Studies on (Na+ + K+)-activated ATPase XLVII. Chemical composition, molecular weight and molar ratio of the subunits of the enzyme from rabbit kidney outer medulla

The amino acid composition of each subunit closely resembles that of the corresponding subunit of the enzyme from dog and lamb kidney, duck salt gland, shark rectal gland and electric eel electroplax, which indicates that there is no specific binding of phospholipids to the subunits.

Abstract

(1) The subunits of a purified preparation of (Na+ + K+)-ATPase from rabbit kidney outer medulla have been completely separated by gel filtration in sodium dodecyl sulfate. During the gel filtration procedure 3–4% of the phospholipids present in the purified enzyme preparation remain bound to the separated subunits even after repeated gel filtration. (2) The composition of the bound phospholipids does not greatly differ from that in the original enzyme preparation, which indicates that there is no specific binding of phospholipids to the subunits. The isolated β-subunit contains twice as much phospholipid per mg protein as the α-subunit. (3) The α-subunit is more hydrophobic (43%) in its amino acid composition than the β-subunit (38%). The α-subunit contains more alanine and histidine and much less tyrosine and lysine than the β-subunit; the other amino acids show little difference. The amino acid composition of each subunit closely resembles that of the corresponding subunit of the enzyme from dog and lamb kidney, duck salt gland, shark rectal gland and electric eel electroplax. (4) Both subunits are glycoproteins, although the β-subunit contains 5-times more carbohydrate per g protein than the α-subunit. Both subunits contain glucosamine, galactose, mannose, sialic acid and also glucose, which appears to be covalently bound. Galactosamine, in small amounts, is only detected in the β-subunit.