Simultaneous measurement of Delta pH and electron transport in chloroplastthylakoids by 9-aminoacridine fluorescence

Electron transport and the electrochemical proton gradient across the thyla koid membrane are two fundamental parameters of photosynthesis. A combinati on of the electron acceptor, ferricyanide and the Delta pH indicator, 9-ami noacridine, was used to measure simultaneously electron transport rates and Delta pH solely by changes in the fluorescence of 9-aminoacridine. This me thod yields values for the rate of electron transport that are comparable w ith those obtained by established methods. Using this method a relationship between the rate of electron transport and Delta pH at various uncoupler c oncentrations or light intensities was obtained. Ln addition, the method wa s used to study the effect of reducing the disulfide bridge in the gamma-su bunit of the chloroplast ATP synthase on the relation of electron transport to Delta pH. When the ATP synthase is reduced and alkylated, the threshold Delta pH at which the ATP synthase becomes leaky to protons is lower compa red with the oxidized enzyme. Proton flow through the enzyme at a lower Del ta pH may be a key step in initiation of ATP synthesis in the reduced enzym e and may be the way by which reduction of the disulfide bridge in the gamm a-subunit enables high rates of ATP synthesis at low Delta pH values.