A modifier of (Na⁺ + K⁺) ATPase in commercial ATP

Abstract

WHEN most (Na⁺ + K⁺)ATPase preparations are allowed to hydrolyse ATP in the presence of different concentrations of Na⁺ and K⁺ ions, but with the sum [Na⁺] plus [K⁺] held constant, the behaviour observed is similar to that shown in the dotted curve of Fig. 1. K⁺ ions activate at low concentrations, and inhibit (by competing with Na⁺ ions at intracellular sites) at high concentrations. There is a broad plateau in the middle of the curve. We reported¹ recently that (Na⁺ + K⁺)ATPase from the outer medulla of pig kidney showed behaviour of a different kind, like that shown in the solid curve of Fig. 1. Here, K⁺ ions apparently inhibit even at relatively low concentrations, so that, as the activating effect of K⁺ ions approaches saturation, the inhibitory effect becomes predominant and causes a sharp fall to an intermediate level of activity. This effect was more pronounced at 20 or 30 °C than at 37 °C, and at pH 7.5 than at pH 6.5. We have now discovered that this peculiar behaviour depends on the use of Sigma ‘Sigma grade’ ATP, and is not seen with Boehringer ATP (see Fig. 1). We report here that many samples of ‘Sigma grade’ ATP contain something which reacts with (Na⁺ + K⁺)ATPase reversibly and with a high affinity, making the enzyme much more sensitive to inhibition by K⁺ ions. The changed response to K⁺ ions, and the sensitivity of this response to pH can account for the inhibition of kidney (Na⁺ + K⁺)ATPase by ‘Sigma grade’ ATP reported by Charney, Silva and Epstein², and the altered pH optimum reported by Katz and Michell³.