title: Biosynthesis of vitamin B12. Transformation of riboflavin 2H-labeled in the 1'R position of 1'S position into 5,6-dimethylbenzimidazole.
abstract: The 5,6-dimethylbenzimidazole moiety of vitamin B12 is formed from riboflavin in aerobic and some aerotolerant bacteria. Thereby C1' of riboflavin is transformed into C2 of the vitamin B12 base. In the present publication a study on this transformation with riboflavin 2H-labeled in the 1'R or 1'S position is described. This study was undertaken in order to find out if one of the two hydrogens at C1' is transferred to C2 of 5,6-dimethylbenzimidazole. The 2H-labeled riboflavin samples were synthesized starting from unlabeled or 1-2H-labeled ribose and 3,4-dimethylaniline yielding N-beta-D-ribopyranosyl-3,4-dimethylaniline. The unlabeled riboside was reduced to N-D-ribityl-3,4-dimethylaniline with sodium cyanoborotrideuteride, the 2H-labeled riboside with sodium cyanoborohydride. The ribityl derivatives were transformed into N-D-ribityl-2-phenylazo-4,5-dimethylaniline, and condensed with barbituric acid to riboflavin. The reduction of the ribosyl compound to the ribityl derivative is only partially stereospecific. Thus the riboflavin synthesized from unlabeled ribose had a 2H ratio of 3/1 (1'R/1'S), the riboflavin obtained from D-[1-2H1]ribose of 1/3 (1'R/1'S). The 2H content in these positions was determined from the 1H-NMR spectra. These spectra showed also that 1 mol 2H/mol riboflavin was present in position 1'. The deuterated riboflavin samples were incubated under aerobic conditions with broken cell preparations of Propionibacterium shermanii. The deuterium content of the 5,6-dimethylbenzimidazole isolated was determined by mass spectrometry and by 1H NMR. These measurements revealed that the hydrogen in the pro-S position at C1' of riboflavin is retained during 5,6-dimethylbenzimidazole formation, and is thus found at C2 of this base.
authors.affiliation: Institut für Biologische Chemie und Ernährungswissenschaft, Universität Hohenheim, Federal Republic of Germany.
authors.forename: B
authors.initials: B
authors.lastname: Lingens
authors.forename: T A
authors.initials: TA
authors.lastname: Schild
authors.forename: B
authors.initials: B
authors.lastname: Vogler
authors.forename: P
authors.initials: P
authors.lastname: Renz
mesh_terms: D001562:Benzimidazoles / Q000737:chemistry*; D005486:Flavin Mononucleotide / Q000737:chemistry; D006859:Hydrogen; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D011424:Propionibacterium / Q000378:metabolism; D012256:Riboflavin / Q000737:chemistry*; D013237:Stereoisomerism; D014805:Vitamin B 12 / Q000096:biosynthesis* / Q000737:chemistry
chemical_list: D001562:Benzimidazoles; C015158:5,6-dimethylbenzimidazole; D005486:Flavin Mononucleotide; D006859:Hydrogen; D014805:Vitamin B 12; D012256:Riboflavin