Papain digestion of the subunits of (Na,K)-ATPase

Role of the Na,K-ATPase beta-subunit in the cellular accumulation and maturation of the enzyme as assessed by glycosylation inhibitors

No functional role could yet be established for the glycosylated beta-subunit of the Na,K-ATPase. In this study, we describe the intracellular processing of the beta-subunit as a glycoprotein in toad bladder cells and the consequences of its structural perturbation with glycosylation inhibitors on the cellular expression of the alpha- and beta-subunits and on the structural and functional maturation of the enzyme. Controlled trypsinolysis of homogenates from pulse-labeled cells reveals that the beta-subunit is subjected to glycosylation-dependent structural rearrangements during its intracellular routing. Inhibition of correct terminal glycosylation of the beta-subunit with deoxynojirimycin or swainsonine has no effect on the trypsin sensitivity of the alpha-subunit, its ability to perform cation-dependent conformation changes or the cellular Na,K-ATPase activity. Acquisition of core-sugars is sufficient for the enzyme to assume its catalytic functions. On the other hand, complete inhibition of glycosylation with tunicamycin leads to a destabilization of both the beta- and the alpha-subunits as judged by their higher trypsin sensitivity. In addition, tunicamycin treatment results in a decrease of the amount of newly synthesized beta- and alpha-subunit indicating that a glycoprotein, possibly the beta-subunit itself, plays a role in the efficient accumulation of the alpha-subunit in the endoplasmic reticulum.

Protective effect of Na+ and K+ against inactivation of (Na+ + K+)-ATPase by high concentrations of 2-mercaptoethanol at high temperatures

Purified dog kidney (Na+ + K+)-ATPase (EC 3.6.1.3) was inactivated with high concentrations of 2-mercaptoethanol at 50-55 degrees C. The inactivation was prevented by NaCl or KCl, with KCl being more effective than NaCl (the former ion being about one order more efficient under a typical set of experimental conditions). A disulfide bond in the beta-subunit of the enzyme protein was prevented from reductive cleavage by NaCl or KCl in accordance with protection of the enzyme activity. Choline chloride did not exert a significant protective effect over a similar concentration range. (Na+ + K+)-ATPase was also inactivated with high concentrations of 2-mercaptoethanol in the presence of low concentrations of dodecyl sulfate. This inactivation was also prevented by NaCl or KCl, with the latter being again more efficient than the former. These results indicate that Na+ and K+ bound to their respective ion-binding sites on the alpha-subunit exert a protective effect on a disulfide bond on the beta-subunit. This suggests some sort of interaction between the alpha- and the beta-subunits.