A COMPARATIVE-STUDY OF NA+ K+-ATPASES OF DUCK SALT-GLAND AND CANINE KIDNEY - IMPLICATIONS FOR THE ENZYMES REACTION-MECHANISM/

Highly purified preparations of duck salt gland and canine kidney Na+/ K+-ATPases with comparable specific activities were used to clarify th e causes of previously reported differences between the substrate-velo city curves of these enzymes, When assays were done under identical co nditions (pH 7.4; 37 degrees C), and a wide range of closely spaced AT P concentrations were used, the curves of both enzymes exhibited inter mediary plateaus, as noted before for the salt gland enzyme. The two e nzymes also had the same numbers of phosphorylation and ouabain bindin g sites, and their catalytic subunits were of the alpha(1) isoform typ e 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 rea ction mechanisms that generate rate equations containing powers of sub strate concentration greater than two, e.g., a mechanism involving an oligomer with more than two protomers. (C) 1995 Academic Press, Inc.