Properties of the chloride-ATPase from Limonium salt glands: activation by, and binding to, specific sugars

Chloride ATPase pumps in nature: do they exist?

Five widely documented mechanisms for chloride transport across biological membranes are known: anion-coupled antiport, Na+ and H(+)-coupled symport, Cl- channels and an electrochemical coupling process. These transport processes for chloride are either secondarily active or are driven by the electrochemical gradient for chloride. Until recently, the evidence in favour of a primary active transport mechanism for chloride has been inconclusive despite numerous reports of cellular Cl(-)-stimulated ATPases coexisting, in the same tissue, with uphill ATP-dependent chloride transport. Cl(-)-stimulated ATPase activity is a ubiquitous property of practically all cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase pump activity. Recent studies of Cl(-) -stimulated ATPase activity and ATP-dependent chloride transport in the same plasma membrane system, including liposomes, strongly suggest a mediation by the ATPase in the net movement of chloride up its electrochemical gradient across the plasma membrane structure. Contemporary evidence points to the existence of Cl(-)-ATPase pumps; however, these primary active transporters exist as either P-, F- or V-type ATPase pumps depending upon the tissue under study.

´Chloride-stimulated ATPase ’ activity in Limonium vulgare Mill

Limonium vulgare Mill, (sea lavender) is a saltmarsh plant in whose leaves are embedded multicellular glands for the excretion of salt. Physiological studies have shown that the basis of the salt secretion mechanism appears to be salt-inducible, ATP-powered pumping of chloride ions out of gland cells. We have been investigating a ‘chloridestimulated’ ATPase activity of solubilized Limonium leafmicrosomes, which we thought was almost certainly part of a chloride pump involved in salt secretion because the properties of the enzyme activity correlated so well with the characteristics of salt secretion. Methods of purifying this activity on hexyl agarose and by selective solubilization with low concentrations of neutral detergent are presented. These and other results have made it necessary to revise our understanding of earlier findings on the purification of the ‘ Cl-stimulated ’ ATPase on Sepharose- N -caproyl galactosamine. Recent results indicate that the apparent ‘ stimulation ’ by chloride can be accounted for by an inhibition of salt-inducible ATPase activity by the ‘control’, non-Cl-, monovalent anion, benzene sulphonate, adopted because it resembles chloride yet did not support salt-secretion. Our results demonstrate some of the dangers in interpreting measurements of ion-stimulated ATPase activity, and make an instructive ‘ cautionary tale’.

The chloride pump: a Cl(-)-translocating P-type ATPase

Three widely documented mechanisms of chloride transport across plasma membranes are anion-coupled antiport, sodium-coupled symport, and an electrochemical coupling process. No direct genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)-stimulated adenosine triphosphate (ATP)ases coexisting in the same tissue with uphill chloride transport that could not be accounted for by the three common chloride transport processes. Cl(-)-stimulated ATPases are a common property of practically all biological cells, with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases.

Improved preparation and assay and some characteristics of Cl--ATPase activity from Limonium vulgare

We present a revised method for the preparation, storage and assay of the Cl--ATPase activity responsible for salt secretion in Limonium vulgare. Altering the centrifugation step improved the yield and linked-enzyme assay for ADP provided a sensitive method for continuous monitoring of Cl--ATPase activity. The activity of untreated membranes was low but fairly stable. Treatment with detergent gave strong stimulation of Cl--ATPase activity ATP between 0.1 and 0.2 mM. Cl- stimulated the activity up to a maximum at 0.13 M Cl-, and the pH optimum was around 6.3 to 6.4.