Identification of Human Thymus-Leukemia-Associated Antigen as a Low-Molecular-Weight Form of Adenosine Deaminase2

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Abstract
In the determination of whether human thymus-leukemia-associated antigen (HThy-L) is a low-molecular-weight form of adenosine deaminase (ADA), both HThy-L and ADA were found to have the same molecular weight of 45,000 as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The antigen and enzyme displayed a phenomenon of complete Identity in immunodiffusion and a high degree of cross-reaction in a competitive radioimmunoassay for HThy-L or ADA. When tested for adenosine-deaminating activity, HThy-L was nearly as active as purified low-molecular-weight ADA from erythrocytes. However, HThy-L and ADA differed in their capacity to combine with the complexing protein isolated from human kidney. Apparently, HThy-L represents a thymic isoenzyme of ADA and is the first antigen to be associated with differentiation of hematopoietic cells for which a functional activity is established.