Oxidation nonenzymatic

Although the direct reaction of a lipoyl group with the thiamin-bound enamine (active aldehyde) is generally accepted, and is supported by recent studies,3153 an alternative must be considered.315 Hexacyanoferrate (III) can replace NAD+ as an oxidant for pyruvate dehydrogenase and is also able to oxidize nonenzymatically thiamin-bound active acetaldehyde... 

Sulfur is readily oxidized, nonenzymatically as well as enzymatically (Scheme 11.21). Chlorpromazine metabolism provides an example of S-oxidation by CYP3A (Scheme 11.22). Chlor promazine is also metabolized by N-oxidation and N-dealkylation pathways, resulting in a multiplicity of excreted products. 

SCHEME 11.21 Sulfur is readily oxidized, nonenzymatically as well as enzymatically. 

Although sulfide (mainly H2S) may be oxidized nonenzymatically to elemental sulfur by giving its electrons to ferricytochrome c, sulfide-cytochrome c... 

The major cause of consumer complaints to the food industry is an objectionable flavor [1]. Objectionable (or unacceptable) flavor may be due to incidental contamination of the food from environmental sources (e.g., air, water, or packaging materials) and is termed tainted, or may arise in the food itself as a result of degradation of some food component (e.g., lipid oxidation, nonenzymatic browning, or enzymatic action) and is considered off-flavored [2], A third canse of unacceptable flavor, which is not generally recognized and is much more poorly defined, is when the flavor is not satisfactory due to the loss of characteristic flavor (evaporation or reaction of characterizing flavor components with the food itsell) [3]. The food has neither been tainted nor developed an off-flavor. This source of an unacceptable flavor is difficult to diagnose and correct. 

Foods contain a number of constituents that may either react with other constituents or simply degrade into off-flavor components. The major reactions leading to off-flavors include lipid oxidation, nonenzymatic browning, enzymatic changes, and photo-catalyzed reactions. This section will present some examples of off-flavors arising in foods from these reactions. 

The mechanism of this degradation has received considerable attention, and for some species the reaction is equivalent to a nonenzymatic Baeyer-Villager reaction, producing first the 17j8-acetate. This functionality can then be hydrolyzed and oxidized to the ketone and may undergo a second Baeyer-Villager reaction to produce a lactone ... 

Nakamura, H., etal. (1989). Structure of dinoflagellate luciferin and its enzymatic and nonenzymatic air-oxidation products./. Am. Chem. Soc. Ill 7607-7611. 

Figure 5.2 Cyclic deracemization process involving sequential enzyme-catalyzed oxidation and nonenzymatic reduction.

Worobey BL (1989) Nonenzymatic biomimetic oxidation systems theory and application to transformation stndies of environmental chemicals. Handbook Environ Chem 2E 58-110. 

SCHEME 10.5 Proposed pathway for the nonenzymatic conversion of BHTOOH to BHT-QM in keratinocytes. BHTOOH is oxidized to a peroxy radical that spontaneously loses oxygen. Two BHT phenoxy radicals then undergo disproportionation. 

However, the metabolic pathways of lutein and zeaxanthin are only beginning to be discovered. Several derivatives of dietary xanthophylls have been identified in the retina, such as 3 -epilutein, meso-zeaxanthin, 3 -oxolutein, and 3-methoxyzeaxanthin, and it has been suggested that they may be formed as a result of nonenzymatic oxidative modifications (Bernstein et al., 2001,2002b Bhosale et al., 2007b Khachik et al., 1997). The macula lutea contains predominantly meso-zeaxanthin (Figure 15.1), which is believed to originate from either oxidative modification or double bond isomerization of dietary lutein (Khachik et al., 1997, 2002). 

In 1996, the first successful combination of an enzymatic with a nonenzymatic transformation within a domino process was reported by Waldmann and coworkers [6]. These authors described a reaction in which functionalized bicy-clo[2.2.2]octenediones were produced by a tyrosinase (from Agaricus bisporus) -catalyzed oxidation of para-substituted phenols, followed by a Diels-Alder reaction with an alkene or enol ether as dienophile. Hence, treatment of phenols such as 8-1 and an electron-rich alkene 8-4 in chloroform with tyrosinase in the presence of oxygen led to the bicyclic cycloadducts 8-5 and 8-6 in moderate to good yield (Scheme 8.1). It can be assumed that, in the first step, the phenol 8-1 is hydroxylated by tyrosinase, generating the catechol intermediate 8-2, which is then again oxidized enzy-... 

The oxidized mediator form, produced in the course of the enzymatic reaction, can nonenzymatically oxidize compounds (including nonphenolic lignin structures) with ionization potentials exceeding the potentials of laccases (Morozova and others 2007). 

The CL reaction of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) (1) is one of the more commonly used nonenzymatic CL reactions and has been extensively studied [49, 57-59], It is well known that the luminol CL reaction is catalyzed by many kinds of substances, e.g., ozone, halogen-Fe complex, hemin, hemoglobin, persulfate, and oxidized transition metals. The most acceptable scheme is shown in Figure 10. Luminol forms a six-membered ring of peroxide (3) from a diazaquinone intermediate (2) and then, by the decomposition of 3, N2 gas and the Si-excited state of the phthalate dianion are produced, yielding... 

In the last decade numerous studies were dedicated to the study of biological role of nonenzymatic free radical oxidation of unsaturated fatty acids into isoprostanes. This task is exclusively difficult due to a huge number of these compounds (maybe many hundreds). Therefore, unfortunately, the study of several isoprostanes is not enough to make final conclusions even about their major functions. F2-isoprostanes were formed in plasma and LDL after the treatment with peroxyl radicals [98], It is interesting that their formation was observed only after endogenous ascorbate and ubiquinone-10 were exhausted, despite the presence of other antioxidants such as urate or a-tocopherol. LDL oxidation was followed by... 

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