Wang C C, Cheng H W
Mol Biochem Parasitol. 1984 Feb;10(2):171-84. doi: 10.1016/0166-6851(84)90005-7.
Trichomonas vaginalis is incapable of de novo pyrimidine biosynthesis because it cannot incorporate bicarbonate, aspartate or orotate into its pyrimidine nucleotides or nucleic acids. The organism can salvage exogenous cytidine greater than uridine greater than uracil and thymidine, and incorporate them into the nucleotide pool. A portion of cytidine is converted to CMP, CDP and CTP by cytidine phosphotransferase and nucleotide kinases. Some cytidine and most of uracil are, however, converted first to uridine by cytidine deaminase and uridine phosphorylase respectively; uridine is then incorporated into UMP, UDP and UTP by uridine phosphotransferase and nucleotide kinases. The two phosphotransferases, found mainly in the non-sedimentable fraction of T. vaginalis, provide the main avenue of pyrimidine salvage. No significant levels of pyrimidine phosphoribosyl transferase or nucleoside kinases can be detected in the extract. T. vaginalis has no appreciable dihydrofolate reductase or thymidylate synthetase; it grows normally in millimolar concentrations of methotrexate, pyrimethamine, or trimethoprim, and cannot incorporate labels from exogenous uracil or uridine into DNA. It has an enzyme thymidine phosphotransferase in the sedimentable fraction which converts thymidine to TMP. Thymidine salvage in T. vaginalis is thus totally isolated from the rest of the pyrimidine salvage.
阴道毛滴虫无法进行嘧啶的从头合成，因为它不能将碳酸氢盐、天冬氨酸或乳清酸掺入其嘧啶核苷酸或核酸中。该生物体能够回收外源性胞苷（大于尿苷大于尿嘧啶和胸苷），并将它们掺入核苷酸池中。一部分胞苷通过胞苷磷酸转移酶和核苷酸激酶转化为CMP、CDP和CTP。然而，一些胞苷和大部分尿嘧啶分别首先通过胞苷脱氨酶和尿苷磷酸化酶转化为尿苷；然后尿苷通过尿苷磷酸转移酶和核苷酸激酶掺入UMP、UDP和UTP中。这两种磷酸转移酶主要存在于阴道毛滴虫的非沉淀部分，是嘧啶补救的主要途径。在提取物中未检测到显著水平的嘧啶磷酸核糖转移酶或核苷激酶。阴道毛滴虫没有明显的二氢叶酸还原酶或胸苷酸合成酶；它在毫摩尔浓度的甲氨蝶呤、乙胺嘧啶或甲氧苄啶中正常生长，并且不能将外源性尿嘧啶或尿苷的标记掺入DNA中。它在可沉淀部分有一种胸苷磷酸转移酶，可将胸苷转化为TMP。因此，阴道毛滴虫中的胸苷补救与嘧啶补救的其他部分完全分离。
