The hydrolysis of adenosine, 2-deoxyadenosine and 6-chloropurine riboside by adenosine deaminase preparations from human spleen has been investigated, and Km and Vm values have been determined. The effect of the substrate on the reaction velocity was followed over a 250-fold range Results showed no deviation from Michaelis-Menten kinetics. The effect of pH on Vm, Vm/Km, and the apparent activation energy was examined. Inactivation of the enzyme by p-chloromercuribenzoate suggests the involvement of one or more SH groups. Competitive inhibition by inosine and the fact that ammonia is not, an inhibitor support a reaction mechanism involving a ternary complex. The apparent activation energy of the a parameter was smaller or less sensitive to temperature than the ß parameter.Ball State UniversityMuncie, IN 47306