NA+, K+-ATPASE ISOFORMS IN THE RETINA

This chapter describes Na+, K+- adenosine triphosphatase (ATPase) isoforms in the retina and explores how the existence of these isoforms might be advantageous for visual function. It also discusses the aspects of enzyme expression in other cells that may contribute to understanding of the functions of the enzyme in retina, especially in photoreceptor cells. Na+, K+-ATPase or Na+, K+-transporting ATPase is the enzyme that forms the sodium pump, ubiquitous in animal cells. The enzyme couples the energy derived from ATP hydrolysis to the transport of sodium and potassium ions. The sodium pump is composed of two subunits, α, the larger catalytic subunit, and β, a smaller glycoprotein. The α subunit contains binding sites for Na+, K+, and ATP, and a site for phosphorylation and a binding site for ouabain and related cardiac glycosides, which specifically inhibit Na+, K+-ATPase. The existence of variant forms of the α subunit of Na+,K+-ATPase was suggested by studies of inhibition of enzyme activity in brain by cardiac glycosides such as ouabain, which revealed at least two binding affinities for the inhibitor. Sweadner identified two forms—namely, α (for the band that had the same mobility as the catalytic subunit in kidney) and α+ (for the additional slower migrating band) in neural tissue in sodium dodecyl sulfate (SDS) gels. The electrophoretic variant α+ is known to consist of two comigrating polypeptides, called α2 and α3. © 1992, Academic Press Inc.