Mobility in the mitochondrial electron transport chain

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Abstract
The role of lateral diffusion in mitochondrial electron transport was investigated by measuring the diffusion coefficients for lipid, cytochrome c and cytochrome oxidase in membranes of giant mitoplasts from cuprizone-fed mice using the technique of fluorescence redistribution after photobleaching (FRAP). The diffusion coefficient of the phospholipid analog N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine is dependent on the technique used to remove the outer mitochondrial membrane. A sonication technique yields mitoplasts with monophasic recovery of the lipid probe (D = 6 .times. 10-9 cm2/s), while digitonin-treated mitochondria show biphasic recoveries (D1 = 5 .times. 10-9 cm2/s; D2 = 1 .times. 10-9 cm2/s). Digitonin appears to incorporate into mitoplasts, giving rise to decreased lipid mobility concomitant with increased rates of electron transfer from succinate to oxygen, in a manner reminiscent of the effects of cholesterol incorporation [Schneider, H., Lemasters, J. J., Hochli, M., .permill. Hackenbrock, C. R. (1980)] FRAP measurements on tetramethylrhodamine cytochrome c modified at lysine-39 and on a mixture of active morpholinorhodamine derivatives of cytochrome c gave diffusion coefficients of (3.5-7) .times. 10-10 cm2/s depending on the assaymedium. With morpholinorhodamine-labeled antibodies purified on a cytochrome oxidase affinity column, the diffusion coefficient for cytochrome oxidase was determined to be 1.5 .times. 10-10 cm2/s. The results are discussed in terms of a dynamic aggregate model in which an equilibrium exists between freely diffusing and associated electron-transfer components.