EFFECT OF pH ON THE ELECTROKINETIC AND LIGHT SCATTERING PROPERTIES OF PEA THYLAKOIDS IN THE PRESENCE OF PHYTOHEMAGGLUTININ

We studied the effect of phytohemagglutinin on the surface charge density and 90° light scattering of pea thylakoids as a function of pH (from pH 6.0 to pH 7.8). These pH-dependent modifications were interpreted in terms of the membrane conformational changes. Lectin induced a decrease in LS level due to the increase in aggregation of the particles at all pH studied. These changes were accompanied by a reduction in the electrophoretic mobility of the particles after lectin treatment at pH 7–7.8 and had an optimum at pH 7.5. Light exposure stimulated the recovery of reduced electrokinetic potential of lectin treated thylakoids (pH 7.5) up to approximately the initial higher negative surface charge density. Acidification induced a smaller decrease in electrokinetic potential and energization of the membrane after lectin treatment and light exposure. Lectin plus light treatment increased the energization of the membrane. Phytohemagglutinin-Phaseolosaxin (PHA-P) influenced the enhancement of ∆pH gradient formation more effectively at alkaline than in acid pH. Screening of the charges of the surface exposed groups (in the presence of Mg2+) and lectin binding decreased the primary ionic-exchange processes through the membrane at all pH tested and significantly activated them at pH 7.0. Alkaline pH determined the strong increase in dark relaxation of thylakoids pretreated with PHA and inhibition of this phase at pH 6.3. The pH-dependent phenomena in the membrane-membrane interactions and the lectin regulated processes of aggregation in the thylakoid membranes at pH range of 6.0 to 7.8 was discussed.