake and K(+) loss was established for several further minutes, together with a slight depolarization of the membrane. There was hardly any pH change in the cytoplasm, because the cytoplasmic buffering capacity was high enough to stabilize the pH for several minutes despite the net H(+) fluxes. The quantitative participation of the several phases of H(+) and K(+) flow depended on the pH of the medium, the ambient Ca(2+) concentration, and the metabolic fate of the transported sugar. The results indicate that the activity of the H(+)-ATPase never fully compensated for H(+) uptake by the sugar-symport system, because at least 10% of symport-caused charge inflow was compensated for by K(+) efflux. The restoration of pH in the cytoplasm and in the medium was probably achieved by metabolic reactions connected to increased glycolysis and respiration.