Direct and indirect effects of valinomycin upon cytochrome c oxidase

Valinomycin binds beef heart cytochrome c oxidase with approximately equimolar stoichiometry (one valinomycin per cytochrome aa3 complex) and a Kd of 2.0-3.0 microM, almost independent of ionic strength, but dependent upon the presence of K+ ions. Its effects are twofold: a red shift in the Soret maximum of the resting enzyme from 419 to 422 nm, with an appreciable delta EmM at 430-410 nm, and an inhibition of catalytic activity of about 50%. Much smaller concentrations of valinomycin are sufficient to induce the release of respiration in cytochrome oxidase vesicles by nigericin. Intermediate levels of valinomycin alone are capable of partially releasing respiratory control, probably by increasing H+ permeability as well as that of K+. The amounts of valinomycin required to bind the enzyme are much greater than those required to collapse delta psi in vesicular systems in a steady state or during proton pulse experiments. We conclude that the two effects--a direct one upon the enzyme and an indirect one via abolition of electrogenic potentials--are of separate origins. The existence of the direct effect does not call for reinterpretation of classical results showing proton translocation, which are adequately explained by the indirect action of valinomycin as an ionophore.