Peroxidase uric acid oxidation

A large number of products, including some of the products of uricase action, are produced from uric acid by peroxidases + enzymatically generated H202. The nonspecific action of peroxidases emphasizes the part played by nonenzymatic reactions of the unstable products of uric acid oxidation. 

Fig. 6. Generalized mechanism of oxidation of uric acid by peroxidase enzymes 25>26)...

There is no doubt that electrochemically xanthine is initially oxidized to uric acid, which is then further oxidized to a bis-imine that undergoes hydrolysis giving ultimately alloxan, allantoin and urea. There is no single enzyme in man that will bring about such a fragmentation of xanthine. However, there are organisms that possess a combination of enzymes, e.g., xanthine oxidase and certain peroxidases, that under conditions comparable to those employed in the... 

Uric acid (29) kits Commercial kits are based on the enzymatic process shown in equation 15, followed by a chromogenic oxidation process catalyzed by peroxidase, similar to equation 27, involving 4-aminoantipyrin (81) and 3,5-dichloro-2-hydroxyben-zenesulfonate (88), measuring at 500 to 520 nm . ... 

Grinstead RR (1960) Oxidation of salicylate by the model peroxidase catalyst iron-ethylenediamine-tetraacetato-iron(lll) acid. J Am Chem Soc 82 3472-3476 Grootveld M, Halliwell B (1987) Measurement of allantoin and uric acid in human body fluids. Bio-chemJ 243 803-808... 

Devices that use uricase in a dry reagent format to measure uric acid have also been described. For example, a multilayer film system employs uricase and peroxidase separated by a semipermeable membrane from a leuco dye that is oxidized to form a colored product. A ceUulose matrix pad system employs uricase, peroxidase, and MBTH as oxygen acceptor the system employs a diluted plasma sample, which helps to reduce interferences, although ascorbic acid was shown to be a significant interferant. A third system incorporates separation of plasma from red cells and uricase, peroxidase, and a substituted phenol to measure uric acid. AH three systems employ a reflectance meter system to facilitate accurate and precise quantitation of the color change. 

Antioxidant compounds include the well-known vitamins C and E, ubiquinone, uric acid, and many others. Antioxidant compounds are sacrificed to oxidation in order to directly protect more important cellular components [1-4, 6]. Antioxidant enzymes, such as superoxide dismutases, glutathione peroxidases, and quinone reductases, act catalytically to convert oxidants to less reactive species [5,6]. The essential cytoplasmic and nuclear proteolytic enzyme, proteasome recognizes and selectively degrades oxidized proteins [6, 8, 9]. Oxidized membrane lipids are recognized and selectively removed by lipases, particularly phospholipase A2 [6, 8,10]. Oxidatively modified DNA is subject to removal or excision repair by a wide series of DNA repair enzymes including endonucleases, glycosylases, polymerases, and ligases [6,8,11 ]. 

The enzymic oxidation of uric acids with peroxidase (type VIII from horseradish peroxidase) has recently been studied and critically compared to the electrochemical oxidation. Peroxidase was studied because this enzyme will oxidize not only uric acid but also most of its N-methyl derivatives. Typical spectra of uric acid obtaine d during the peroxidase-catalyzed oxidation of uric acid are shown in Figure 27. Curve 1 is the spectrum of uric acid. Addition of hydrogen peroxide and peroxidase causes the initial rapid decrease of the spectrum because of dilution. However, it is quite clear from Figure 27... 

Figure 27. Spectrum of 200 fiM uric acid undergoing oxidation in the presence of 200 tiM H2O2 and 0.7 /iM peroxidase in 0.5 M NaCl puls 5 mM Na2HP04, pH 7.5. Curve...

Observed First-order Rate Constants for Reaction of the UV-Absorbing Intermediate Formed on Enzymic (Peroxidase) and Electrochemical Oxidation of Uric Acid Derivatives at pH 7 ... 

Such results reveal that upon electrochemical and enzymic (peroxidase) oxidation of uric acid and uric acid derivatives a uv-absorbing intermediate is formed. The spectral, kinetic, and electrochemical behavior of this intermediate whether generated enzymically or electrochemically are identical. Analytical data on the trapped and derivatized electrochemical intermediate are in accord with the uv-absorbing intermediate having an imine-alochol structure (i.e.. Ill, Figure 25). 

For comparitive purposes, it is worthwhile reviewing the information available from purely biochemical studies of the peroxidase-catalyzed oxidation of uric acids. In fact, more detailed studies have been carried out on the uricase-catalyzed oxidation of uric acid. " However, this enzyme is virtually specific for uric acid and its mechanism has not been studied using the electrochemical approach outlined earlier for the case of the peroxidase enzymes. 

Paul and Avi-Dor " found that 1-methyl uric acid is oxidized in the presence of horseradish peroxidase and H2O2. This reaction is most rapid when the uric acid molecule is uncharged. According to these workers the primary oxidation product obtained at pH 3-5 could be reversibly transformed into two other spectrophotometrically active forms by changing the pH to >6 or >1. Depending on the pH, the primary oxidation product was proposed to undergo a nonenzymic decomposition to allantoin (pH 3-6) or alloxan (pH[Pg.181]

Canellakis and co-workers examined the oxidation of uric acid with the catalase-HaOa, lactoperoxidase-H202, verdoperoxidase-H202, and horseradish peroxidase-H202 systems. On the basis of this investigation it was proposed that at around pH 7 uric acid (I, Figure 29) is oxidized, via one or... 

The reaction schemes shown in Eq. (19) and Figure 29 were thrown into some doubt when Howell and Wyngaarden found that 7-methyl (21) and 9-methyl uric acid (22) were oxidized in the presence of methemoglobin (a hemeprotein or peroxidase enzyme)-H202 or horseradish peroxidase-H202 but that 1,3,7,9-tetramethyl uric acid (23) was not. These workers thus... 

Interference studies. In the conventional electrochemical method for H2O2 determination based on the oxidation at either platinum or carbon electrodes, electroactive compounds such as ascorbic acid, acetaminophen, and uric acid interfere. Various approaches, such as the incorporation of peroxidases or... 

Volk, K.J., Yost, R.A., Brajter-Toth, A. (1990) On-line Mass Spectrometric Investigation of the Peroxidase-catalysed Oxidation of Uric Acid. J. Pharm. Biomed. Anal. 8 205-215. 

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