Na,K-ATPases of the lens epithelium and fiber cell: formation of catalytic cycle intermediates and Na+: K+ exchange.Exp Eye Res. 1994 Jun; 58(6):705-18.EE
Previous studies suggest that an alpha 2-related isoform of the catalytic subunit is predominant in the lens fiber cells. The alpha 1 isoform is predominant in the lens epithelium (Garner, Horwitz and Enomoto, 1992). Data are presented to show that strophanthidin-sensitive K+ transport is sustained by both of these lens Na,K-ATPases. The K50 for strophanthidin inhibition of K+ transport was 1.4 +/- 0.5 x 10(-6) M for the alpha 1 isoform of the epithelium, 1.3 +/- 0.6 x 10(-7) M for the alpha 2-related isoform of the lens fiber cells. Na+ accumulation in cultured bovine lenses was strophanthidin sensitive. The K50 values for strophanthidin-sensitive Na+ accumulation were similar to those obtained for K+ transport. ARP turnover by the lens fiber cell Na,K-ATPase (1700 +/- 600 min-1) was lower than ATP turnover by the lens epithelium Na,K-ATPase (8000 +/0 1000 min-1). Both lens Na,K-ATPases form the (ouabain + Mg(2+) + phosphate)-dependent phosphoenzyme. Both lens Na,K-ATPases form the (ATP + Na(+) + Mg2+)-dependent phosphorylated intermediate. K+ does not effectively dephosphorylate the Na,K-ATPase of the lens fibers. K+ does cause dephosphorylation of the Na,K-ATPase of the lens epithelium. Interaction of the Na,K-ATPase with Mg2+ would appear to cause the monovalent cation insensitivity. The lens fiber cell Na,K-ATPase, like the lens epithelium Na,K-ATPase occludes two K+ ions. However, between the two major Na,K-ATPases of the lens, there would appear to be differences in the ATP dissolution of the K-occluded state.