NA,K-ATPASES OF THE LENS EPITHELIUM AND FIBER CELL - FORMATION OF CATALYTIC CYCLE INTERMEDIATES AND NA+K+ EXCHANGE

Previous studies suggest that an α2-related isoform of the catalytic subunit is predominant in the lens fiber cells. The α1 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 × 10-6 M for the α1 isoform of the epithelium, 1·3 ± 0·6 × 10-7 M for the α2-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. ATP 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 ±1000 min-1). Both lens Na,K-ATPases form the (ouabain + Mg2+ + 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 maior Na,K-ATPases of the lens, there would appear to be differences in the ATP dissolution of the K-occluded state. © 1994 Academic Press. All rights reserved.