Search

We studied the ability of purine compounds to restore the proliferation of concanavalin-A-stimulated rat T-lymphocytes under conditions of purine de novo synthesis inhibition and, on the other hand, the inhibition by purine nucleosides of the response of these cells to a mitogenic stimulation under conditions of normal purine de novo synthesis. The use of 50 microM azaserine, a potent inhibitor of purine de novo synthesis, allowed us to define the physiologically active salvage pathways of purine bases, ribo- and deoxyribonucleosides in concanavalin-A-stimulated rat T-lymphocytes. Except for guanylic compounds, all purines completely restored cell proliferation at a concentration of 50 microM. Guanine, guanosine and 2'-deoxyguanosine at concentrations up to 500 microM did not allow us to restore more than 50% of the cell proliferation. In conditions of normal purine de novo synthesis, the addition of 1000 microM adenine, adenosine, 2'-deoxyadenosine or 100 microM 2'-deoxyguanosine inhibited rat T-lymphocyte proliferation. The differences between the degree of inhibition of cell proliferation could be explained only in part by the differences between the capacities of salvage of these compounds. Furthermore, the fact that 2'-deoxyguanosine toxicity was dependent and 2'-deoxyadenosine toxicity independent on the activation state of the cells provided more evidence that the biochemical mechanisms of inhibition of cell proliferation should be different for these two nucleosides.

5'-Deoxy-5'-methylthioadenosine, a by-product of polyamine metabolism, is a potent inhibitor of cell proliferation. MTA phosphorylase cleaves MTA into adenine and 5'-methylthioribose-1-P. We studied MTA inhibition and salvage into purine compounds and methionine in concanavalin A-stimulated rat T lymphocytes and in Raji cells. When de novo purine synthesis was inhibited by azaserine (20 microM), low concentrations of MTA, (less than or equal to 20 microM), were able to completely restore cell proliferation in both types of cells. When cells were cultured in a methionine-free medium, MTA (15 microM) completely fulfilled the methionine requirement of Raji cells but only 50% of that of rat T lymphocytes. MTA displayed a dose-dependent inhibition of the proliferation of both types of cells, but in the case of MTA salvage into purines or methionine, the curves were shifted to higher MTA concentrations. In vitro studies by Backlund et al. (Backlund, P.S., Chang, C.P. and Smith, R.A. (1982) J. Biol. Chem. 257, 4196-4202) on rat liver homogenates, suggested that the last step of MTA salvage into methionine may be the transamination of 2-keto-4-methylthiobutyrate to methionine. We present evidence that this is a step physiologically efficient in intact cells.