The 50 kDa subunit is glycosylated and could be a previously undescribed P type ATPase subunit. Although the available sequence evidence is not conclusive, its N terminal sequence does not correspond to any previously reported subunit . As shown in Fig. 3, the distribution of the Na and Na K ATPase differs through distinct guinea pig kidney segments. Both enzymes are well expressed in the outer cortex, but Na ATPase expression is lower in the inner regions of the kidney and absent in the medulla. In the intestine, the Na ATPase is mainly expressed in villous and surface cells. In the crypt region , the enzyme seems to have an intracellular distribution. This particular renal and intestinal distribution probably has to do with the physiological role of this enzyme in sodium transport in these epithelia. Furthermore, IgY polyclonal antibodies raised against the purified Na and Na K ATPases differentially recognize these enzymes. Antibodies raised against the purified Na ATPase inhibit the Mg2 dependent ouabain insensitive Na stimulated ATPase activity without effect on the Na K ATPase, while antibodies raised against the purified Na K ATPase inhibit this enzyme without effect on the Na ATPase .
Na ATPase forms a phosphorylated intermediate The Na ATPase can be classified among the P type ATPases. Its Mg2 dependence, sensitivity to vanadate and capacity to Nutlin-3 selleck form a phosphorylated intermediate from ATP or Pi are the strongest pieces of evidence for this classification. It can be phosphorylated from inorganic phosphate in an ion sensitive reaction stabilized by furosemide . In that article, a phosphorylated polypeptide of about 100 kDa was identified for the first time as directly associated with the Na ATPase. In 2005, del Castillo et al. reported a phosphorylated intermediate obtained from ATP associated with the purified Na ATPase. The phosphorylation was Mg2 dependent, vanadate sensitive and stimulated by Na with two different Km values . The stimulatory effect was specific for Na and independent of anions. Phosphorylation was insensitive to ouabain but stimulated by furosemide with an EC50 of 1.8 0.54 mM. In addition, 0.
5 mM ADP partially inhibited it. Phosphorylation was also sensitive to alkaline pH and hydroxylamine, suggesting an acyl phosphate bond associated with the 100 kDa polypeptide of the enzyme. A minimum reaction cycle for the Na ATPase was proposed in which the enzyme has an E1 form that can be phosphorylated from ATP in the presence of Mg2 and Na , producing the E1.P.Na form, sensitive to ADP. Furosemide stabilizes the E1.P.Na Tenofovir form. The enzyme then changes to the E2.P.Na form, insensitive to ADP, which is susceptible to dephosphorylation. A conformational change induces Na translocation through the membrane.