Plasma membrane ATPase activity of IGC 3507III grown in the presence of the lipophilic acid octanoic acid [4-50 mg I (0.03–0.35 mM), pH 4.0] was 1.5-fold higher than that in cells grown in its absence. The for ATP, the pH profile and the sensitivity to orthovanadate of the basal and the activated forms of the membrane ATPase were identical. This activation was closely associated with a decrease in the biomass yield and an increase in the ethanol yield, and was rapidly reversed after removal of the acid. However, the activated level was preserved when membranes were extracted and subjected to manipulations which eliminated or decreased octanoic acid incorporation in the plasma membrane. The activity of the basal plasma membrane ATPase in the total membrane fraction was slightly increased by incubation at pH 6.5 with octanoic acid at 100 mg I or less (2.4 mg acid form plus 97.6 mg octanoate ion I). However, destruction of the permeability barrier between the enzyme and its substrate could not explain the activation. A role for plasma membrane ATPase activation in the regulation of the intracellular pH (pH) of cells grown with octanoic acid was not proven.


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