Salt‐induced cooperativity in ATPase activity of plasma membrane‐enriched fractions from cultured Citrus cells: kinetic evidence

ATPase activity was studied in plasma membrane‐enriched fractions prepared from cultured Citrus sinensis L. cv. Osbeck cells. In general, properties of the plasma membrane ATPase from cultured cells, such as optimal pH and temperature. V max and K m were similar to those already observed in higher plants. The effects of high salt concentrations on ATPase activity were studied in membrane fractions derived from salt‐sensitive and salt‐tolerant cells grown in the presence or absence of salt. NaCl did not have an in vivo effect on V max and the apparent K m value for ATP. However, high concentrations of NaCl, or KCl, added in vitro, induced cooperativity in the enzyme and reduced the affinity of the enzyme for its substrate. Isoosmolar concentrations of sucrose or choline chloride failed to do so. Our results suggest that the plasma membrane ATPase of Citrus cells has more than one substrate‐binding site on the native form of the enzyme which interact in the presence of salt and act independently in its absence.