Electrocyte (Na(+),K(+))ATPase inhibition induced by zinc is reverted by dithiothreitol

The Mg(2+)-dependent (Na(+),K(+))ATPase maintains several cellular processes and is essential for cell excitability. In view of the importance of the enzyme activity, the interaction and binding affinities to substrates and metal ions have been studied. We determined the effect of Zinc ion (Zn(2+)) on the (Na(+),K(+))ATPase activity present in both conducting (non-innervated) and post-synaptic (innervated) membranes of electrocyte from Electrophorus electricus (L.). Zn(2+) is involved in many biological functions and is present in pre-synaptic nerve terminals. This metal, which has affinity for thiol groups, acted as a potent competitive inhibitor of (Na(+),K(+))ATPase of both membrane fractions, which were obtained by differential centrifugation of the E. electricus main electric organ homogenate. We tried to recover the enzyme activity using dithiothreitol, a reducing agent. Kinetic analysis showed that dithiothreitol acted as a non-essential non-competitive activator of (Na(+),K(+))ATPase from both membrane fractions and was able to revert the Zn(2+) inhibition at mM concentrations. In the presence of dithiothreitol, this metal behaved as a competitive inhibitor of (Na(+),K(+))ATPase in the non-innervated membrane fractions and presented a non-competitive inhibition of (Na(+),K(+))ATPase in innervated membrane fractions. This difference may be attributed to formation of a Zn-dithiothreitol complex, as well as the involvement of other binding sites for both agents. The consequences of the enzyme inhibition by Zn(2+) may be considered in regard to its neurotoxic effects.