Structural and enzymic aspects of the hydrolysis of adenosine triphosphate by membranes of kidney cortex and erythrocytes.

The concept of an intimate linkage between active cationmovements across cell membranes and adenosine-triphosphatase activity is strongly supported by the parallelism between the effects of cations, notably Na+ and K+, and of cardiac glycosides, such as ouabain, on these two processes. The adenosine triphosphatases are activated by suitable concentrations of Na+ and K+ ions, this activation being counteracted by low concentrations of glycosides that also inhibit the coupled active transport of Na+ and K+ ions. This postulated involvement with cation movements clearly requires the adenosine-triphosphatase activity to be located in cell membranes and to possess the overall spatial asymmetry characteristic of the transport system. The erythrocyte provides excellent material for investigations of these aspects of the problem, and the situation has been more clearly defined in erythrocytes (Glynn, 1962; Whittam, 1962 b) than in other tissues, such as brain and kidney, where localization of the activity depends on fractional centrifugation of homogenates. It thus seemed important to determine the distribution of this enzymic activity among subcellular fractions of kidney-cortex homogenate and to prepare what appears to be a membrane fraction. A partial fractionation of the erythrocyte membrane was also attempted by preparing elinin, the main lipoprotein component ofthemembrane (Moskowitz, Dandliker, Calvin & Evans, 1950; Moskowitz & Calvin, 1952). Previous work has shown that cell-debris fractions from rabbit kidney-cortex homogenate possess adenosine-triphosphatase activity that is inhibited by ouabain (Wheeler & Whittam, 1962; Kinsolving, Post & Beaver, 1963; Taylor, 1963). In an attempt to clarify the nature of the enzyme system and its possible role in other transport processes, the effects of a variety of inhibitors have been studied. In the interests of clarity and brevity we shall call the component of the adenosine-triphosphatase activity that is observed in the presence of Mg2+ ions alone, or Mg2+ ions, K+ ions, Na+ ions and ouabain, the '(Mg)-adenosine triphosphatase'; and, similarly, the component that requires Mg2+, K+ and Na+ ions, and is inhibited by ouabain, the '(NaK)-adenosine triphosphatase'. METHODS