Two Isoenzymes of Na+, K+-ATPase Have Different Kinetics of K+Dephosphorylation in Normal Cat and Human Brain Cortex

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Abstract
Analysis of purified Na+, K+‐ATPase from cat and human cortex by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis reveals two large catalytic subunits called α(‐) (lower molecular weight) and α(+) (higher molecular weight). Differences in K+ dephosphorylation of these two molecular forms have been investigated by measuring the phosphorylation level of each protein after their separation on sodium dodecyl sulfate gels. In the presence of Na+, Mg2+, and ATP, both subunits are phosphorylated. Increasing concentrations (from 0 to 3 mM) of K+ induce progressive dephosphorylation of both α‐subunits, although the phosphoprotein content of α(‐) is decreased significantly less than that of α(+). Ka values of α(‐) for K+ are 40% and 50% greater in cat and human cortex, respectively, than values of α(+), α(‐) and α(+) are thought to be localized in specific cell types of the brain: α(‐) is the exclusive form of nonneuronal cells (astrocytes), whereas α(+) is the only form of axolemma. Our results support the hypothesis that glial and neuronal Na+, K+‐ATPases are different molecular entities differing at least by their K+ sensitivity. Results are discussed in relation to the role of glial cells in the regulation of extracellular K+ in brain.