Hypersensitivity of an enzyme reaction to solvent water

The hydrolytic activity of calf intestinal adenosine deaminase is reduced sharply, but reversibly, in the presence of added methanol, ethanol, acetonitrile, or dioxane. This decrease in kcat/Km appears to be related to diminished water content in the presence of each of these cosolvents. No agreement between cosolvents is observed if enzyme activity is plotted as a function of viscosity or dielectric constant; nor do these cosolvents act as conventional reversible inhibitors. The Km value of adenosine and the Ki values of a substrate analogue (6-dimethylaminopurine ribonucleoside) and a powerful competitive inhibitor (6-hydroxy-1,6-dihydropurine ribonucleoside) increase with decreasing solvent water content, but kcat is unaffected. Values of 1/Km and 1/Ki increase with roughly the 9th power of the concentration of water and show no sign of approaching a maximum value as the concentration of water approaches 55 M. These results are consistent with an equilibrium between an abundant, inactive, relatively dehydrated form of the enzyme and a rare, relatively hydrated form of the enzyme. Only the hydrated form of the enzyme, containing at least nine more water molecules than the dehydrated form, appears to be capable of binding substrates or competitive inhibitors. Possible physiological consequences of this behavior, in a tissue in which water is transported in large quantities, are considered.