Concanavalin A-induced proliferation of rat T-lymphocytes is completely inhibited by 10−5 M pyrazofurin, a potent inhibitor of pyrimidine de novo synthesis, as judged by cell viability and [3H]thymidine incorporation. Proliferation is completely restored by 5 × 10−5 M uridine. Cytidine, deoxycytidine, deoxyuridine and thymidine 10 × 10−5 M each, fail to re-establish proliferation but produce an isotropic dilution of [3H]thymidine uptake in DNA. Bases (cytosine, uracil and thymine) neither restore proliferation nor induce isotopic dilution. The unexpected inability of cytidine to reverse de novo pyrimidine synthesis inhibition suggests a lack of cytidine deaminase activity in rat T-lymphocytes. This is confirmed by a direct sensitive radioisotopic assay (<0.001 nmol.min−1.10−6 cells).
