Hydrodynamic properties of solubilized (Na+ + K+)-ATPase from rectal glands of Squalus acanthias.

(1) (Na+ + K+)-ATPas from the rectal glands of Squalus acanthias, solubilized in octaethylene glycol dodecyl monoether (2 mg detergent/mg protein), retains its activity for days when stored at 0 degrees C both with and without 20% glycerol. Glycerol protects partially against inactivation at higher temperatures. (2) Solubilization leads to a decrease in the amount of lipids bound per mg protein. 50 mol phospholipids and 40 mol cholesterol are bound per 265 000 g protein. 90% of the phospholipid is phosphatidylcholine (72%) and phosphatidylethanolamine (18%) and there is about 1 mol acidic phospholipid per 265 000 g. In addition, the protein has about 27 000 g carbohydrate as hexose, hexosamine and sialic acid bound per 265 000 g. (3) The calculation of the molecular weight from an In C vs. r2 plot obtained by sedimentation equilibrium centrifugaton in the presence of 560 microM detergent gives a molecular weight of the protein part of the active solubilized enzyme of 265 000 using the measured values for bound detergent, lipid (phospholipid + cholesterol) and carbohydrate. The sedimentation coefficient (S20,w) is 10.1 S, giving a Stokes' radius of 77 A. (4) An increase in detergent concentration to 56 mM dissociates the 10.1 S particle into particles with a sedimentation coefficient of 5.8 S and a molecular weight of 139 000 (Stokes' radius, 66 A). In the presence of this detergent concentration the enzyme is inactive. (5) The molecular weights of the soijm dodecyl sulphate-solubilized, isolated alpha- and beta-chains are found to be 106 000 and 40 000, respectively. (6) It is concluded that the active solubilized enzyme is an (alpha beta)2 structure and that it dissociates into an enzymatically inactive alpha beta structure when the detergent-to-protein ratio is increased.