Ascorbic acid-like effect of the soluble fraction of rat brain on adenosine triphosphatases and its relation to catecholamines and chelating agents

The inhibitory effect of the soluble fraction of rat brain on the Na⁺, K⁺-ATPase, Mg²⁺-ATPase and K⁺-p-nitrophenylphosphatase activities of rat brain subcellular particles and its antagonization by catecholamines have been studied. The soluble fraction could be replaced by l-ascorbic acid in the process, but dehydroascorbic acid or cytoplasmic reducing agents such as l-cysteine and l-glutathione did not show the same effect. Catecholamines could be replaced by chelating agents; EDTA, EGTA, o-phenanthroline and α,α'-dipyridyl prevented the inhibition by the soluble fraction and by l-ascorbic acid. The inhibition of the enzyme activities was not prevented by catecholamines or chelators when they were added after preincubation of the enzyme preparation in the presence of soluble fraction or of l-ascorbic acid. The effect of EGTA was eliminated by an excess of Ca²⁺ and the effect of EDTA by an excess of Ni²⁺. Excesses of Ca²⁺, Mn²⁺ and Co²⁺ did not influence the effect of EDTA. It is supposed that a reoxidizable heavy metal bound to the membrane structures is responsible for the inhibitory effect of the soluble fraction and of l-ascorbic acid. Catecholamines may act by chelating this metal. The possible role of iron in the phenomenon has been investigated and discussed.