THE SUBCELLULAR DISTRIBUTION AND NUCLEOTIDE SPECIFICITIES OF Na⁺, K⁺‐STIMULATED ADENOSINE TRIPHOSPHATASE AND [¹⁴C]ADENOSINE DIPHOSPHATE‐ADENOSINE TRIPHOSPHATE EXCHANGE REACTIONS IN RAT BRAIN

Abstract— (1) The subcellular distributions of Na‐K ATPase and [ 14 C]ADP‐ATP exchange activities were studied in rat brain. The data presented are not consistent with a discrete localization of these enzymes in any given fraction, but nerve endings and microsomes had similar specific activities. The supernatant fraction had the highest exchange and the lowest Na‐K ATPase activities, measured at a concentration of 3 m m ‐MgCl 2 . (2) Nucleotide specificity of the Na‐K ATPase was determined in all fractions, and this enzyme system showed an absolute requirement for ATP. The [ 14 C]ADP‐ATP exchange, measured at 3m m ‐MgCl 2 , possessed broader specificity and also was active toward ITP, UTP and GTP; this serves to differentiate it from the Na‐K ATPase. (3) Treatment of nerve ending fractions with NaI medium removed the bulk of the [ 14 C]ADP‐ATP exchange activity without loss in Na‐K ATPase activity. (4) The exchange activity in NaI‐insoluble fractions was insensitive to NaCl in the presence of 3 m m ‐MgCl 2 , but it was stimulated 502‐820 percent at low MgCl 2 concentrations, a finding which may be consistent with the postulated role of this exchange reaction in the Na‐K ATPase system.