Phospholipid requirements for the reconstitution of Complex-III vesicles exhibiting respiratory control (electron-transport control) were studied. Vesicles prepared from pure phosphatidylethanolamine gave maximal control ratios. Phosphatidylcholine alone did not support respiratory control, although these vesicles were capable of maintaining stable K+-diffusion gradients. Apparently Complex III cannot insert into a bilayer of phosphatidylcholine. Formation of mixed phosphatidylcholine/phosphatidylethanolamine (6:1, w/w) vesicles was sufficient, however, to allow Complex-III insertion and to restore respiratory control. Mixtures of acidic phospholipids with either phosphatidylethanolamine or phosphatidylcholine did not improve respiratory control over that obtained with pure phosphatidylethanolamine. Phosphatidylethanolamine from bovine heart mitochondria, soya beans or Escherichia coli was equally effective in reconstituting respiratory control, suggesting that the specificity is referable to the head group and not to the fatty-acid moiety.

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