The relipidation of delipidated Na,K-ATPase. An analysis of complex formation with dioleoylphosphatidylcholine and with dioleoylphosphatidylethanolamine.

Enzymatically inactive, delipidated Na,K-ATPase from dogfish rectal glands was titrated with dioleoylphosphatidylcholine and with dioleoylphosphatidylethanolamine. The process of relipidation has the following characteristic properties. Enzymatic activities reappear independently of each other: first the phosphatase, then the ATPase. The properties of the phosphatase regenerated depend on the ratio of lipid/protein used; the ATPase seems to be independent of this ratio. The simplest model that is consistent with the above results and with the shapes of the titration curves, has the following requirements. Firstly, the enzyme is composed of two subunits that, as far as lipid binding is concerned, are identical and independent of each other. Secondly, lipid adds onto the enzyme as preformed clumps of 25 molecules of phosphatidylcholine or 18 molecules of phosphatidylethanolamine. Thirdly, each subunit binds two clumps of lipid, and binding shows positive cooperativity. Fourthly, when either subunit becomes saturated with lipid, the enzyme exhibits one form of phosphatase. Fifthly, when both subunits are saturated with lipid, the enzyme exhibits a second form of phosphatase and ATPase. The data and their analysis according to this model lead to the suggestion that Na,K-ATPase is a functional dimer, the interaction between subunits being influenced by the Na+ and K+ concentrations in the medium: K+ favouring the functional independence of the subunits and Na+ favouring their functional interaction.