(Na+K+)-activated ATPase in human cornea

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Abstract
Distribution and principal characteristics of (Na+K+)-activated ATPase in human cornea were investigated. (Na+K+)-ATPase was present in both epithelium and endothelium, whereas the corneal stroma did not exhibit significant enzyme activity. In homogenates specific activity of the (Na+K+)-ATPase was 2.3-fold higher in endothelium than in epithelium. Calculation of total enzyme activity revealed a 6.1-fold higher content of (Na+K+)-ATPase in the epithelium. In the epithelium a 7-fold enrichment of (Na+K+)-ATPase compared to the homogenate was obtained in the 150–1500×g av fraction. Maximum enrichment in the endothelium was 3.5-fold and was achieved in the 1500–2500×g av fraction. Both fractions showed, however, the same specific activity. The pH-optimum of (Na+K+)-ATPase in the 150–1500×g av fraction ranged from 8.0–8.2 in both epithelium and endothelium. In the epithelial 150–1500×g av fraction the apparentK m-values were 4.0 mM for Na+, 2.8 mM for K+ and 0.12 mM for Mg2+ · ATP in equimolar concentrations. The inhibition constant of epithelial (Na+K+)-ATPase for ouabain was determined asK i=3.3×10−7 M. The present data support the view that control of corneal hydration in man is a function of both endothelium and epithelium.

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