Ascorbic acid oxidase inhibitors

Copper-Free Ascorbic Add Oxidase. A totally different specific ascorbic acid oxidase has been prepared from fungal spores. This enzyme is not sensitive to copper inhibitors but is inhibited by sulfhydryl reagents. It does not attack ascorbic acid analogs. Like the copper-containing enzyme, fungal ascorbic acid oxidase gives rates proportional to oxygen tension. 

An ascorbic acid oxidase (AO) occurs in wheat flour (Table 15.25), which oxidizes L-threo- and D-erythro-2iSCOYhic acid at comparable rates. In addition, a substance has been found in flour extracts which oxidizes L- /ir 6>-ascorbic acid at pH 10 at a maximal rate. In comparison with AO, this activity does not decrease on incubation with proteases nor is it inhibited by the addition of the AO inhibitors KCN and NaF. It obviously catalyzes a nonenzymatic oxidation of ascorbic acid. 

It has been suggested that there may be some benefit of using tryptophan in selected patients, particularly those with psychomotor retardation (3). Unfortunately, most of these reports have appeared as letters to the editors of journals (4-6) or as preliminary communications (7). In addition to the possible absence of any consistent effect, there are many plausible reasons to explain the variability in response. Tryptophan has been given in both the racemic and monomeric (levorotatory) forms, both alone and together with a number of substances intended to increase the synthesis or availability of serotonin, including monoamine oxidase (MAO) inhibitors (8), potassium or carbohydrate supplements (9), and co-enzymes such as pyridoxine or ascorbic acid (10). It has also been... 

The results of enzymatic determinations of ceruloplasmin are often expressed in arbitrary units, and the values judged in the light of a series of results obtained in normal subjects by the same method. Expression of the enzyme activity in milligrams of ceruloplasmin per unit volume of serum is also possible. The relation between oxidase activity and the amount of ceruloplasmin in serum can be determined by measuring in parallel samples of sera both the oxidase activity and the change of optical density at 610 mix before and after the addition of ascorbic acid or cyanide. On the basis of the known absorbancy index, the ceruloplasmin concentration can be calculated (see Section 2.2.1) and the relation between it and the enzyme activity determined. Alternatively, purified human ceruloplasmin can be used for standardization of the enzymatic method. The ceruloplasmin content of the purified preparation can be determined colorimetrically or, in the case of a highly purified preparation, by nitrogen analysis. Predetermined increments of ceruloplasmin can then be added to aliquots of a selected serum. It is convenient to select a serum with relatively low ceruloplasmin level to start with. Serum of a patient with Wilson s disease, some of whom have no measurable amount of enzyme activity, would be ideal for the purpose however, Walshe (W5) has recently found an inhibitor in these sera. 

The biochemical role of ascorbic acid in maintaining the pHPP oxidase reaction in vivo when extra tyrosine is fed is similar to its role in the usual in vitro assays. The mechanism of this effect is still obscure. It is shared by the other stereoisomers of ascorbic acid and by certain other compounds such as dichlorophenolindophenol. It consists of maintaining the reaction against an inhibition which develops without ascorbic acid. It has been suggested (HI, Z1) that an inhibitor is formed from the substrate, or alternatively, that the inhibition is a reaction inactivation of the enzyme. Evidence in favor of the latter is the reversal of the inhibition by incubation of the enzyme with dichlorophenolindophenol (Kll). 

Ferric iron must be reduced to Fe for absorption by the mucosal cells in the proximal duodenum, the primary site of absorption. The most effective enhancer of iron absorption is ascorbic acid, especially with food of low iron bioavailability where a 10-fold, or greater, enhancement has been observed (8). Inhibitors of iron absorption include tea, soy protein, bran, and other fibers. Gastric and intestinal juices assist in solubilizing and releasing bound Fe in food. Absorbed Fe is reoxidized to Fe by ferroxidase(s) before it is bound by apotransferrin or apoferritin (storage). Ferroxidase activity has been demonstrated with xanthine oxidase and cemloplasmin [11-13]. 

Golan-Goldhirsh and colleagues (71,72) report that sodium bisulfite, reduced glutathione, and ascorbic acid caused a 50% inactivation of mushroom polyphenol oxidase after 28, 106, and 130 min, respectively, at 5 mM concentration. Dithiothreitol was a more effective inhibitor, causing 50% inactivation at 0.1 mM after only 70 min. 

Intermediate formation of qulnone derivatives and cytochrome oxidase via cytochrome C CBonner, 1957 Aberg, 1958 Mapson, 1958 Chlnoy, 1962 Chlnoy et al., 1969 1969 a 1970). In some cases these enzyme systems may readily oxidize all AA to DHA upon grinding while In other cases natural Inhibitors, e g, oxalic acid, strongly retard this process CAberg, 1958 Mapson, 1958). Hence a method for the determination of ascorbic acid utilization (AAU was developed ... 

Sondey S.M., Seib P.A., El-Atawy Y.S. Control of enzymatic browning in apple with ascorbic acid derivatives, polyphenol oxidase inhibitors, and complexing agents. Journal of Food Science, 53 997-1002 (1989). 

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