Multiple Effects of Lysophosphatidylcholine on the Activity of the Plasma Membrane H + ‐ATPase of Radish Seedlings *

We analyzed the effect of lysophosphatidylcholine (lysoPC) on the activity of the plasma membrane (PM) H + ‐AT‐Pase measured at pH 6.3 or 7.5 in inside‐out PM vesicles isolated from germinating radish seeds. LysoPC stimulated PM H + ‐ATPase at both pHs, but the dependence of the effect on lysoPC concentration was different: at pH 6.3 maximal stimulation was observed with 40 to 200 μg ml −1 lysoPC, while at pH 7.5 a sharp peak of activation was observed at about 50 μg ml −1 lysoPC, higher concentrations becoming dramatically inhibitory; this inhibitory effect was considerably reduced in the presence of 10% (v/v) glycerol. In trypsin‐treared PM lysoPC stimulated the H + ‐ATPase activity assayed at pH 6.3, but only marginally that assayed at pH 7.5. LysoPC increased both V max (from 190 to 280nmol min −1 mg −1 prot) and apparent K M (from 0.15 to 0.3 mM) of the H + ‐ATPase at pH 6.3, while it increased V max (from 120 to 230 nmol min −1 mg −1 prot) and decreased apparent K m (from 0.8 to 0.4 mM) at pH 7.5. Low concentrations of Nacetylimidazole (10 to 50 mM), which modifies tyrosine residues, abolished the stimulation by lysoPC of the PM H + ‐ATPase activity at pH 7.5, but not that observed at pH 6.3. These results indicate that lysoPC influences the PM H + ‐ATPase through different mechanisms, and that its effect can only partly be ascribed to its ability to hamper the inhibitory interaction of the regulatory C‐terminal domain with the catalytic site. N‐acety‐limidazole did not affect the stimulation of PM H + ‐ATPase by controlled trypsin treatment or by fusicoccin, indicating that the requirement for the tyrosine residue(s) modified by low Nacetylimidazole concentrations is specific for lysoPC‐induced displacement of the C‐terminal domain.