Tri-n-butyltin chloride is a specific inhibitor that binds stoicheiometrically and irreversibly with the ATP-synthesizing sites of chloroplasts. Titration of Euglena chloroplasts with tri-n-butyltin chloride shows about six ATP-synthesizing sites per molecule of cytochrome c552 or per 380 molecules of chlorophyll. This system was used to study the possibility of linkage between individual enzyme chains of the energy-conservation pathway or the possible existence of a common pool of an intermediate in this pathway. The inhibition of ATP synthesis by tri-n-butyltin chloride at low rates of electron transport (low light-intensities, NADP+ or ferricyanide as electron acceptor) agrees with a kinetic model of two to four ATP-synthesizing sites per energy-conservation chain. At high rates of electron transport, however, the results occasionally agree with a model of 20 or more sites per chain. The results are interpreted as indicating the absence of a common intermediate pool, but the presence of a limited degree of linkage between individual chains. It also indicates the presence of two energy-conservation sites in these chloroplasts.
