KINETIC PROPERTIES OF ADENOSINE DEAMINASE IN MIXED AQUEOUS SOLVENTS

Hoagland and Fisher showed that chicken duodenal adenosine deaminase exhibits a rate equation over a very wide range of substrate and product concentrations in a neutral aqueous environment (see eq 1 of text). Since this reaction is irreversible and inosine and adenosine are the only substrate or product variables that can be used, it was difficult to get more information about the mechanism using standard steady-state kinetic techniques. Consequently a detailed study was made of this system in a variety of solvents at many concentrations, temperatures, and pH values. The same rate equation held under all of these conditions even though reaction rates were changed several orders of magnitude. Since in all probability such a detailed study would have allowed other terms in the rate equation to appear if they were present, we have concluded that there are no other terms and have proposed a mechanism based upon this assumption. Studies in a variety of solvents have allowed us to conclude that water is not a limiting substrate in this system and we have assumed that it is saturating. An ordered-sequence mechanism with a ternary complex is suggested which satisfies all the known properties of this system. The kinetic parameters (α, β, and γ) were drastically changed by the various solvents, e.g., a decreased 1000-fold in the presence of 70% v/v ethylene glycol without irreversible inactivation of the enzyme. Specific properties of the solvent seem to induce these enormous changes without affecting the conformation of the enzyme. Log β is a linear function of the reciprocal of dielectric constant and log α appears to be a linear function of internal pressure. Log α could not be correlated with any bulk solvent property examined. © 1969, American Chemical Society. All rights reserved.