HYPERSENSITIVITY OF AN ENZYME REACTION TO SOLVENT WATER

The hydrolytic activity of calf intestinal adenosine deaminase is redu ced sharply, but reversibly, in the presence of added methanol, ethano l, acetonitrile, or dioxane. This decrease in k(cat)/K(m) appears to b e related to diminished water content in the presence of each of these cosolvents. No agreement between cosolvents is observed if enzyme act ivity is plotted as a function of viscosity or dielectric constant; no r do these cosolvents act as conventional reversible inhibitors. The K (m) value of adenosine and the K(i) values of a substrate analogue (6- dimethylaminopurine ribonucleoside) and a powerful competitive inhibit or (6-hydroxy-1,6-dihydropurine ribonucleoside) increase with decreasi ng solvent water content, but k(cat) is unaffected. Values of 1/K(m) a nd 1/K(i) increase with roughly the 9th power of the concentration of water and show no sign of approaching a maximum value as the concentra tion of water approaches 55 M. These results are consistent with an eq uilibrium between an abundant, inactive, relatively dehydrated form of the enzyme and a rare, relatively hydrated form of the enzyme. Only t he hydrated form of the enzyme, containing at least nine more water mo lecules than the dehydrated form, appears to be capable of binding sub strates or competitive inhibitors. Possible physiological consequences of this behavior, in a tissue in which water is transported in large quantities, are considered.