Thiophosphoryl guanine nucleotide analogues inhibit the (Na+,K+)-ATPase

The effects of several guanine nucleotide analogues on (Na+,K+)-ATPase activity of membranes isolated from several tissues were analyzed to determine if a G-protein might be involved in the hormonal regulation of the (Na+,K+)-ATPase. Submillimolar concentrations of GTP gamma S, but not GMPPNP, inhibit rat skeletal muscle and axolemma, but not kidney, (Na+,K+)-ATPase activity. Furthermore, GDP beta S does not reverse GTP gamma S inhibition, but rather itself slightly inhibits (Na+,K+)-ATPase activity. Dithiothreitol can block and reverse GTP gamma S inhibition of skeletal muscle (Na+,K+)-ATPase; the results obtained with axolemma membranes are complicated by the inhibition of (Na+,K+)-ATPase activity in these membranes by DTT. Results showing that high membrane concentrations can mute the inhibitory action of GTP gamma S suggest that a minor contaminant in GTP gamma S preparations is responsible for inhibiting (Na+,K+)-ATPase activity. Neither vanadate, a heavy metal, GDP, phosphate, nor thiophosphate, however, is responsible for this inhibition, and the inhibitory activity elutes with GTP gamma S from Sephadex G-10 columns. It is concluded that GTP gamma S or a structural derivative of GTP gamma S inhibits the (Na+,K+)-ATPase, in a tissue-specific manner, not by interaction with a G-protein as a GTP analogue, but through a direct chemical interaction with the (Na+,K+)-ATPase or some regulatory protein. The terminal SH group of the nucleotide analogue is probably required for this interaction.