Ordering of bulk membrane lipid or protein promotes activity of plasma membrane Mg2+ATPase

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Abstract
Treatment of liver plasma membranes with phospholipase A2 or high doses of concanavalin [Con]A enhances the activity of Mg2+ ATPase assayed at temperatures greater than 30.degree. C. The effects of the 2 treatments are not additive. Both the removal of phospholipids and binding of the lectin increase the degree of polarization of fluorescence of the lipid-soluble fluorophores, diphenylhexatriene and .beta.-parinaric acid, suggesting that decreased lipid fluidity may activate Mg2+ ATPase. In fact modification of lipid fluidity by reconstitution of phospholipase-treated membranes with phosphatidylcholines of defined fatty acid composition or by addition of cis-vaccenic acid showed a strong inverse correlation between Mg2+ ATPase activity and lipid fluidity as monitored by fluorescence polarization. Despite the ability of Con A to nonspecifically order membrane lipid, its effect on Mg2+ ATPase is apparently not mediated in this manner because other enzyme-activating lectins such as Ricinus communis agglutinin and wheat germ agglutinin are without effect on lipid fluidity. The facts that lectins of lower valency than tetravalent native Con A such as divalent succinyl Con A are far less effective in activating the enzyme and that paraformaldehyde treatment also activates suggests that cross-linking of membrane proteins is responsible. The diminution in activity of this membrane enzyme due to the disordering effect of heat in the physiological temperature range can be counteracted by isothermally increasing the order of either membrane lipid or protein.

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