Purine Metabolizing Enzymes of Lymphocyte Cell Populations: Correlation between AMP-Deaminase Activity and dATP Accumulation in Murine Lymphocytes

Deoxyadenosine and its nucleotides are thought to be involved in the pathogenesis of the immunodeficient state associated with inherited adenosine deaminase (ADA) deficiency (1–3) and in the lymphotoxicity that occurs during the treatment of leukemia with deoxycoformycin, an ADA inhibitor (1–3). Lymphotoxicity can be simulated in vitro by treating lymphoid cells with deoxyadenosine in the presence of an ADA inhibitor. Such a treatment of lymphoblasts results in intracellular accumulation of deoxy-ATP (dATP), inhibition of DNA synthesis, ATP depletion, leading to cell toxicity; different lymphoid cells display very different sensitivity to this deoxyadenosine cytotoxicity (4, 5).

The mechanism of dATP-induced cytotoxicity is still poorly understood; one of the first hypotheses was the blockade by dATP of ribonucleotide reductase which would result in an inhibition of DNA synthesis due to the shortage of deoxynucleotide precursors (4–6). More recently it was claimed that a dATP-induced inhibition of RNA synthesis could provide an alternative explanation for deoxyadenosine toxicity in lymphoid cells (7). Both hypotheses appear correlated to the cell capacity to accumulate dATP; nevertheless, as suggested by Sylwestrowicz (8), this cytotoxicity depends not only on intracellular dATP synthesis but also on the capacity of the cell to degrade accumulated dATP.

Both AMP-deaminase (AMP-DA), which deaminates AMP and dAMP, and cytosolic 5′-nucleotidase (c5′N), which hydrolyzes intracellular mononucleotides, are involved in nucleotide catabolism. Contrary to other enzymes of nucleotide metabolism, these enzymes have scarcely been studied in lymphoid cells. It was only reported that T lymphoblasts display AMP-DA activities 5- to 10-fold lower than those of B lymphoblasts and that different AMP-DA isoenzymes could be expressed in these lymphoblasts (9). It was also shown that AMP-DA activity is greatly altered during the in vitro differentiation of muscle cells (10).