Ascorbic acid in animal tissue

This system cannot now be fitted into known pathways of electron transport, principally because a system that oxidizes ascorbic acid in animal tissues is unknown. The fact is clear, however, that ascorbic acid is oxidized to dehydroascorbic acid in animal tissues and that dehydro-ascorbic add can be reduced. At the very least, this system emphasizes the potential eflBciency of a form of ascorbic acid as an electron acceptor (K3). 

The distribution of ascorbic acid in animal tissues has been reviewed by Giroud (1939). Cellular tissues contain a greater concentration of the... 

Shimazono, N., and Mano, Y., 1961, Enzymatic studies on the metabolism of uronic and aldonic acids related to L-ascorbic acid in animal tissues, Ann. N.Y. Acad. Sci. 92 91-104. 

G Barja de Quiroga, M Ldpez-Torres, R Perez-Campo, C Rojas. Simultaneous determination of two antioxidants, uric and ascorbic acid, in animal tissue by high-performance liquid chromatography. Anal Biochem 199 81-85, 1991. 

Carr, R.S., Bally, M.B., Thomas, P., and Neff, J.M., Comparison of methods for determination of ascorbic acid in animal tissues. Anal. Chem., 55, 1229-1232, 1983. 

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. 

Selected methods for the determination of ascorbic acid in animal cells, tissues, and fluids, 1 "Methods in Enzymology, D. B. McCormick and L. D. Wright, eds., Academic Press, New York, Vol. 62, Part D, pp. 3-11. 

The synthesis of ascorbic acid in animals has also been linked with thiamine and riboflavin (Kennaway and Daff, 1946 Roy et al., 1946). With both these deficiencies the ascorbic acid content of the tissue of rats and mice has been shown to be low. The stimulation of synthesis shown by a normal animal after chloretone treatment was not observed in animals deficient in either thiamine or riboflavin. The claim was made... 

The existence of a DHA reductase enzyme in animal or human tissue has been a theoretical possibility. Recently, Park and Levine have purified and expressed a glutaredoxin with dehydroascorbic acid-reducing activity from human neutrophils (29). They found that glutaredoxin was responsible for most of the protein-mediated DHA reduction in lysates. DHA reduction was at least fivefold greater in neutrophil lysates than in myeloid tumor cell lysates where glutaredoxin could not be detected (29). However, it may be still possible that more than one enzyme participates in the process (30). On the other hand, it seems that a nonenzymatic, direct reaction between GSH and DHA is the major physiologically relevant process underlying the reduction of DHA to ascorbic acid in mammalian tissues (26). 

N5,N10-methenyltetrahydrofolate (with ascorbic acid) was adjusted to neutral pH, autoclaved, and stored at -20° C prior to column purification on DEAE and G-15 Sephadex. These labeled products are the biologically active diastereomers, and they are used to study the metabolism of folinic acid in cells, tissues, and animals. 

In its biochemical functions, ascorbic acid acts as a regulator in tissue respiration and tends to serve as an antioxidant in vitro by reducing oxidizing chemicals. The effectiveness of ascorbic acid as an antioxidant when added to various processed food products, such as meats, is described in entry on Antioxidants. In plant tissues, the related glutathione system of oxidation and reduction is fairly widely distributed and there is evidence that election transfer reactions involving ascorbic acid are characteristic of animal systems. Peroxidase systems also may involve reactions with ascorbic acid In plants, either of two copper-protein enzymes are commonly involved in the oxidation of ascorbic acid. 

As indicated in Figure 2, when minces of tumor obtained from normal and ascorbic acid-deficient animals were incubated with C14-pro-line, much more radioactivity was incorporated into the collagen of the normal tissue. When the specific radioactivities of the isolated imino acids were examined (Table HI), several conclusions were possible. In the experiments with granuloma from normal animals and C14-proline, the values for hydroxyproline were not far from those of proline. With the scorbutic granulomas, the specific activity of the isolated proline was not greatly reduced, compared to that obtained from the normal granuloma. In contrast, the specific activity of the hydroxyproline isolated from the deficient tissues was markedly reduced. Similar results were obtained in the studies with tritiated proline. Thus, the specific activity of the proline isolated from the deficient granuloma was only moderately reduced, whereas the specific activity of the hydroxyproline was extremely low. This observation may be explained in terms of a dual-pathway mechanism of proline incorporation, considered below. 

Prompt stabilization of ascorbic acid is especially important in the case of plasma or serum samples. Metaphosphoric acid is often used for this purpose because it also serves as a protein precipitant. Such properties are desirable in the inactivation of oxidase and the catalytic eflFect of copper. Oxalic acid is an attractive stabilizer for ascorbic acid analysis because of its lower cost and greater stability however, it is not a protein precipitant, therefore, it has a limited use for the extraction of animal tissues. The use of ethylenediaminetetraacetic acid (EDTA) in addition to the metaphosphoric acid has been recommended (96). EDTA would chelate divalent cations, and a study has shown it will stabilize ascorbic acid in the presence of copper for several days (96). Perchloric acid has been used also but because of its inherent dangerous properties its use is generally avoided. Trichloroacetic acid and EDTA also seem appropriate extractants for ascorbate in plant materials (97). 

For ascorbic acid analysis, metaphosphoric acid is very useful in the inactivation of the catalytic effect of ascorbic acid oxidase as well as other catalytic oxidizing agents discussed previously. Foods such as fruits and vegetables also have a tendency to have a larger proportion of dehydroascorbic acid than animal tissues consequently, methods that assay for only the reduced form of ascorbate may provide misleading low values. 

The most important recent addition to our knowledge of ascorbic acid is the series of stepwise enzyme reactions by which it is made in animal tissues and through which it is related to the reactions of other carbohydrates in the body. The biosynthetic pathways of ascorbic acid in plants have been partially elucidated, but less successfully, since it appears that two different pathways may operate (L21). These important advances have been the subject of a number of reviews (M7, B38, B39, K5). 

The possible existence of bound forms of ascorbic add in animal and plant tissues forms one of the most contradictory subjects in the biochemical literature. The very high concentrations of ascorbic acid within both plant and animal cells provide reasons to look for such combined forms. The realization that most of the water-soluble vitamins function... 

The specific enzymatic catalysis of ascorbic acid oxidation is known in plants but not in animal tissues. 

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