Molecular species of a soluble nucleoside diphosphokinase related to the Na + ,K + -ATPase

Soluble nucleosidediphosphokinase (NDK) activity is prepared from purified kidney membranes rich in Na⁺, K⁺-ATPase. The soluble NDK differs from the bound activity by the lack of sensitivity to Na⁺, K⁺ and ouabain. Sucrose gradient centrifugation of the soluble NDK shows the presence of three major species with molecular weights of 21,000, 92,000 and 138,000. Increasing the urea concentration in the sucrose gradient centrifugation decreases the proportion of the enzyme in the heavier form. This effect of urea is reversible. These data suggest that the higher molecular weight forms are tetramers and hexamers of the 21,000 dalton form of the soluble NDK.