Effects of tricyclohexylhydroxytin on the kinetics of adenosine triphosphatase system and protection by thiol reagents

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Abstract
Tricyclohexylhydroxytin, commonly known as Plictran® inhibited Na+, K+ -ATPase activity of rat brain synaptosomes in a concentration-dependent manner with median inhibitory concentration (IC-50) of 2 μM. Both K+ -stimulated para-nitrophenylphosphatase and [3-H]-ouabain binding to synaptosomes were also inhibited by Plictran with IC-50 values of 11 and 30 μM, respectively. Altered pH and Na+, K+ -ATPase activity curves demonstrated comparable inhibition in buffered neutral and alkaline pH ranges, and no inhibition was observed in acidic pH. The inhibition of Na+, K+ -ATPase was independent of temperature. Kinetic studies of substrate (ATP) activation of Na+, K+ -ATPase indicated uncompetitive inhibition. Results also showed noncompetitive inhibition for p-nitrophenylphosphate and uncompetitive inhibition for K+ activations of p-nitrophenylphosphatase. Preincubation of synaptosomes with dithiothreitol, a sulfhydryl (SH) agent, resulted in the complete protection of Plictran inhibition of Na+, K+ -ATPase, K+ -para-nitrophenylphosphatase, and [3-H]-ouabain binding. The protection was specific and concentration dependent since cysteine and glutathione did not afford protection. These results indicate that Plictran inhibited Na+, K+ -ATPase by interacting with dephosphorylation of the enzyme-phosphoryl complex and exerted a similar effect to that of SH-blocking agents.