Triphenyltin as an Inhibitor of Membrane-Bound Pyrophosphatase ofRhodospirillum rubrum

The effect of triphenyltin on the activity of membrane-bound pyrophosphatase ofRhodospirillum rubrumwas investigated. Triphenyltin inhibits the hydrolysis of chromatophore membrane-bound pyrophosphatase in a pH-dependent pattern, being maximal at pH 9–10. At basic pH values, the inhibition produced by this organotin on membrane-bound pyrophosphatase is very similar to that produced on the chromatrophore H+ATPase (I50= 14.4 and 10 μM, respectively). Detergent-solubilized membrane-bound pyrophosphatase is also inhibited by triphenyltin, but the cytoplasmic enzyme ofR. rubrumis inhibited only slightly. The inhibitory effect of triphenyltin on membrane-bound pyrophosphatase is the same with Mg-PPior Zn-PPi, and is dependent on the chromatophore membrane concentration. Triphenyltin modified mainly theVmaxof the enzyme, and only slightly itsKm.Free Mg2+does not reverse the inhibition. Reducing agents prevent triphenyltin inhibition of the membrane-bound pyrophosphatase, but their effect is due to an alteration of the inhibitor, and not to a modification of thiol groups of the enzyme. The most likely site for triphenyltin inhibition in chromatophore membrane-bound pyrophosphatase is a component either within or closely associated with the membrane.