Bound ascorbic acid

FIGURE 19. Active site of ascorbate peroxidase (lOAF) with the heme anchored to the protein via Hisl63, containing a bound ascorbic acid substrate (pink molecule in Plate XLIII). Ascorbate is H-bonded to the 6-propionate group of the heme and to Lys30 and Argl72 (orange)... 

After the characterization of vitamin C as ascorbic acid, it was observed that the content of ascorbic acid in some vegetables (83,84) increases when boiled or cooked. The increase is believed to be caused by liberation of bound ascorbic acid (85,86,87). The name ascorbigen (87) was given to that substance that was later separated (88) and synthesized (89,90,91). Ascorbigen was synthesized either from 3-hydroxy indole and ascorbic acid or from indole, formaldehyde, and ascorbic acid. 

The possibility that some fraction of the ascorbic add in animal tissues is bound instead of free has continued to attract interest. It can be accepted as established that there is no bound ascorbic add in blood serum (S8). The ascorbic acid present in serum is freely dialyz-able and is not increased by various hydrolytic procedures when measured by the 2,4-dinitrophenylhydrazine method. The increases after hydrolysis reported earlier may be attributed to the production of other materials which readed with the 2,6-dichlorophenolindophenol used for titration. The possibility of bound ascorbic acid is, therefore, restricted to other tissues. 

Somewhat similar evidence of a bound form of ascorbic acid was observed in the developing sea urchin egg after the stage of gastrulation (Bl). The bound ascorbic acid was not extracted by cold water or KCl solution and only partially by hot water or cold trichloroacetic acid. But hke the fractions studied in the liver of animals, it was readily extracted by 10 minutes treatment with hot trichloroacetic or metaphosphoric acids. 

Ascorbic acid within the cells was localized in the cytoplasm (G3). More recent studies in rat liver and ox adrenals by the method of differential centrifugation have shown that most of the ascorbic acid was in the soluble and microsomal fractions (G2). The sum of ascorbic acid in the different fractions exceeded that of the whole tissue analyses by 40 %, suggesting to these workers the existence of bound ascorbic acid not liberated by extraction of the whole tissue. The fact of some localization in microsomes deserves confirmation. 

H9. Hastings, W. H., and Spencer, C. F., Determinations of free and bound ascorbic acid in fishery products. J. Marine Research (Sears Foundation) 11 241-244 (1952). 

L9. Lewis, W. R., and Chiang, J. L., The influence of dietary intake of ascorbic acid and protein on bound ascorbic acid in guinea pig. Biochem. Biophys. Research Commutes. 2, 464-467 (1960). 

Sargent, F., and Golden, R., Failure to find bound ascorbic acid in plasma. J. Biol. Chem. 188, 773-781 (1951). 

STRUCTURE AND PROPERTIES OF FREE AND BOUND ASCORBIC ACID... 

CHEMICAL STRUCTURE OF BOUND ASCORBIC ACID OR ASCORBIGEN (ASG)... 

The existence of bound ascorbic acid was reported in the liver of the pig by Summerwell and Sealock (1952) but their method was criticized by Lewis et al (I960). Later,... 

Sodium ascorbate and ascorbic acid esters, such as ascorbyl 6-palmitate (5-111) and ascorbyl 2-phosphate (5-113), are fully bioavailable, while ascorbyl 2-sulfate (5-114) is a completely inactive vitamin form. Phosphate and sulfate are about 20 times more stable to oxidation than the free acid. D-lsoascorbic acid (5-107) shows only 5-20% activity, 6-deoxy-L-ascorbic acid (5-115), found in fungi, has about 30% activity and the bound ascorbic acid form ascorbigen has 15-20% of the activity of ascorbic acid. Ascorbic acid 2-0-P-D-glucoside, systematic name 2-0-(P-D-glucopyranosyl)-L-ascorbic acid (5-116), has the same biological activity as ascorbic acid and is also stable against oxidation. 

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