Purification and some properties of adenosine deaminase from human thymus

Two fractions of adenosine deaminase (ADA) were separated by ion-exchange chromatography and purified to homogeneity from human thymus tissue by a combination of conventional biochemical methods and affinity chromatography. Some of the physical, chemical and serological properties of the two fractions were compared to those of erythrocyte ADA. All three proteins had apparent molecular weights of about 45,000. They exhibited similar amino acid composition, specific enzymatic activities, Km values for adenosine and antigenic activities as determined by radioimmunoassay. A small portion of ADA isolated from thymus did not bind to complexing protein whereas all of the erythrocyte ADA was bound by this protein. So far, this has been the only difference found between thymic and erythrocyte ADA.