Inhibition by uranyl nitrate of adenosine triphosphatases derived from animal and human tissues

Inhibition of adenosine triphosphatase (ATPase) by uranyl nitrate (UO₂²⁺ or U⁶⁺) was studied in microsomal fractions and tissue homogenates of several organs and species. U⁶⁺ inhibited ouabain-sensitive (Na⁺ + K⁺-dependent) ATPase and ouabain-insensitive (Mg²⁺-dependent) ATPase with I50 of 2 × 10^?5 to 2 × 10^?4m. Higher concentrations of U⁶⁺ were required to inhibit the enzyme in homogenates than in microsomal fractions. Mg²⁺ ATPase was somewhat more sensitive to U⁶⁺ than was Na⁺ + K⁺ ATPase when data were corrected for protein content of enzyme preparations. The inhibition of Na⁺ + K⁺ ATPase, but not Mg²⁺ ATPase, was markedly antagonized by Na⁺. This suggests that U⁶⁺ may inhibit Na⁺ + K⁺ ATPase at the Na⁺ site on the enzyme, whereas ouabain inhibits at the K⁺ site. ATP decreased and Mg²⁺ increased the inhibition of both enzymes. K⁺ had no effect. The remaining studies were done with Na⁺ + K⁺ ATPase. Increasing pH enhanced inhibition. The enzyme was protected by bovine serum albumin and citric acid. Ascorbic acid increased inhibition possibly by reducing U⁶⁺ to U⁴⁺, thus rendering the new ionic species reactive with sulfhydryl groups in addition to organic anions.