Control of adenosine deaminase levels in human lymphoblasts

Abstract

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 i 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 lymphoblasts extracts, the enzyme is apparently identical based on K m for adenosine and deoxyadenosine, K i for inosine, V max for adenosine, S 20w, 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 studies. (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 (t 1 2 ) for the enzyme degradation is 19 and 39 hr, respectively, for CCRF-CEM and RPMI-8402, while the t 1 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) is 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 imunosupressive 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.