The shape of the substrate curve during ATP hydrolysis by duck salt gl
and Na+,K+-ATPase was found to depend on experimental conditions. Unde
r optimal conditions (pH 7.4, 37 degrees C) the curve has an intermedi
ary plateau in the range of 0.7-0.9 mM ATP. Both acidification of the
medium and decrease in incubation temperature (below 20 degrees C) tra
nsform this curve into a simple hyperbole characteristic of GTP or UTP
hydrolysis, having no modifying effect on the enzyme. We recently sug
gested that the deviation from Michaelis-Menten kinetics during ATP hy
drolysis is explained by the formation of short-lived oligomeric compl
etes during the hydrolytic cycle of the enzyme. We interpret the data
in terms of the idea that the studied factors affect interprotomer int
eractions in the Na+,K+-ATPase oligomers.