A spin label study of the lipid boundary layer of mitochondrial NADH-ubiquinone oxidoreductase.

Mitochondrial NADH-ubiquinone oxidoreductase (Complex I) is a lipoprotein enzyme containing phosphatidylcholine (PC), phosphatidylethanolamine (PE) and cardiolipin. Enzyme preparations containing endogenous cardiolipin and a range of either soyabean PC or dimyristoylphosphatidylcholine (DMPC) concentrations have been made. Using a spin-labelled fatty acid, two probe environments differing in mobility have been shown to be present. The fatty acid probe has a relative binding constant (or partition coefficient between lipid and protein) of unity. The boundary layer or lipid annulus reported by the probe has a value of approx. 300 lipid molecules per molecule of enzyme FMN in preparations containing soyabean PC, or DMPC above the phase transition temperature of the latter. In soyabean PC-replaced enzyme the apparent size of the boundary layer is independent of temperature between 30 degrees C and 14 degrees C but shows a modest increase to about 400 lipid molecules per molecule of FMN between 14 degrees C and 2 degrees C. Complex I replaced with high concentrations of DMPC gives non-linear Arrhenius plots of NADH-ubiquinone oxidoreductase activity. The results of the ESR experiments show that both boundary layer and bulk lipid must be motionally restricted for this to occur. Thus, the change in activity is probably not caused by an effect exerted directly on the catalytic activity of the enzyme but is more likely due to restriction of free diffusion of ubiquinone to its site of reduction.