LONG CHAIN FATTY ACID INHIBITION OF SODIUM PLUS POTASSIUM‐ACTIVATED ADENOSINE TRIPHOSPHATASE FROM RAT HEART

Abstract

Long chain fatty acids were found to inhibit (Na ⁺ +K⁺)– ATPase prepared from rat heart. Unsaturated and polyunsaturated fatty acids were more inhibitory than saturated fatty acids with myristic acid being the most inhibitory saturated fatty acid tested and linoleic the most inhibitory unsaturated fatty acid. As an example of fatty acid modification of the enzyme, inhibition of (Na⁺+K⁺)–ATPase by oleate was examined. When compared to ouabain, inhibition of (Na ⁺ +K⁺)– ATPase by oleate was found to be similar in that both were dependent on K⁺ concentration, but, in contrast to the almost instantaneous inhibition by ouabain, oleate inhibition was a slow process requiring over 20 min incubation at 37° to produce maximum inhibition. Inhibition of rat heart (Na ⁺+ K ⁺) –ATPase by oleate was found to be readily reversible by washout. In the presence of albumin an oleate/albumin molar ratio greater than 75 was required for inhibition to occur. The activity of rat heart (Na ⁺+K⁺)-ATPase had a temperature optimum above 40° and a discontinuous Arrhenius' plot with a transition temperature of 25°. In the presence of oleate, however, the enzyme's optimum temperature decreased to below 40°, the activation energy of the reaction at temperatures below 25° was lowered from 24.7 kcal/mol to 12.6 kcal/mol and tire enzyme had a linear Arrhenius' plot. The possibility of in vivo inhibition of cardiac (Na ⁺+K⁺)– ATPase under conditions of elevated fatty acids is discussed.