Net synthesis of ATP by reversal of the sodium pump.

Abstract

A FEW years ago, Garrahan and Glynn1,2 described experiments in which very steep concentration gradients of Na+ and K+ across the membranes of human red cell ghosts led to a ouabain-sensitive incorporation of 32P-labelled inorganic phosphate into ATP, apparently by a reversal of the cation transport system. These findings have been confirmed and extended in experiments on ghosts3,4 and in experiments on intact red cells5–7 in which the incorporation of phosphate was shown to parallel the ouabain-sensitive downhill movements of the cations. Although the interpretation of these experiments seems clear, attempts in a number of laboratories to demonstrate a net ouabain-sensitive synthesis of ATP at the expense of downhill ion movements have not been successful. The failure of these attempts can be attributed to the relatively low rate of ouabain-sensitive phosphate incorporation in human cells, ATP breakdown by processes unconnected with the pump being rapid compared with the expected rate of formation of ATP by reversal of the pump. Nevertheless, in the absence of a demonstrable increase in the amount of ATP, it is difficult to be certain that the observed incorporation of radioactivity is brought about by the formation of new ATP rather than by some kind of exchange reaction. The finding that guinea-pig red cells showed a surprisingly large ouabain-sensitive efflux of K+ into K+ -free high-Na+ media8 suggested that it would be worth looking for a net ouabain-sensitive synthesis of ATP in these cells, and we now report that such a synthesis does in fact occur.