Irby R B, Adair W L
Department of Biochemistry and Molecular Biology, University of South Florida College of Medicine, Tampa 33612.
J Biol Chem. 1994 Sep 30;269(39):23981-7.
Biosynthesis of molybdopterin was followed in the yeast, Pichia canadensis, using labeled precursors. High performance liquid chromatography analysis of extracts from cells labeled with [U-14C]guanosine showed that the label was incorporated into the molybdopterin oxidation product, dephospho Form A. Dephospho Form A isolated from cells labeled with [U-14C,5'-3H]guanosine was devoid of tritium, indicating partial loss of the ribose moiety of guanosine during the synthesis of molybdopterin. In vivo labeling of P. canadensis using [7-14C]neopterin and [6,7,1-14C]hydroxymethylpterin led to label from both compounds appearing in dephospho Form A as well as in folic acid in wild type cells. When these labeled precursors were incubated with P. canadensis mutants blocked in molybdopterin synthesis, only folic acid was labeled. These results suggest a shared pathway in the biosyntheses of molybdopterin and folic acid. [6-14C]Glucose labeling experiments led to exclusive incorporation into the 4'-position of dephospho Form A but not in folic acid. It is proposed that molybdopterin synthesis branches from the folic acid biosynthetic pathway at dihydrohydroxymethylpterin and that a 3-carbon phosphorylated compound such as glyceraldehyde 3-phosphate may condense with dihydrohydroxymethylpterin to form the 4-carbon side chain precursor to molybdopterin.
利用标记前体对加拿大毕赤酵母中的钼蝶呤生物合成进行了追踪研究。对用[U-¹⁴C]鸟苷标记的细胞提取物进行高效液相色谱分析，结果表明该标记被掺入到钼蝶呤氧化产物脱磷酸形式A中。从用[U-¹⁴C,5'-³H]鸟苷标记的细胞中分离得到的脱磷酸形式A不含氚，这表明在钼蝶呤合成过程中鸟苷的核糖部分有部分损失。利用[7-¹⁴C]新蝶呤和[6,7,1-¹⁴C]羟甲基蝶呤对加拿大毕赤酵母进行体内标记，结果显示在野生型细胞中，这两种化合物的标记都出现在脱磷酸形式A以及叶酸中。当这些标记前体与钼蝶呤合成受阻的加拿大毕赤酵母突变体一起孵育时，只有叶酸被标记。这些结果表明钼蝶呤和叶酸的生物合成存在共同途径。[6-¹⁴C]葡萄糖标记实验导致标记只掺入到脱磷酸形式A的4'-位，而未掺入叶酸中。有人提出，钼蝶呤合成在二氢羟甲基蝶呤处从叶酸生物合成途径分支出来，并且一种三碳磷酸化化合物，如3-磷酸甘油醛，可能与二氢羟甲基蝶呤缩合形成钼蝶呤的四碳侧链前体。
