On the Mechanism of the Effect of Ionic Strength on Crystalline Aldolase Activity.

During the course of studies made with rat liver enzymes, it was found that the activity of aldolase was lower when assayed in the presence of phosphate buffer than with tris(hydroxymethyl)aminomethane buffer in the incubation system (1). A similar effect had been observed with crystalline aldolase from rabbit muscle and was attributed to an enhancement of the enzyme activity (2, 3). However, since aldolase requires the presence of at least one phosphate group in its substrate (4, 5), an alternative possibility might be that the phosphate molecule acted as inhibitor of the enzyme activity. On the other hand, there is conflicting information concerning the optimal pH in enzyme assays in which Tris buffer is included (2, 6, 7). The aut,hors involved in the controversy did not take into account the variations in ionic strength introduced by the adjustments to a given pH. This paper provides further information on the specific roles of phosphate and Tris in regard to aldolase activity; it also shows an inhibitory effect of ionic strength on the rate of catalysis, which is suggested to be due to a distortion of the tertiary structure of the protein.