A comparative study of Na+/K(+)-ATPases of duck salt gland and canine kidney: implications for the enzyme's reaction mechanism.

Highly purified preparations of duck salt gland and canine kidney Na+/K(+)-ATPases with comparable specific activities were used to clarify the causes of previously reported differences between the substrate-velocity curves of these enzymes. When assays were done under identical conditions (pH 7.4; 37 degrees C), and a wide range of closely spaced ATP concentrations were used, the curves of both enzymes exhibited intermediary plateaus, as noted before for the salt gland enzyme. The two enzymes also had the same numbers of phosphorylation and ouabain binding sites, and their catalytic subunits were of the alpha 1 isoform type as revealed by immunostaining with specific antibodies. The findings suggest that the substrate-velocity curves of all widely used Na+/K(+)-ATPases may contain an intermediary plateau which is diagnostic of reaction mechanisms that generate rate equations containing powers of substrate concentration greater than two, e.g., a mechanism involving an oligomer with more than two protomers.