The interaction of monovalent cations with the sodium pump of low‐potassium goat erythrocytes

Publisher Website
Google Scholar
Abstract
The activation by Na ions and the effect of the anti-L antibody on the Na pump of low K type (LK) erythrocytes, were studied by measuring the ouabain-sensitive ATPase activity of red cell membranes of LK goats. The experimental data were 1st corrected for incomplete occupation of the external K sites of the pump, using a saturation function obtained from influx experiments. Double-reciprocal plots of the corrected rates against Na concentration at various fixed K concentrations, yield a pattern of competitive K inhibition when it is assumed that 3 equivalent Na sites take part in the internal activation of LK-(Na + K)-ATPase. The dissociation constant of Na at each site (Km) lies between 10-20 mM and that of K as competitive inhibitor (Ki), between 1.5-4.5 mM. The maximal rate of hydrolysis of LK goat (Na + K)-ATPase is not different from those usually obtained with the high-K type (HK) red cell enzyme. Then, the low pumping rate of LK erythrocytes in physiological conditions is only reflecting the poor Na affinity, both absolute and relative, at the internal Na sites of their Na pumps. The stimulation of the ouabain-sensitive ATPase activity by sensitization of the membranes with anti-L serum, is mediated by a 3-fold reduction of the Km/Ki ratio at each site. Km decreases by a factor of 10, but there is also a smaller diminution of Ki. The maximal rate of hydrolysis is unchanged by the anti-L treatment. The least-squares fitting of the pooled data by the rate equation, converges better with less than 3 and more than 2 equivalent Na sites. The affinity sequence at 2 external K sites of the LK goat erythrocyte Na pump, determined in the presence of 100 mM external Na, is Rb > K > Cs. It is obtained from the concentration dependence in influx experiments, and is the same as reported for human red cells. Cubic-root Dixon plots of the corrected ouabain-sensitive ATPase activity against the concentration of K and its congeners, show the sequence Tli > K > Rb > Na > Cs for the affinities at the internal cation sites of the LK Na pump. Anti-L treatment decreases the relative magnitude of Na and Cs selectivities, it being not certain whether a Rb-Na transition then occurs. The results are discussed in terms of possible mechanisms whereby the Na pump of LK and HK red cells may adjust the properties of their cation sites upon translocation of monovalent cations.