Diketo gulonic acid

The ease of oxidation of reduced ascorbic acid is the basis for a simple method of analysis by dye titration (58j. Ascorbic acid as it occurs in citrus juice is in the reduced form. When subjected to oxidation, ascorbic acid changes to the dehydro form. Dehydroascorbic acid has nearly the same physiological activity as the reduced form and is easily converted to the latter. Further oxidation of the dehydroascorbic acid converts it to 2,3-diketo-gulonic acid. This reaction is irreversible, and the oxidized product is devoid of biological activity. These reactions are shown in Figure 2. Nearly 90 percent or more of the vitamin C found in citrus juice and citrus products is in the reduced form (Table X) (59). 

R7. Roe, J. H., Cbemical determination of ascorbic, debydroascorbic and diketo-gulonic acids. Methods of Biochem. Anal. 1, 115 (1954). 

The rate of the uncatalyzed reaction is, however, too slow to be of much consequence, the half-time period for the reduction of dehydroascorbic acid by GSH at physiological temperatures and at pH values and in concentrations usually found in vivo being of the order of 15 minutes, whereas under the same conditions the conversion of DHA to 2,3-diketo-gulonic acid has a half life of only 2 minutes (Ball, 1937). 

Diketo-gluconic acid (24) 2-Keto-L-gulonic acid (2-KGA)... 

M. Otsuka, T. Kurata, and N. Arakawa, Isolation and characterization of an intermediate product in the degradation of 2,3-diketo-L-gulonic acid, Agric. Biol. Chem., 1986, 50, 531-533. 

L-ascorbic acid (Figure 9-9) is a lactone (internal ester of a hydroxycarboxylic acid) and is characterized by the enediol group, which makes it a strongly reducing compound. The D form has no biological activity. One of the isomers, D-isoascorbic acid, or erythorbic acid, is produced commercially for use as a food additive. L-ascorbic acid is readily and reversibly oxidized to dehydro-L-ascorbic acid (Figure 9-10), which retains vitamin C activity. This compound can be further oxidized to diketo-L-gulonic acid, in a... 

L-threonic acid, and 2,3-diketo-L-gulonic acid. No oxalic acid can be detected in bread. 

Micromodifications. Click et al. (G4) described a method for the determination of L-ascorbic, dehydro-L-ascorbic, and diketo-L-gulonic acids in microgram quantities of tissue, such as adrenal tissue, with the phenylhydrazine method. They also found slightly lower values by incubation during coupling at 100°C for 5 minutes but false, high values at 100° C for ten minutes, as compared to the values obtained at 37°G for 6 hours. 

S. Kamiya (n-ascorhic acid degradation by bacteria. I-IV. J. Vitaminol. 6, 217-235, 240-245, 1960) studied the degradation of L-ascorbic acid by anaerobically adapted cells of a species of Micrococcus. After conversion to diketo-L-gulonic acid, pyruvic acid was formed. 

D6. Damron, C. M., Monies, M. M., and Roe, J. H., Metabolism of n-ascorbic acid, dehydro-L-ascorbic acid, and diketo-i.-gulonic acid in the guinea pigs. J. Biol. Chetn. 196, 599-606 (1952). 

In alkaline solutions the oxidation of 5-keto-D-gluconic acid, dehydro-L-ascorbic acid, and 2,3-diketo-L-gulonic acid are completely inhibited by borate which would indicate the reaction of these ligands with borate (54). 

The X-ray structure of the tetra-acetate of dimeric dehydro-ascorblc acid is referred to in Chapter 23, and the hydrolysis products of the acid (2,3-diketo-L-gulonic acid L—threo-hex-2,3-dlulosonlc acid) have been concluded to contain the 6-lactone... 

Oxalate is a terminal product of normal metabolism. When [ " C]oxalate is injected, most of the labeled compound is excreted, and only a small fraction of the radioactivity is recovered in bone and muscle. There are only two immediate precursors of oxalic acid in normal metabolic pathways glyoxylic acid and 2,3-diketo-L-gulonic acid. The first of these compounds is the product of amino acid oxidation (serine and glycine), and the second is derived from the oxidation of ascorbic acid (see Fig. 3-25). 

The intricacies of the metabolic pathway that leads from ascorbic acid to oxalate are not clear. Ascorbic acid is oxidized to yield dehydro-L-ascorbic add this compound is further hydrated to yield 2,3-diketo-L-gulonic acid. The intermediates between 2,3-diketo-L-gulonic acid and oxalate are not known. L-Xylulose, L-xylose, and L-xylosone have been eliminated. An enzyme was partially purified from rat kidney it catalyzes the decarboxylation of 2,3-diketo-L-gulonic acid to yield L-xylonic and L-lyxonic acids [105]. 

The 2,3-diketo-L-gulonic acid (XIV) is formed spontaneously on dissolution of dehydroascorbic acid I48). Dehydroascorbic acid may be stored as a methanol complex, which is far more stable than the parent substance 148a). 

Otsuka et al. (1986) used TLC to isolate and partially characterize degradation products of 2,3-diketo-L-gulonic acid (intermediates in the biosynthesis of ascorbic acid) the products were characterized by different spectrometric methods. Mandrou et al. (1988) devised a TLC procedure to determine ascorbic and dehydroascorbic acids in fruit juices and wine the sample was reacted with 2,4-dinitrophenylhydrazine to form the osazones the osazones were spotted on the TLC plate and quantified by. in situ densitometry at 494 nm. 

Polarographic studies of 2,3-diketo-L-gulonic acid (i.e. L-r/ireo-hex-2,3-diulosonic acid, the hydrolysis product of dehydroascorbic acid) and the 2,3- and 3,4-endiol forms of its 1,5-lactone have been reported. On condensation with a-phenylenediamine, these compounds form a single product, 2-(2-aminophenyl)-3-(L-f/ireo-triitol-l-yl)quinoxaline, that gives a well defined cathodic wave suitable for analytical purposes. "... 

About 3% of the body s vitamin C pool, which is 20-50 mg/kg body weight, is excreted in the urine as ascorbic acid, dehydroascorbic acid (a combined total of 25%) and their metabolites, 2,3-diketo-L-gulonic acid (20%) and oxalic acid (55%). An increase in excreted oxalic acid occurs only with a very high intake of ascorbic acid. Scurvy is caused by a dietary deficiency of ascorbic acid. 

S. oleracea utilized L-ascorbic acid and dehydro-L-ascorbic acid (but not diketo-L-gulonate) for oxalic acid biosynthesis (Table VI) (29). In these experiments, l-[1- C] ascorbic acid was converted to labeled oxalic acid. The fate of the C4 fragment from L-ascorbic acid or dehydro-L-ascorbic acid was not determined. Similar results (Table VI) were obtained with O. stricta and O. oregana. Conversion to labeled oxalate proceeded in the dark as well as in the light when detached leaves were given L-[1- C]ascorbic acid. Hordeum vulgare (barley), a nonaccumulator of oxalate, converted only 2% of the in l-[1- C] ascorbic acid to oxalic acid under conditions identical to those used in studies with Oxalis species. 

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