The anaerobic parasitic protozoa Tritrichomonas foetus is found incapable of de novo pyrimidine biosynthesis by its failure to incorporate bicarbonate, aspartate, or orotate into pyrimidine nucleotides or nucleic acids. Uracil phosphoribosyltransferase in the cytoplasm provides the major pyrimidine salvage for the parasite. Exogenous uridine and cytidine are mostly converted to uracil by uridine phosphorylase and cytidine deaminase in T.foetu prior to incorporation. T. foetus cannot incorporate labels from exogenous uracil or uridine into DNA; it has no detectable dihydrofolate reductase or thymidylate synthetase and is resistant to methotrexate, pyrimethamine, trimethoprim, and 5-bromovinyldeoxyuridine at millimolar concentrations. It has an enzyme thymidine phosphotransferase in cellular fraction pelleting at 100,000 x g that can convert exogenous thymidine to TMP via a phosphate donor such as p-nitrophenyl phosphate or nucleoside 5'-monophosphate. Thymidine salvage in T. foetus is thus totally dissociated from other pyrimidine salvage.It has become apparent in recent years that parasitic protozoa are generally incapable of de novo synthesis of purine nucleotides. Trypanosoma cruzi (1), Leishmania donovani (2), Plasmodium lophurae (3), Eimeria tenella (4), and Trichomonas vaginalis (5), to name but a few examples, depend on specific networks of salvage pathways to fulfill their purine requirements. Because of this deficiency in metabolic activities, rational approaches to controlling some of the parasites have been possible. Allopurinol exhibits antitrypanosomal and antileishmanial activities because it is recognized by the parasite salvage enzymes as a hypoxanthine analog (6, 7). Allopurinol riboside (8), formycin B (9), and 4-thiopyrazolopyrimidine riboside (10) have antileishmanial activities because of the nucleoside phosphotransferase in leishmania, which incorporates the compounds into parasite's nucleotide pool.De novo pyrimidine biosynthesis, on the other hand, takes place in most of the parasitic protozoa. Recently, it has been reported that the anaerobic flagellates Trichomonas vaginalis and Giardia lamblia may not, however, have even the capability of pyrimidine de novo synthesis. The former lacks aspartate transcarbamoylase, dihydroorotase, dihydroorotate dehydrogenase, and orotate phosphoribosyltransferase in its crude extract (11), whereas the latter indicates no incorporation of aspartate into the cold trichloroacetic acid-insoluble fraction (12). These results suggest that anaerobic flagellates may differ from other protozoan parasites in lacking both purine and pyrimidine de novo synthetic abilities and thus may offer even more opportunities for chemotherapeutic attack.To verify these possibilities, we studied pyrimidine metabolism in Tritrichomonasfoetus, a Precursor Incorporation into the Nucleotide Pool. T. foetus cells were washed, suspended in phosphate-buffered saline, pH 7.2 (Pi/NaCl)/20 mM glucose to a final cell density of 108/ml, and incubated at 370C. A radiolabeled subst...
