POLYAMINE BINDING TO PLASMA-MEMBRANE VESICLES ISOLATED FROM ZUCCHINI HYPOCOTYLS

The general features of [C-14]spermidine binding to plasmalemma vesicl es isolated from zucchini (Cucurbita pepo L.) etiolated hypocotyls are reported in the present paper. The specific interaction of the polyam ine with the plasma membranes was reversible and thermolabile, since i t decreased by about 50% in the assay performed at 40 degrees C compar ed to that carried out on ice. On the contrary, nonspecific binding wa s unaffected by temperature. Specific spermidine binding showed a pH d ependence with a maximum at pH 8.0 and it reached saturation between 0 .75 and 1 mM external spermidine concentration. The value of the disso ciation constant calculated from Scatchard analysis was 4.4 x 10(-5) M . Specific spermidine interaction appeared to be sensitive to detergen ts and was markedly reduced by the presence of divalent cations, such as Mg2+ and Ca2+, whereas it was stimulated by monovalent cations. Pol yamine binding sites were highly sensitive to pronase treatment. Compe tition experiments, performed using a series of compounds structurally related to spermidine, may provide some indication of the characteris tics of spermidine binding sites. The results presented here suggest t hat specific spermidine binding occurs mainly with the protein compone nt of the plasma membrane.