Nonenzymatic Transformation of Riboflavin into 5, 6-Dimethylbenzimidazole

Abstract

Riboflavin, the biosynthetic precursor of the 5, 6-dimethylbenzimidazole moiety of vitamin B12, is transformed non-enzymatically into 5, 6-dimethylbenzimidazole in small yield on treatment with 1 N or 5 N NaOH at 100 °C. Besides 5, 6-dimethylbenzimidazole 1, 2-diamino-4, 5-dimethylbenzene, 1, 2-dihydro-6, 7-dimethyl-2-keto-1-D-ribityl-3-quinoxaline carboxylic acid and N-1-D-ribitylamino-2-amino-4, 5-dimethylbenzene can be detected. When [1'-14C] riboflavin is used the 5, 6-dimethylbenzimidazole contains about 75 per cent of the specific radioactivity of riboflavin. N-1-D-ribityl-amino-2-amino-4, 5-dimethylbenzene is transformed into 5, 6-dimethylbenzimidazole more efficiently than riboflavin. Oxygen enhances the yield of 5, 6-dimethylbenzimidazole and 1, 2-diamino-4, 5-dimethylbenzene from riboflavin as well as from N-1-D-ribitylamino-2-amino-4, 5-dimethylbenzene. 1, 2-diamino-4, 5-dimethylbenzene reacts together with formaldehyde but not with formate to form 5, 6-dimethylbenzimidazole under alkaline conditions at 100 °C. It is therefore suggested that the nonenzymatic reaction of riboflavin proceeds via N-1-D-ribityl-amino-2-amino-4, 5-dimethylbenzene and 1, 2-diamino-4, 5-dimethylbenzene, and that the latter reacts with formaldehyde preferably formed by oxidative degradation of C-1' of the ribityl side chain to form 5, 6-dimethylbenzimidazole via its unstable imidazoline derivative. The possible relevance of these results for the enzymatic process is discussed. © 1977, Verlag der Zeitschrift für Naturforschung. All rights reserved.