THE EFFECT OF MERCURY AND ALUMINUM ON SODIUM-POTASSIUM-MG2+ DEPENDENT-ADENOSINE TRIPHOSPHATASE-ACTIVITY OF ELECTROPHORUS-ELECTRICUS (L) ELECTROCYTE

1. Na+,K+-ATPase, which mediates the active transport of Na+ and K+ across the plasma membrane, is found in equivalent amounts in both plasma membranes of the electrocyte, the anterior, non-innervated (fraction P-2) and the posterior, innervated (fraction P-3) obtained by differential centrifugation of Electrophorus electricus (L.) electric organ. 2. The kinetic effects of Hg2+ and Al3+, described as neurotoxic metals, on the Na+,K+-ATPase activity of the two membrane fractions (P-2 and P-3) were analysed with respect to Na+ and K+ ions, after the I-50 estimation of each metal. 3. Mercury is a potent Na+,K+-ATPase inhibitor in the nanomolar range. In all cases, it behaved as a mixed partial hyperbolic inhibitor. 4. Aluminum was shown to be a poor enzyme inhibitor. Changing the K+ concentration, it behaved as a mixed linear inhibitor (P-2 fraction) and as a non-essential mixed activator (fraction P-3). Aluminum behaved as a partial hyperbolic inhibitor for both P-2 and P-3 fractions with respect to Na+ concentration. 5. The observation of the variable kinetic behaviour of P-2 and P-3 led us to attribute these differences to the Na+,K+-ATPase electrocyte isoenzymes which occur in different proportions in these fractions (Gomes-Quintana et a[., 1992 Comp. biochem. Physiol. 103B/3 623-628).