Reconstitution of mitochondrial nicotinamide nucleotide transhydrogenase from beef heart with synthetic phospholipids

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

The interaction of mitochondrial transhydrogenase with derivatives of coenzyme A

The 2,4-dinitrophenyl derivative of dephospho-CoA and the 7-nitrobenzofurazan-4-yl derivative of CoA are competitive inhibitors (Ki 3 microM and 2.6 microM respectively) of mitochondrial transhydrogenase with regard to NAD+ and NADPH respectively. The 7-nitrobenzofurazan-4-yl derivative of dephospho-CoA is a competitive inhibitor with regard to both transhydrogenase substrates with the same Ki equal to 0.3 microM. The pattern of transhydrogenase inhibition with the 7-nitrobenzofurazan-4-yl derivative of dephospho-CoA indicates that one molecule of the inhibitor binds simultaneously to both the NADP(H) and the NAD(H) binding sites of the enzyme. This result is evidence of the short distance between the NADP(H) and the NAD(H) binding sites.

Phospholipid requirements for the reconstitution of complex-III vesicles exhibiting controlled electron transport

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.