THE ADENOSINE DIPHOSPHATE‐ADENOSINE TRIPHOSPHATE EXCHANGE REACTION OF EXTRACTED CEREBRAL MICROSOMES

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Abstract
—Microsomal fractions prepared from guinea pig cerebral cortex manifested ADP‐ATP exchange activity, 40–99 per cent of which was extractable by dilute salt solutions. All of the (Na+, K+)‐ATPase activity remained in the particulate material. The unextracted ADP‐ATP exchange activity was stimulated six to seven fold by a non‐ionic detergent (Lubrol W). When pre‐extracted microsomes were sedimented in a sucrose density gradient, the ADP‐ATP exchange activity was more widely distributed than (Na+, K+)‐ATPase or adenylate kinase activities. The ADP‐ATP exchange activity of microsomes extracted with NaI was stimulated by Na+ ions when the Mg2+ concentration in the reaction mixture was low (0·2 mm). The Na+ stimulation of exchange activity was more variable than was the stimulation of phosphate formation by Na+ plus K+. The Na+‐stimulated ADP‐ATP exchange reaction of extracted microsomes may be a component of the (Na+, K+)‐ATPase system, which has not been freed from adenylate kinase or possibly other contributing enzyme systems.

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