PERTURBATION OF PHOSPHOLIPID MEMBRANES BY GIBBERELLINS

SUMMARY Differential scanning calorimetry and electron spin resonance have been used to characterize the association of gibberellic acid4 and gibberellic acid, (a GA4/GA7 mixture) with phospholipid membranes. The GA4/GA7 mixture lowered the temperature and reduced the co-operativity of the main phase transition of liposomes prepared from dipalmitoylphosphatidylcholine (DPPC), distearylphosphatidylcholine (DSPC) and dipalmitoylphosphatidylglycerol (DPPG), and eliminated the pretransition. The reduction in co-operativity of the main transition was not accompanied by a reduction in transition enthalpy, indicating that the gibberellins simply perturb the bilayer rather than complex with the phospholipid molecules. Perturbation was markedly greater at pH values approaching the pKa of the gibberellin carboxyl group than at pH 7. Moreover, the negatively charged DPPG was less susceptible to perturbation than the zwitterionic phosphatidylcholines. Perturbation by GA8, a physiologically inert gibberellin, was virtually imperceptible. Electron spin resonance of GA4/GA7-treated liposomes of phosphatidylcholine (PC) and phosphatidylglycerol (PG) confirmed the results obtained by differential scanning calorimetry and also indicated that the gibberellins associate with the surface of the phospholipid membranes rather than penetrate into the interior of the bilayer. The data are consistent with the proposal that gibberellins can influence membrane behaviour by modulating the physical properties of the lipid bilayer.