(Na+K+)-activated ATPase in human cornea. Distribution within the cornea and properties of the enzyme from epithelial cells.
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 X gav fraction. Maximum enrichment in the endothelium was 3.5-fold and was achieved in the 1500-2500 X gav fraction. Both fractions showed, however, the same specific activity. The pH-optimum of (Na+K+)-ATPase in the 150-1500 X gav fraction ranged from 8.0-8.2 in both epithelium and endothelium. In the epithelial 150-1500 X gav fraction the apparent Km-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 as Ki = 3.3 X 10(-7) M. The present data support the view that control of corneal hydration in man is a function of both endothelium and epithelium.
D000251:Adenosine Triphosphatases / Q000032:analysis / Q000037:antagonists & inhibitors / Q000378:metabolism*; D001693:Biological Transport, Active; D003315:Cornea / Q000201:enzymology*; D004727:Endothelium / Q000201:enzymology; D004848:Epithelium / Q000201:enzymology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D013347:Subcellular Fractions / Q000201:enzymology
D010042:Ouabain; D012964:Sodium; D000251:Adenosine Triphosphatases; D008274:Magnesium