Non-enzymic oxidation

LPO causes non-enzymic oxidation of unsaturated lipids, probably acting through its haem group the heat-denatured enzyme is more active than the native enzyme. 

The Maillard reaction is best known as the phenomenon responsible for the browning of untreated foodstuffs, by non-enzymic oxidation. It involves the condensation of amino acids on sugars, both aldoses (glucose) and ketoses (fructose) (Figure 8.27). When aldoses are involved, the primary condensation products are aldimines (Rj = H) (or Schiff base), while ketoses produce ketimine (Ri H). Due to their enoliz-able character, these imines develop according to two tautomeric equilibriums into enaminol and... 

Fig. 8.27. Maillard reaction involved in the non-enzymic oxidative browning of plant tissues, (a) Formation of an imine by an amino acid reacting with an aldose (Ri = H) or ketose (Ri H). (b) Enolization of the imine to enaminol, then to an Amadori (Ri = H) or Heyns (Ri H) intermediate, (c) Breaking of the preceding intermediates, with the appearance of a reductone in redox equilibrium with an a-dicarbonylated compound, responsible for the non-enzymic oxidation phenomenon...

Glutamate-mediated calcium influx results in stimulation arachidonic acid release from neural membrane glycerophospholipids. This release is catalyzed by CPLA2 and PLC/DAG-lipase pathway (McIntosh et al., 1998 Schuhmann et al., 2003 Shohami et al., 1987, 1989 Wei et al., 1982 Dhillon et al., 1996 Homayoun et al., 1997, 2000). Arachidonic acid release occurs in traumatic as well as fluid percussion models of brain injury (FPI). Enzymic oxidation of arachidonic acid generates prostaglandins, leukotrienes, and thromoboxanes whereas non-enzymic oxidation produces isoprostanes and ROS which include superoxide and hydroxyl radicals (Farooqui and Horrocks, 2007). 

Fig, 17.13 The formation of a-acetohydroxy acids and their non-enzymic oxidative decarboxylation to diacetyl and pentane dione. 

Unsaturated oils often contain small amounts of hydroxy and/or epoxy acids after prolonged storage, probably as a result of enzymic and/or non-enzymic oxidation (Sections 10.2 and 10.3). 

Saito, I., Y. Chujo, H. Shimazu, M. Yamane, T. Matsuura, and H. J. Cahnmann Non Enzymic Oxidation of p-Hydroxyphenylpyruvic Acid with Singlet Oxygen to Homogentisic Acid. A Model for the Action of p-Hydroxyphenylpyruvate Hydroxylase. J. Amer. Chem. Soc. 97, 5272 (1975). 

The deficiency of enzymes downstream of bydroxymetbylbilane causes the accumulation of intermediates that are diverted by non-enzymic oxidation to form several porphyrins which, when exposed to light, form singlet oxygen Oj". This is cytotoxic, causing photosensitivity on exposure to sunlight. 

Ascorbic and erythorbic acids in vinification are less deleterious to the aroma of the resultant wine than SO2 (Ewart et al., 1987). It is only when they are used in conjunction with SO2, however, that protection against oxidation is assured. As in the absence of adequate levels of free SO2, some oxidative reactions are promoted by these additives. The mode of reaction is similar to that of the non-enzymic oxidation and therefore can lead to more rapid accumulation of hydrogen peroxide (Simpson, 1980). 

OXIDATION OF THIOLS TO THIYL RADICALS BY NON-RADICAL NON-ENZYME OXIDANTS... 

Saito, 1., Chujio, Y, Shimazu, H., Yamane, M., Matsuura, T, and Cahnmann, H. J., Non-enzymic oxidation of p-hydroxyphenylpyruvic acid with singlet oxygen to homogentsic acid a model for the action ofp-hydroxyphenylpyruvichydroxylase,/. Am. Chem. Soc., 97,5272, 1975. 

Vitamin K is the cofactor for the carboxylation of glutamate residues in the post-synthetic modification of proteins to form the unusual amino acid y-carboxygluta-mate (Gla), which chelates the calcium ion. Initially, vitamin K hydroquinone is oxidized to the epoxide (Figure 45-8), which activates a glutamate residue in the protein substrate to a carbanion, that reacts non-enzymically with carbon dioxide to form y-carboxyglut-amate. Vitamin K epoxide is reduced to the quinone by a warfarin-sensitive reductase, and the quinone is reduced to the active hydroquinone by either the same warfarin-sensitive reductase or a warfarin-insensitive... 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

The selective oxidation of alcohols to the corresponding aldehydes and ketones is of prime importance for organic synthesis, and various types of reagents have been described that achieve this transformation selectively and efficiently [141,142]. However, the number of sub-stoichiometric, nontoxic, non-hazardous oxidation systems has been relatively Umited. As copper enzymes such as galactose oxidase are known to catalyze this oxidation reaction, bioinspired homogeneous catalysts based on copper species have also been developed in recent years. 

The ease with which the hexosyl-4-ulose derivative 109 undergoes dehydration was demonstrated in a non-enzymic, model reaction. Oxidation of methyl /8-D-galactopyranoside with oxygen and a platinum catalyst, followed by hydrogenation over the same catalyst, resulted in a 35% yield of methyl /8-D-fucopyranoside,423 presumably formed through reactions analogous to those in Fig. 3. 

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

More definite evidence for the transient existence of the un-cyclized l-(jS-aminoethyl)-3,4-benzoquinones has been obtained recently by Kodja and Bouchilloux,77 78 who noted that a transient yellow color (Amax ca. 385 mp) was occasionally observed during the enzymic oxidations of catecholamines (particularly in unbuffered systems at low temperatures). This phenomenon was probably due to the formation of the transient o-quinones. (The absorption maximum of o-benzoquinone, the effective chromophore of the open-chain quinones, is known to occur at ca. 390 mp.79) An absorption maximum at 390 mp is characteristic of the formation of the dopa-quinone chromophore during oxidation of small C -terminal tyrosine peptides in the presence of tyrosinase.37 48 Similar spectroscopic features were observed when the oxidations were carried out with lead dioxide in sulfuric acid solutions (pH> 1). If the initial oxidation was carried out for a short period of time, it was possible to regenerate the original catecholamines by reduction (e.g. with sodium bisulfite, potassium iodide, and zinc powder) and to show that the 385 mp peak disappeared.77,78 Kodja and Bouchilloux were also able to identify 2,4-dinitrophenylhydrazones of several of the intermediate non-cyclized quinones by paper chromatography and spectroscopy (Amax n weakly acid solution ca. 350 mp with a shoulder at ca. 410 mp).77,78... 

Secondary alicyclic amines, such as pyrrolidine and piperidine, have many properties typical of the corresponding aliphatic amines. Metabolic oxidation at secondary alicyclic nitrogens results in the formation of hydroxylamines, which may then undergo enzymic or non-enzymic conversion to nitrones, and in some cases to nitroxide radicals. For example, the 2-substituted piperidino derivative (- )-anabasine (1), a tobacco alkaloid, is metabolized initially to a hydroxylamine (2) and then to the nitrone (3), when incubated with liver and... 

The non enzymes, the lipoxygenases, catalyse the oxidation of 1,4-diene fatty acids to alkyl hydroperoxides and the slow step of the reaction involves H atom abstraction from the carbon adjacent to the two double bonds of the fatty acid by a Fe(OH)3 species. This mechanism has now been shown to be correct by use of the lipoxygenase model (244).219 Two papers discussed earlier are relevant to this section.138,175... 

The system protecting organisms from free radical excess comprises enzymes with oxide reductive activity, non-enzyme proteins, polypeptides, water and oil-soluble vitamins, SH-containing amino acids, flavonoids, carotinoids, etc. [40], Most of these compounds prevent oxidative stress evolution by interrupting chain oxidative reactions. That is why these substances are called substances with antiradical activity as well as antioxidants (AO). Foodstuff, nutrients and some drugs are sources of most antioxidants. 

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