Riboflavin dihydro

One-electron oxidation of the adenine moiety of DNA and 2 -deoxyadenos-ine (dAdo) (45) gives rise to related purine radical cations 46 that may undergo either hydration to generate 8-hydroxy-7,8-dihydroadenyl radicals (47) or deprotonation to give rise to the 6-aminyl radicals 50. The formation of 8-oxo-7,8-dihydro-2 -deoxyadenosine (8-oxodAdo) (48) and 4,6-diamino-5-formamidopyrimidine (FapyAde) (49) is likely explained in terms of oxidation and reduction of 8-hydroxy-7,8-dihydroadenyl precursor radicals 47, respectively [90]. Another modified nucleoside that was found to be generated upon type I mediated one-electron oxidation of 45 by photoexcited riboflavin and menadione is 2 -deoxyinosine (51) [29]. The latter nucleoside is likely to arise from deamination of 6-aminyl radicals (50). Overall, the yield of formation of 8-oxodAdo 48 and FapyAde 49 upon one-electron oxidation of DNA is about 10-fold-lower than that of 8-oxodGuo 44 and FapyGua 43, similar to OH radical mediated reactions [91]. 

Riboflavin undergoes a reversible and overall two-electron reduction process, in two overlapping one-electron steps. Both the final product and the one-electron reduction intermediate show acid base equlibria in the pH range 6-7. Thus a number of species take part in the redox process. Experimental investigation aimed at deriving the related equlibrium constants involves generation of the dihydro-... 

Figure 15-8 Absorption spectrum of neutral, uncharged riboflavin (A), the riboflavin anion (B), and reduced to the dihydro form (Fig. 15-7) by the action of light in the presence of EDTA (C). A solution of 1.1 x 1CF4 M riboflavin containing 0.01 M EDTA was placed 11.5 cm from a 40-W incandescent lamp for 30 min.

The attention of biochemists was first attracted to flavins as a result of their color and fluorescence. The study of spectral properties of flavins (Fig. 15-8) has been of importance in understanding these coenzymes. The biochemical role of the flavin coenzymes was first recognized through studies of the "old yellow enzyme"144 145 which was shown by Theorell to contain riboflavin 5 -phosphate. By 1938, FAD was recognized as the coenzyme of a different yellow protein, D-amino acid oxidase of kidney tissue. Like the pyridine nucleotides, the new flavin coenzymes were reduced by dithionite to nearly colorless dihydro forms (Figs. 15-7 and 15-8) revealing the chemical basis for their function as hydrogen carriers. 

The central ring of 1-deazaflavins remains a pyrazine in X, a di-hydropyrazine in the two-electron-reduced form, XI, and continues to dominate the chemistry with oxygen. Like the parent riboflavins, and unlike the 5-deazaflavins, the dihydro- 1-deaza system, XI, is reoxidized by 02 in a fraction of a second in air-saturated solutions (Table II) the semiquinone is accessible and 1-deazaFAD enzymes show full catalytic competence with flavoprotein dehydrogenases and oxidases (24). Turnover numbers vary from about 1% to 100% that of cognate FAD-enzymes but this variation reflects the -280 mV vs. —200 mV E° values, respectively, for 1-deazariboflavin vs. riboflavin. The redox steps may or may not limit Vmax with a given enzyme (15, 24). 

A cofartor can be any chemical required by an enzyme, that is, a metal ion, coenzyme, lipid, or accessory pmtein. A coenzyme is a small organic molecule required by an enzyme that participates in the chemistry of catalysis. Most of the coenzymes work according to the following principle. They shuttle back and forth between two or more different forms. I lere, one of the forms may be considered to be the coenzymatically active form, and the other may be seen as requiring regeneration to the active form. This is the case for folate, vitamin B, riboflavin-and niacin-based cofactors, ascorbic acid, and vitamin K. The coenzymatically active (inactive) forms of three of these coenzymes are tetrahydrofolate (dihydro-folate), ascorbic acid (dehydroascorbic acid), and vitamin KH2 (vitamin K). 

Riboflavin is incorporated into another complex co-enzyme, flavin adenine dinucleotide (FAD). This is involved in enzyme-catalysed reductions of carbon-carbon double bonds, and the reverse. By accepting two hydrogens, the co-enzyme is converted into a dihydro derivative (FADH2), the driving force for this being the relief of the unfavoured interaction between the polarised, opposed C=N bonds. 

Kasai, H., Yamaizumi, Z., Berger, M., and Cadet, J. (1992) Photosensitized formation of 7,8-dihydro-8-oxo-2 -deoxyguanosine (8-hydroxy-2 -deoxyguanosine) in DNA by riboflavin a non singlet oxygen mediated reaction. J. Am. Chem. Soc., 114, 9692-9694. 

Riboflavin [ 1 -deoxy-1 -(3,4-dihydro-7,8-dimethyl-2,4-dioxobenzo[g]pteridine-10(2/ -yl)-D-ribitol, lac-toflavin, vitamin B2]. 

The chemical names of riboflavin (RF) are 3,10-dihydro-7,8-dimethyl-10-[(2S,3S,4R) 2,3,4,5-tetrahydroxypentyl]benzo[g]pteridine-2,4-dione or 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine. RF or lactoflavin is known as vitamin B2. The chemical formula is Ci7H2oN40 and its CAS number is 83-88-5. 

III) and urea (II), the quinoxalinecarboxylic acid (III) is decarboxylated to 1-ribityl-l, 2-dihydro-2-keto-6,7-dimethylquinoxaline (IV), which in turn is oxidized to l-ribityl-2,3-diketo-l, 2,3,4-tetrahydro-6,7-dimethylquinoxaline (V). The first two of these proposed reactions would correspond to the chemical degradation of riboflavin or lumiflavin by dilute alkali and heat (cf. Fig. 1, A, B, C). Preliminary experiments with the appropriate quinoxalinecarboxylic acid and its lactam did not lead to their conversion to l-ribityl-2,3-diketo-l,2,3,4-tetrahydro-6,7-dimethylquinoxaline, although the chemical oxidation by hydrogen peroxide of 1-ribityl-l,2-dihydro-2-keto-6,7-dimethylquinoxaline-3-carboxylic acid has been indicated to occur. As a result, the possibility has been considered that products other than those pictured resulting in compounds (III) and... 

Haley and Lambooy 120) exposed riboflavin-2-C to L. casei for 40 days and found that 16% of the flavin was destroyed only 0.17% of the radioactivity appeared as carbon dioxide. An unidentified radioactive nonfluorescent material was found in the cells which was not urea, oxaluric acid, lactic acid, uracil, alloxan, barbituric acid, lumichrome, lumiflavin, 1,2-dihydro-2-keto -1 - (d -1 - ribityl) -6,7- diraethylquinoxaline - 3 - carboxy-ureide 118, 119), or 1,2-dihydro-2-keto-1-methyl-6,7-dimethyl-quinoxa-line-3-carboxyureide. 

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