Abstract: | Dibutyltin-3-hydroxyflavone bromide [Bu2SnBr-(of)] is a fluorescent inhibitor (excitation max, 395 nm; emission max., 450 nm) of mitochondrial F1F0–ATPase which does not inhibit F1–ATPase. Bu2SnBr(of) binding to mitochondria and submitochondrial particles results in a 10-fold fluorescence enhancement which correlates with the amount of F1F0–ATPase in the inner membrane. Enhancement is not affected by respiratory-chain substrates, ATP, uncoupling agents, ionophores or respiratory-chain inhibitors. It is reversed by tributyltin chloride (Bu3SnCl), indicating competition for a common triorganotin-binding site on the F0 segment of F1F0–ATPase. Enhancement is not reversed by dialkyltins, monoalkyltins, tributyl-lead acetate, efrapeptin or oligomycin. Bu2SnBr(of) is thus a new class of fluorescent probe of the F0 segment of F1F0–ATPase which titrates F0. |