´Chloride-stimulated ATPase ’ activity in Limonium vulgare Mill

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Abstract
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’.