Inhibition of (Na/K)-ATPase by electrophilic substances: functional implications

The effect of electrophilic substances: p-bromophenylisothiocyanate (PBITC); fluoresceinisothiocyanate (FITC); [4-isothiocyanatophenyl-(6-thioureidohexyl)-carbamoylmethyl] -ATP (ATPITC); 2,4,6-trinitrobezenesulfonic acid (TNBS); 1-(5-nitro-2-furyl)-2-phenylsulfonyl-2-furylcarbonyl ethylene (FE1); 1-(5-phenylsulfonyl-2-furyl)-2-phenylsulfonyl-2-furylcarb onyl ethylene (FE2) and 1-(5-phenylsulfonyl-2-furyl)-2-phenylsulfonyl-2-tienocarb onyl ethylene (FE3) on the sarcolemmal (Na/K)-ATPase isolated from guinea-pig hearts was studied. FITC and PBITC were found to inhibit competitively the activation of (Na/K)-ATPase by ATP. Being for the enzyme inhibitor and substrate at the same time ATPITC does not offered clear kinetic behavior. However, the activation of (Na/K)-ATPase by sodium and potassium ions was inhibited non-competitively by all three isothiocyanates. These data indicated that isothiocyanates may interact predominantly in the ATP-binding site of the enzyme molecule. In contrary to isothiocyanates TNBS and FE1 (FE2 and FE3 were ineffective) inhibited the activation of (Na/K)-ATPase by ATP non-competitively i.e., their interaction in the ATP-binding site seemed to be improbable. Nevertheless, TNBS and FE1 both manifested affinities to that moiety of (Na/K)-ATPase molecule which is binding potassium. More specific was the effect of FE1 that showed clearly competitive inhibition of potassium-stimulation of the enzyme activity. FE1 exerted also an ouabain-like effect on the mechanical activity of isolated perfused guinea-pig heart. This result indicates that FE1 seems to exert a selective inhibition of the (Na/K)-ATPase not only in vitro but also in integrated cardiac tissue.