The adenosine diphosphate--adenosine triposphate-excange reaction of cerebral microsomes and its relation to he sodium ion-stimulatd adenosine-triphosphatase reaction.

Microsomes from guinea-pig cerebral cortex contain a system capable of exchanging ADP with ATP at rates of about 20mumoles/mg. of protein/hr. The ADP-ATP-exchange reaction requires Mg(2+) for activity. The reaction is not stimulated by Na(+) or K(+) and is not inhibited by ouabain, in contrast with the Na(+)-plus-K(+)-stimulated adenosine triphosphatase. The pH optimum also differs from that of the adenosine triphosphatase. The ADP-ATP-exchange reaction is stimulated two- to three-fold by non-ionic, anionic and cationic detergents, even when these agents are inhibiting the adenosine-triphosphatase reaction. This reaction may represent a component of the Na(+)-plus-K(+)-stimulated adenosine-triphosphatase reaction but is more likely to be due to other enzyme systems present in microsomal subfractions.