Inhibition by Triphenyltin Chloride of a Tightly‐Bound Membrane Component Involved in Photophosphorylation

At very low concentrations (< 1 μM) triphenyltin chloride inhibits ATP formation and coupled electron transport in isolated spinach chloroplasts. Basal (−P i ) and uncoupled electron transport are not affected by triphenyltin. The membrane‐bound ATP ⇌ P i exchange and Mg 2+ ‐dependent ATPase activities of chloroplasts are also completely sensitive to triphenyltin, although the Ca 2+ ‐ dependent and Mg 2+ ‐dependent ATPase activities of the isolated coupling factor protein are insensitive to triphenyltin. The light‐driven proton pump in chloroplasts is stimulated (up to 60 %) by low levels of triphenyltin. Indeed, the amount of triphenyltin necessary to inhibit ATP formation or stimulate proton uptake is dependent upon the amount of chloroplasts present in the reaction mixture, with an apparent stoichiometry of 2 — 2.5 triphenyltin molecules/100 chlorophyll molecules at 50: < inhibition of ATP formation and half‐maximal stimulation of proton uptake. Chloroplasts partially stripped of coupling factor by an EDTA wash are no longer able to accumulate protons in the light. However, low levels of triphenyltin can effectively restore this ability. The amount of triphenyltin required for the restoration of net proton uptake is also dependent upon the amount of chloroplasts, with a stoichiometry of 4 — 5 triphenyltin molecules/100 chlorophyll molecules at 50% reconstitution. On the basis of this and other evidence it is concluded that triphenyltin chloride inhibits phosphorylation, ATP + P i exchange and membrane‐bound ATPase activities in chloroplasts by specifically blocking the transport of protons through a membrane‐bound carrier or channel located in a hydrophobic region of the membrane at or near the functional binding site for the coupling factor.