Ascorbic Acid in excess

Disposition in the Body. Readily absorbed after oral administration the proportion of a dose absorbed tends to decrease with increasing dose it is widely distributed in the body tissues. The concentration of ascorbic acid is higher in leucocytes and platelets than in erythrocytes and plasma. Ascorbic acid is metabolised to dehydroascorbic acid, 2,3-diketogulonic acid, oxalate, and carbon dioxide some conjugation with sulphate occurs to form ascorbate-3-sulphate. Ascorbic acid in excess of the body s requirements is rapidly eliminated in the urine. About 85% of an intravenous dose, given to subjects previously saturated with the vitamin, is excreted in the urine in 24 hours, with about 70% of the dose excreted unchanged and 15% as dehydroascorbic acid and diketogulonic acid. The amount normally present in the body is in excess of 1.5 g. 

Massive doses of ascorbic acid—Intakes of ascorbic acid in excess of 1,000 mg/day (1 g/day) or more have been reported to have some effect in reducing the frequency and severity of symptoms of colds and flu. To date, the results of research work have generally shown that the benefits of large doses of vitamin C are too small to justify recommending routine intake of large amounts. But further studies are needed. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

The oxidation of ascorbic acid in the presence of an excess of oxygen (saturation) was assumed to follow a first order reaction [2, 3,5]. The reaction rate can be expressed as ... 

The reaction of DCIP with ascorbic acid is shown in Figure El 1.3. Ascorbic acid reduces the indicator dye from an oxidized form (red in acid) to a reduced form (colorless in acid). The procedure is simple, beginning with dissolution of the sample to be tested in metaphosphoric acid. An aliquot of the sample is then titrated directly with a solution of DCIP. Although the original DCIP solution is blue, it becomes light red in the acid solution. Upon reaction with ascorbic acid in the sample, the dye becomes colorless. Titration is continued until there is a very slight excess of dye added (faint pink color remains in the acid solution). 

Peptide Modification lodination was carried out on a stainless steel probe target by adding 0.1 % aq. I2 (1 pi) to the dried peptide (ca. 1 pmol). The reaction was stopped after 1 minute by addition of ascorbic acid and the MALDI matrix, a-cyano cinnamic acid in excess. Esterification with ethanol was carried out using the method of Hunt et al. (15), where an acetylchloride and ethanol solution (1 6, v v) was added (5 pi) to the peptide dried in a microcentrifuge tube (ca. 1 pmol). After incubation for 15 minutes at room temperature a 2 mM p-mercaptoethanol (in ethanol) solution was added (1 pi) and the mixture was dried. The matrix, a-cyano-4-hydroxycinnamic acid (2 pi), was added to the micro tube and after 5 minutes 1 pi of this matrix was removed and applied to a target. 

Like laccase and ceruloplasmin, ascorbate oxidase is an acidic protein, with aspartic acid and glutamic acid in excess over histidine, lysine, and arginine. For the amino acid composition see the detailed data in References 3 and 18. Unlike laccase, ascorbate oxidase has a relatively low carbohydrate content, 2.4% vs. approximately 45% (29). 

In humans there is a Na -dependent active transport system with a Km of about 1 mM. Absorption is very eflBcient at low intakes of ascorbic acid and becomes poor as stomach levels of ascorbic acid increase. The upper level of ascorbic acid in the blood is limited by kidney clearance with Tm of 1.5 mg/100 mL. Where intestinal absorption is excessive the eflSciency of the kidney clearance improves. Transfer of ascorbic acid into the central nervous system and other tissues is a facilitated saturable process. Therefore, control at all levels appears to sharply limit maximum levels of ascorbate in tissues. 

Within the body, ascorbic acid and minerals have two additional levels of important interaction in the tissue storage and turnover of ascorbic acid and in the synthesis of tissues and organs. The apparent decrease in the half-life of ascorbic acid in the presence of excess iron is an example of the former interaction, whereas the simultaneous participation of vitamin C, calcium, and phosphorus in the formation of growing bone is an example of the latter. 

Effects of Excess Tissue Iron on Ascorbic Acid Metabolism. Epidemiological observations among the Bantu of South Africa showed an apparent association of clinical scurvy in adult males with hemosiderosis common to this group. Both plasma clearance of ascorbic acid and urinary excretion of ascorbic acid were altered in severe iron overload plasma clearance was increased and urinary excretion was decreased in siderotic subjects (40,41), The evidence was interpreted as a demonstration of enhanced oxidative catabolism of ascorbic acid in the presence of excess tissue iron. 

Further evidence of like character in support of the participation of coenzyme I in reactions associated with the reduction of dehydroascorbic acid has been advanced by Waygood (1950). Cell-free extracts of wheat seedlings were found to contain a malic dehydrogenase enzyme, reducing coenzyme I, as well as ascorbic oxidase and peroxidase enzymes. When to such extracts malic acid, coenzyme I, and ascorbic acid were added, together with a fixative for the oxalacetate formed in the reaction, the system absorbed oxygen in excess of that required for the complete oxidation of ascorbic acid. In this system methylene blue could replace ascorbic acid. 

Sometimes the proposed method is an indirect procedure to monitor the excess of oxidant or the color change of an auxiliary reagent such as the method described for the assay of azathioprine, either in pure form or in pharmaceutical formulations the methods are based on the oxidation of the drug with excess N-bromosuccinimide or chloramine-T and determining the consumed reagent by the decrease in color intensity of celestine blue or gall-ocyanine, respectively. Another example is the determination of ascorbic acid in fruit juice and pharmaceuticals on the basis of its inhibition effect on the reaction between hydrochloric acid and bro-mate. The decolorization of methyl orange due to the reaction products was used to monitor the reaction at 510 nm. 

Figure 10.2 Cyclic voltammogram of oxidation of 2.5x10" m dopamine in the presence of 100-fold excess of ascorbic acid, in phosphate buffer, pH = 7, obtained on (a) bare carbon fibre microdisc electrode and (b) the same electrode modified with semipermeable membrane poly(4-HBS/2-AP) (100 1). Curves ( ) were obtained in the background electrolyte (phosphate buffer, pH = 7).

It is well established that stress affects the ascorbic acid concentration in various components of the body. What mechanism causes stress is unimportant because similar effects are obtained with prolonged exposure to cold, heat, or X-irradiation, excessive oxygen tension, or even a simple injection of saline. Stress increases the excretion of ascorbic acid in the urine and the plasma levels of ascorbic acid in the blood (in the early stages of stress), and reduces the ascorbic acid level of the adrenal. Increased urinary excretion of dehydroascorbic and diketonic acids usually is associated with ascorbic acid in the urine. 

The estimated total amount of ascorbic acid in the human body tissues is on average 1500 mg (maximum 5 g) when completely saturated. Any excess of vitamin... 

Since ascorbic acid is water-soluble and very easily oxidized, loss of this factor in the storage and preparation of food may be great. Destruction is more rapid in neutral and alkaline solutions than in acid solutions. In patients who have achlorhydria and in those who have received alkaline medication, ascorbic acid may be destroyed in the upper intestinal tract. The vitamin is absorbed readily, and the levels of ascorbic acid in plasma are related to recent dietary intake. Tissue saturation is achieved easily with large doses of ascorbic acid, and amounts in excess of this are excreted in the urine. It has been estimated that when the tissues of an adult human being are saturated the body contains about 5 g. of ascorbic acid. 

Quite a complicated derivatization process was introduced by Bilic (34). Two-step sequential derivatization was applied to the simultaneous assay of both ascorbic acid and DHA by HPLC. First, the sample was derivatized with 4-ethoxy-l,2-phenylenediamine, followed by isolation of the products of derivatization and removal of excess of reagent by small C18 silica gel and cation exchange columns. Second, the ascorbic acid in the sample was oxidized to DHA and then derivatized with 4-methoxy-l,2-phenylenediamine. In the first step, ascorbic acid remains constant during derivatization of DHA. This was obtained by performing the derivatization of DHA at pH 2. The methoxyquinoxaline (corresponding ascorbic acid) and ethoxyquinoxaUne (corresponding DHA) derivatives were separated by reversed-phase HPLC. 

Sudden death sometimes occurs in severe scurvy, so prompt and aggressive treatment is necessary. The usual treatment consists of ascorbic acid in doses as high as 250 mg each, four times a day, for a week. The purpose of such high levels is to achieve rapid saturation of the body fluids with ascorbic acid so as to hasten the healing of diseased tissues. Excesses of the vitamin are excreted in the urine, so there is little... 

Hiromi, K., C. Kuwamoto, and M. Ohnishi. 1980. A rapid sensitive method for the determination of ascorbic acid in the excess of 2,6-dichlorophenolindophenol usinga stopped-flow apparatus. Anal. Biochem. 101 421-426. 

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