| Summary: | Mitochondrial oxidative phosphorylation is the chief site of eukaryotic energy transduction, accounting for up to 90% of cellular oxygen consumption. During the process, protons pumped out across the mitochondrial inner membrane return down their electrochemical gradient to synthesise ATP, a universal energy intermediate. Protons may also passively leak back in, by-passing ATP synthesis. This proton leak is an important physiological process, accounting for up to 25-30% of the standard metabolic rate of an animal such as the rat. Three major determinants of metabolic rate - body mass, thyroid hormones, and phylogeny, all correlate with mitochondiral proton leak, and also with mitochondrial phospholipid fatty acid composition. It is hypothesised that the mitochondrial inner membrane's phospholipid fatty acid composition may play a role in determining its proton permeability, though the mechanism of this is unknown. In the current investigation, novel and modified methods were used to isolate mitochondiral inner membrane phospholipids from 8 species of animal of different metabolic rate, and reconstitute them into liposomes without the use of detergents. The fatty acid composition and proton permeability of these liposomes was determined. The results suggest that only 5% of mitochondrial proton leak is through the bulk phospholipid bilayer portion of the inner membrane. Further, they indicate that the fatty acid composition of a pure phospholipid bilayer does not affect its proton permeability. Thus, if phospholipid fatty acid composition does play a role in determining the proton permeability of the mitochondrial inner membrane, any effects must be mediated through the other components of the inner membrane, such as the proteins. The results have implications for the basis of standard metabolic rate, and also for the biophysical understanding of proton leak mechanisms in pure phospholipid bilayers.
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