A longitudinal cross-over feeding design was used to investigate the relationship of dietary lipid composition to the membrane lipid environment and activity of mitochondrial ATPase in vivo. Rats were fed a polyunsaturated fatty-acid-rich oil (soya-bean oil) for 12 days, crossed-over to a monounsaturated fatty-acid-rich oil (rapeseed oil) for the next 11 days, then returned to soya-bean oil for 11 more days. Additional rats were fed either soya-bean oil or rapeseed oil throughout. Rats fed rapeseed oil had lower rates of ATPase-catalysed ATP/[32P]Pi exchange than rats fed soya-bean oil. Arrhenius plots showed higher transition temperature (Tt) and activation energy (Ea) for rats fed rapeseed oil. Switching from soya-bean oil to rapeseed oil was dynamically followed by changes in the thermotropic and kinetic properties of the mitochondrial ATPase exchange reaction. Returning to soya-bean oil reversed these changes. The rapid and reversible modulation of Tt caused by a change of the type of fat ingested suggests that membrane physicochemical properties are not under rigid intrinsic control but are continually modified by the profile of exogenously derived fatty acids. The studies suggest that in vivo the activity of mitochondrial ATPase is in part determined by dietary lipid via its influence on the microenvironment of the enzyme. The rapidity and ready reversibility of changes observed for this subcellular-membrane-bound enzyme suggest that dietary fatty-acid balance may be an important determinant of other membrane functions in the body.

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