Conversion of human erythrocyte-adenosine deaminase activity to different tissue-specific isozymes. Evidence for a common catalytic unit.

Adenosine deaminase activity resides in various characteristic isozymes in red blood cells (RBC-ADA) and other tissues. Absence of RBC-ADA has been reported in a proportion of patients with autosomally inherited severe combined immunodeficiency (SCID). We have previously reported that the tissue isozymes of ADA are also deficient in children with SCID and RBC-ADA deficiency, although these isozymes differ from RBC-ADA in molecular weight, accessible SH groups, and electrophoretic mobility. The deficiency of all types of ADA in SCID implies that a catalytic unit of ADA in each isozyme is coded by the same structural gene. The relationship of RBC-ADA and the different tissue ADA isozymes is the subject of this paper. Incubation of RBC-ADA with ADA-deficient liver, kidney, and fibroblast extracts resulted in the appearance of new isozymes of ADA. These newly generated isozymes had the physicochemical and electrophoretic characteristics of the tissue-specific isozymes obtained from normal tissues. The electrophoretic mobility of the isozyme generated appeared to depend upon the tissue utilized and corresponded to the electrophoretic mobilities of the ADA isozymes found naturally in each of the different tissues. Additionally, the genetically determined polymorphism exhibited by RBC-ADA could be detected in the isozyme generated. Incubation with normal kidney also caused conversion of the RBC isozyme to the kidney form. These findings further support the concept that the catalytic activity of each of the several forms of the ADA enzyme resides in a single molecule coded at the same genetic locus as is defective in one form of SCID. The tissue-specific isozymes, which differ in electrophoretic mobility and molecular weight, are generated by interaction of the RBC catalytic unit with tissue-specific factors present in the different tissues of normal humans and patients.