Schulze B, Vogler B, Renz P
Institut für Biologische Chemie und Ernährungswissenschaft der Universität Hohenheim, Stuttgart, Germany.
Eur J Biochem. 1998 Jun 15;254(3):620-5. doi: 10.1046/j.1432-1327.1998.2540620.x.
In anaerobic bacteria 5-hydroxybenzimidazole and 5-hydroxy-6-methylbenzimidazole are precursors of the 5,6-dimethylbenzimidazole moiety of vitamin B12. In order to elucidate the pathway from these bases to vitamin B12, experiments on the transformation of 5-hydroxy-6-methylbenzimidazole, of 5-hydroxy-6-methylbenzimidazole-alpha-D-ribofuranoside, of 5-hydroxybenzimidazolylcobamide and of 5-hydroxy-6-methylbenzimidazolylcobamide into vitamin B12 were carried out. The vitamin B12 synthesized by the anaerobe Eubacterium limosum in the presence of 5-hydroxy-6-methylbenzimidazole and L-[methyl-13C]methionine was subjected to NMR spectroscopy. It revealed that the methyl group at C5 of the 5,6-dimethylbenzimidazole moiety was 13C labeled, whereas the methyl group at C6 was unlabeled. This shows that the transformation of 5-hydroxy-6-methylbenzimidazole into the base moiety of vitamin B12 occurs regiospecifically. 5-Hydroxy-6-methylbenzimidazole-alpha-D-ribofuranoside as well as 5-hydroxybenzimidazolylcobamide and 5-hydroxy-6-methylbenzimidazolylcobamide were also transformed into vitamin B12 by E. limosum. When 5-hydroxy-6-methylbenzimidazolylcobamide 13C labeled at C2 of the base part and 14C labeled in the ribose was used for this experiment, the vitamin B12 obtained from this cobamide was 13C and 14C labeled in the same positions. This demonstrates that the alpha-glycosidic bond of the precursor cobamide is not split during the formation of vitamin B12. It can be deduced from these results that the precursor bases are transformed regiospecifically into their alpha-nucleotides, and partially into their cobamides. The alpha-nucleotides are then transformed into alpha-ribazole-5'-phosphate and, subsequently, into vitamin B12. Most likely the cobamides are degraded to the alpha-nucleotides before being used for the biosynthesis of vitamin B12. A pathway for the latter process is suggested.
在厌氧细菌中，5-羟基苯并咪唑和5-羟基-6-甲基苯并咪唑是维生素B12的5,6-二甲基苯并咪唑部分的前体。为了阐明从这些碱基到维生素B12的途径，进行了关于5-羟基-6-甲基苯并咪唑、5-羟基-6-甲基苯并咪唑-α-D-呋喃核糖苷、5-羟基苯并咪唑钴胺素和5-羟基-6-甲基苯并咪唑钴胺素转化为维生素B12的实验。由厌氧的黏质真杆菌在5-羟基-6-甲基苯并咪唑和L-[甲基-13C]甲硫氨酸存在下合成的维生素B12进行了核磁共振光谱分析。结果表明，5,6-二甲基苯并咪唑部分C5位的甲基被13C标记，而C6位的甲基未被标记。这表明5-羟基-6-甲基苯并咪唑向维生素B12的碱基部分的转化具有区域特异性。5-羟基-6-甲基苯并咪唑-α-D-呋喃核糖苷以及5-羟基苯并咪唑钴胺素和5-羟基-6-甲基苯并咪唑钴胺素也被黏质真杆菌转化为维生素B12。当使用在碱基部分的C2位标记13C且在核糖中标记14C的5-羟基-6-甲基苯并咪唑钴胺素进行该实验时，从该钴胺素获得的维生素B12在相同位置被13C和14C标记。这表明在前体钴胺素形成维生素B12的过程中，α-糖苷键未断裂。从这些结果可以推断，前体碱基被区域特异性地转化为它们的α-核苷酸，并部分转化为它们的钴胺素。然后α-核苷酸被转化为α-核酮糖-5'-磷酸，随后转化为维生素B12。很可能钴胺素在用于维生素B12的生物合成之前被降解为α-核苷酸。提出了后一过程的途径。
