Na,K-ATPases of the lens epithelium and fiber cell: formation of catalytic cycle intermediates and Na+: K+ exchange.
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.
Garner
M H
MH
University of Texas Southwestern Medical Center at Dallas, Department of Ophthalmology 75235-8592.
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002417:Cattle; D046508:Culture Techniques; D004305:Dose-Response Relationship, Drug; D006868:Hydrolysis; D007908:Lens, Crystalline / Q000201:enzymology / Q000378:metabolism*; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism*; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology*; D013327:Strophanthidin / Q000494:pharmacology
D013327:Strophanthidin; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium; D011188:Potassium