Membrane lipid fatty acids and regulation of membrane-bound enzymes. Allosteric behaviour of erythrocyte Mg2+-ATPase, (Na+ + K2+)-ATPase and acetylcholineasterase from rats fed different fat-supplemented diets

Studies were carried out to determine the Hill coefficients for the inhibition by F^? of the erythrocyte membrane-bound Mg²⁺-ATPase, $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ and acetylcholinesterase from rats fed with seven different diets. Five groups were fed with different natural fats or oil supplements, one with a hydrogenated fat supplement and the other with fat-free diet. The responses of the red cell fatty acids to dietary fats were recorded. The value of $\text{n}$ for the inhibition by F^? of the three enzymes revealed a particular and different behaviour in each group. Correlations between the fatty acid compositions of erythrocyte membranes and cooperativity of each enzyme were calculated. The results indicate that neither the essential fatty acid family nor the non-essential ones are particularly involved in the allosteric phenomena. The increase of the double bond index/saturation ratio of fatty acids, which is taken as indicative of membrane fluidity, was accompanied in an inverse manner by changes in allosteric transitions of the $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ and acetylcholinesterase, whereas the Mg²⁺-ATPase was not dependent on this ratio. Diminution of membrane fluidity, carried out by in vitro increase of its cholesterol content, yields confirmatory results of this regulatory mechanism since the value of $\text{n}$ for acetylcholinesterase shifted as predicted.These facts indicate that the membrane fluidity is a physiological regulator for the allosteric behaviour of the membrane-bound enzymes and that each enzyme exhibits a particular behaviour in this phenomenon.