Control of adenosine deaminase levels in human lymphoblasts

High levels of adenosine deaminase (ADA) activity have been associated with normal T cell differentiation and T cell disease, such as acute lymphoblastic leukemia; however, possible mechanisms controlling the level of this enzyme have not been explored. In this study, the properties and rate of turnover of ADA are compared in cultured human T and B lymphoblast cell lines. (1) Relative to B lymphoblasts, the level of ADA activity in extracts of T lymphoblast cell lines (MOLT-4, RPMI-8402, CCRF-CEM and CCRF-HSB-2) is elevated 7- to 14-fold and differs by 2-fold among the T-cell lines. (2) In T and B lymphoblast extracts, the enzyme is apparently identical based on Km for adenosine and deoxyadenosine, Ki for inosine, Vmax for adenosine, S20w, isoelectric pH, and heat stability. Further, by radioimmunoassay the quantity of ADA immunoreactive protein is proportional to the level of enzyme activity in all cell lines studied. (3) Using a purification and selective immunoprecipitation technique, the enzyme turnover could be assessed in cell lines labeled with [35S]methionine. The apparent rate of ADA synthesis, relative to total protein, is 2-fold faster in both T cell lines (RPMI-8402 and CCRF-CEM) than in the B cell lines (MGL-8 and GM-130). The apparent half-life (t1/2) for the enzyme degradation is 19 and 39 hr, respectively, for CCRF-CEM and RPMI-8402, while the t1/2 for both B cell lines is 7-9 hr. From the net rate of synthesis and degradation, the T cell lines exhibit a 6- and 12-fold difference in ADA turnover relative to B cells, consistent with the observed differences in enzyme activity. (4) The level of ADA (activity and/or protein) in cultured T or B lymphoblasts is not influenced by either substrates or products of the ADA reaction or an ADA inhibitor or a selected group of immunosuppressive drugs added to these cells in culture. These studies indicate that while ADA is apparently identical in all T and B lymphoblasts, alterations in both the rate of ADA synthesis and degradation lead to its accumulation and high steady-state level in T cells.