Topology of Cytochrome C Oxidase-Containing Proteoliposomes: Probes, Proteins and PH Gradients

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Abstract
Cytochrome c oxidase-containing proteoliposomes (COV) prepared by cosonication show random orientation (45:55 in:out) of incorporated oxidase molecules; dialysed COV show 30:70 (in:out). Prepared COV show a pH gradient with an internal pH typically more acid than the medium. Such passive pH gradients probably reflect a Donnan distribution of anions such as chloride. The fluorescent pH probe 4-heptadecyl-7-hydroxycoumarin (HDHC) distributes between the two lipid leaflets at a ratio of between 30:70 and 33:67 (in:out) in cosonicated COV as measured by acid/base responses and quenching by p-xylene-b/s-pyridinium bromide. The HDHC pK was 8.25 in lauryl maltoside micelles, but membrane-bound HDHC showed a continuum of values ranging from 8.25 to 10.5. Maximum fluorescence in alkali was greater in lauryl maltoside than in COV. Active ΔpH gradients (alkaline inside) were generated by reductant and cytochrome c with aerobic oxidase-containing proteoliposomes ± valinomycin and nigericin. The gradients exceed 1.0 pH unit at low fluxes, higher than with water-soluble probes. ΔpH maintained between the bulk phases far from the membrane may be less than that at the lipid/water interface. With valinomycin (ΔΨ = 0), which accelerates ΔpH formation, ΔpH saturates at 1.0–1.2 units. Almost all the ΔΨ across the membrane can be converted into ΔpH by slow cation movement in the absence of ionophores. A gradient of either -90 mV (ΔΨ) or 1.0 pH unit (ΔpH) diminishes oxidase turnover by 80–90%. Control exerted by thermodynamically equivalent gradients is more effective with ΔpH than with ΔΨ. Differences between COV and mitochondria may be due to different rate-limiting electron transfer steps in the two systems.