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

Summary 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.