Human tonsillar lymphocytes as targets for immunosuppressive and anticancer drugs

As has been shown earlier by us, the metabolism of extracellular deoxycytidine (dCyd) is 2-3 times higher in follicular and in PNA+ cells than in other cells. Deoxycytidine kinase (dCK) is one of the most important target enzymes for anti-proliferative drugs such as arabinosile-cytosine (ara-C), 2-Cl-deoxyadenosine (CdA). Neither the dCK activity nor the polypeptide correlates with the S phase of the cells, as thymidine kinase (TK1) does in tonsils. The newly developed anti-leukemic drug CdA, and also BrdA, are also phosphorylated by dCK and both effectively inhibit the 3H-dThd incorporation into DNA in tonsillar lymphocytes. A new molecular mechanism has been developed for CdA; it inhibits the interconversion of dCyd into dThd nucleotides. Analysis of the pools after 3H-dCyd labeling showed a decrease of the dUMP labeling. The inhibition of dCMP deaminase by the corresponding monophosphates (Cl-dAMP) in the cells has been suggested. CdA cannot be deaminated by adenosine deaminase (ADA), thus providing a good tool to investigate the importance of that enzyme during differentiation of the lymphoid cells. Elucidation of the nucleoside metabolism during the normal differentiation process might be the only way to get information about the same pathways in malignant transformations, i.e., in leukemias.