SODIUM-POTASSIUM-ACTIVATED ATPASE AND POTASSIUM-ACTIVATED P-NITROPHENYLPHOSPHATASE: A COMPARISON OF THEIR SUBCELLULAR LOCALIZATIONS IN RAT BRAIN.

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The association of a Na+-K+-activated ATPase (ATP phosphohydrolase, EC 3.6.1.4.) with the active transport of these cations is supported by substantial evidence.l-4 An effect of sodiuml ions on a brain ATPase was first reported by Utter.5 Further work has led to the concept that the Na+-K+-ATPase activity may be a resultant of transphosphorylations within the cell membrane which could couple phosphate bond energy directly to cation transport.6-s Consequently, the enzyme system would be an integral component of specialized membranes. Several workers have suggested that the final step in the ATPase reaction sequence may involve a K+-activated phosphatase6 9 and in particular, a phosphoprotein phosphatase.6 Since phenyl phosphate serves as substrate for some phosphoprotein phosphatasesl? and a K+-activated p-nitrophenylphosphatase occurs in brain,ll the structural association of a K+-activated phosphatase with the Na+-K+-ATPase in well-defined subcellular units would support their possible functional relationship. However, the operationally defined subcellular fractions of brain as prepared in various laboratories have produced discordant accounts of ATPase localizations: Deul and McIlwainl2 found Na+-activated ATPase mainly in cerebral microsomes; Schwartz et al.13 found 75 per cent of the Na+-activated ATPase associated with microsomal material other than ribosomes; JirnefeltL4 studied brain homogenates in 0.25 Mf sucrose in which the major ATPase activity was found in a heavy microsomal fraction recovered after centrifuging at 25,000 g X 45 min. In a more recent paper by Hayashi et al.,15 a large percentage of activity was found in a nuclear fraction containing cell debris and intact cells and also in the mitochondrial and microsomlal fractions.