Molecular Properties of Purified (Sodium + Potassium)- activated Adenosine Triphosphatases and Their Subunits from the Rectal Gland of Squalus acanthias and the Electric Organ of Electrophorus elect&us*

The chemical properties of two highly purified preparations of (sodium + potassium)-activated adenosine triphosphatase (NaK ATPase) and their subunits have been compared. One preparation is derived from the rectal gland of the spiny dogfish shark, Squatus acanfhias and the other preparation is derived from the electric organ of the electric eel, Elecfrophorus e2ectricus. Ouabain binding and phosphorylation from [T-~~P]ATP for both enzymes ranged from 4000 to 4300 pmol per mg of protein. This gives a stoichiometry for ouabain binding and phosphorylation of 1:l for both enzymes. The molar ratios of catalytic subunit to glycoprotein was 2: 1 for both enzymes, suggesting a minimum molecular weight of 250,000, which agrees with the molecular weight obtained by radiation inactivation. Assuming that only one of the two catalytic subunits is phosphorylated and binds ouabain per (sodium + potassium)-activated adenosine triphosphatase molecule the data on phosphorylation and ouabain binding also give a molecular weight of 250,000. The data on phosphorylation, ouabain binding, subunit composition, and molecular weight based on radiation inactivation are thus all internally consistent. A technique has been developed for isolation of pure catalytic subunit and glycoprotein in good yields by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A variety of chemical studies have been carried out with the purified subunits. The amino acid composition of the catalytic subunit was different from that of the glycoprotein, but the amino acid composition of each of the two subunits was essentially the same for both species. HOWever, the NH*-terminal amino acid for the catalytic subunit was alanine for the rectal gland enzyme and serine for the electric organ enzyme, suggesting some differences in ammo acid sequences for the two species. The NH*-terminal amino