Garrett C E, Coderre J A, Meek T D, Garvey E P, Claman D M, Beverley S M, Santi D V
Mol Biochem Parasitol. 1984 Apr;11:257-65. doi: 10.1016/0166-6851(84)90070-7.
Thymidylate synthetase and dihydrofolate reductase exist as a bifunctional protein in a number of species of protozoa which span diverse groups of the subkingdom. The enzymes copurify upon gel filtration and on affinity chromatography columns specific for dihydrofolate reductase. The bifunctional protein has been found in species of Crithidia, Leishmania, Trypanosoma, Plasmodium, Eimeria, Tetrahymena and Euglena. For reasons unknown, neither enzyme could be detected in Entamoeba histolytica or E. invadens. Since neither enzyme has yet been found as a separate protein in protozoa, it is likely that the bifunctional protein is widespread among these primitive eukaryotes. In most cases, the apparent size of the native protein is approximately twice that of the subunit possessing thymidylate synthetase. Further, with one exception, the subunit sizes are close to the sum of the subunit sizes of the separate enzymes found in other sources.
胸苷酸合成酶和二氢叶酸还原酶在许多原生动物物种中以双功能蛋白的形式存在，这些原生动物跨越了亚界的不同类群。在凝胶过滤以及针对二氢叶酸还原酶的亲和色谱柱上，这两种酶会共同纯化。已在克氏锥虫、利什曼原虫、锥虫、疟原虫、艾美球虫、四膜虫和眼虫等物种中发现了这种双功能蛋白。出于未知原因，在溶组织内阿米巴或侵袭内阿米巴中均未检测到这两种酶。由于在原生动物中尚未发现这两种酶以单独的蛋白形式存在，因此这种双功能蛋白很可能在这些原始真核生物中广泛存在。在大多数情况下，天然蛋白的表观大小约为具有胸苷酸合成酶的亚基大小的两倍。此外，除了一个例外，亚基大小接近在其他来源中发现的单独酶的亚基大小之和。
