MERCURY BLOCKS NA-K-ATPASE BY A LIGAND-DEPENDENT AND REVERSIBLE MECHANISM

An inhibitory receptor for cardioactive steroids such as digoxin and ouabain is located at the extracellular surface of the Na-K-adenosinetriphosphatase (ATPase) molecule. Besides cardioactive steroids, mercury is a potent inhibitor of the Na-K-ATPase activity. The half-maximal inhibitory concentration (IC50), determined within 30 min at 37-degrees-C at 1-mu-g protein/ml, was 200 nM, despite the presence of 1 mM EDTA; the IC50 decreased with increasing protein/inhibitor ratio, and it reached 2.7-mu-M at 0.1 mg protein/ml and 20-mu-M at 1 mg protein/ml. The IC50 for Na-K-ATPase inhibition by the diuretic compound mersalyl was 4 and 5-mu-M for the nondiuretic p-chloromercuribenzenesulfonic acid at 0.1 mg protein/ml. The IC50 for HgCl2 inhibition was modulated by the presence of EDTA as well as by the pump ligands Mg, Na, K, and ATP. The E2 conformation of the Na-K-ATPase molecule was more sensitive to HgCl2 than the E1 conformation. The mercury antidote 2,3-dimercapto-1-propanesulfonic acid was able to reactivate approximately 70% of the blocked enzyme. In conclusion, a metal-binding domain of the Na-K-ATPase molecule with particular high affinity for Hg(II) was described functionally in the present work. Therefore Na-K-ATPase belongs to the metal-binding proteins. Metals may modulate the cellular expression and activity of the system by interacting with its metal-binding interface.