Abstract
Our previous studies showed that when ethylmercurithiosalicylate (thimerosal) interacts with the transport ATPase of the guinea pig kidney under specified conditions, the Na + + K +-dependent ATPase activity is inhibited, while the Na +-dependent ATPase, the Na + + ATP-dependent phosphorylation of the enzyme, and the K +-dependent discharge of the phosphoenzyme seem to be unaffected. Here we describe other properties of the thimerosal-treated enzyme: Na +-dependent ADP-ATP exchange, Na +-dependent UTPase, and K +-dependent p-nitrophenylphosphatase activities of the modified enzyme are not inhibited. Kinetics of the Na + effect on the UTPase activities of the native and the modified enzyme are the same. However, K + has a greater inhibitory effect on the Na +-UTPase of the modified enzyme than on the Na +-UTPase of the native enzyme. The increase in the apparent affinity of the thimerosal-treated enzyme for K + is also evident from the kinetics of the K + effect on p-nitrophenylphosphatase. Neither the native enzyme nor the modified enzyme catalyzes a P 1-ATP exchange. The uninhibited activities of the thimerosal-treated enzyme are sensitive to ouabain. These data provide further support for those reaction mechanisms in which the existence of two ATP sites within the enzyme is assumed.
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