Effect of Li+ and Ba2+ on the electrocyte membrane-bound (Na+ + K+)-ATPase

Membrane-bound (Na+ + K+)-ATPase activity from the non-innervated and innervated faces of Electrophorus electricus (L.) electric organ, obtained by differential centrifugation, was measured using AChE as an enzyme marker for membranes derived from the post-synaptic area (fraction P3) of the electrocyte. The effect of Li+ and Ba2+ on (Na+ + K+)-ATPase activity of the two membrane fractions (P2 and P3) was analysed with respect to K+ and Mg2+ ions, after the I50 estimation. The kinetics of the reactions with these cations were investigated showing that Li+ inhibits P2 uncompetitively and for P3 presented a mixed type inhibition. Ba2+ behaved as an hyperbolic mixed type inhibitor for P2 and a linear mixed type inhibitor for P3 fraction.