Vitamin redox potential

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

The ascorbate radical is one of the radicals that do not react readily with 02, but it reacts with 02 ". The product of this reaction is not yet known. There are other radicals that have similar properties such as phenoxyl-type radicals. A prominent member of this group is the vitamin E radical. In the phenoxyl radical series, addition as well as ET have been discussed (Jonsson et al. 1993 d Alessandro et al. 2000). The reaction of the tyrosyl radical with 02 is an example showing that addition is the main route despite of its relatively high redox potential [reactions (97)—(99) only one pathway is shown Jin et al. 1993],... 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

In order to more directly investigate the regulation of metabolism by the redox potential of the cell, we have recently labeled the intracellular pools of pyridine nucleotides in the liver, using the C-labeled vitamins, nicotinic acid and nicotinamide ... 

Flavoenzymes are widespread in nature and are involved in many different chemical reactions. Flavoenzymes contain a flavin mononucleotide (FMN) or more often a flavin adenine dinucleotide (FAD) as redox-active prosthetic group. Both cofactors are synthesized from riboflavin (vitamin B2) by microorganisms and plants. Most flavoenzymes bind the flavin cofactor in a noncovalent mode (1). In about 10% of aU flavoenzymes, the isoalloxazine ring of the flavin is covalently linked to the polypeptide chain (2, 3). Covalent binding increases the redox potential of the flavin and its oxidation power, but it may also be beneficial for protein stability, especially in flavin-deficient environments. 

Physiological Function. The mechanism by which L-ascorbic acid benefits an insect is unknown. The vitamin is found in many tissues where it probably plays a variety of roles related to its redox potential. Besides the possible general function of detoxifying superoxide and hydrogen peroxide, L-ascorbic acid may be involved in metabolic processes such as tyrosine metabolism, collagen formation, steroid synthesis, detoxification reactions, phagostimulation, or neuromodulation. At this time one can only speculate about the function of vitamin C in some specific tissues. 

This new strategy consists of the synthesis of molecules that possess hydroxyl groups in such positions that a radical species can be stabilized by mesomery. This feature is found in natural flavonol such as kaemferol that possesses undeniable antioxidant properties. The first published example is a lipophilic analogue of vitamin C, Fig (13) [40]. In this paper, we have proved that the synthesised substituted 2-hydroxyfuran-2-one is a true ascorbic acid analogue. A radical anion that gives very similar data has been generated under the same conditions as for ascorbic acid with a stability somewhat lower and a redox potential lower than those of ascorbyl radical. Its antioxidant properties are also similar to that of ascorbic acid but it inhibits LDL peroxidation induced by Cu2+ or AAPH more efficiently probably due to a higher lipophilicity. 

The decrease in redox potential produced by lamps with a solar emission spectrum, or fluorescent tubes, corresponds to the bleaching of white wines by reducing vitamin B2 to its (colorless) form (Figure 8.29). The redox potential of a Champagne may drop by over 100 mV. The confusion between reduction flavor and sunlight flavor is, therefore, quite legitimate. 

Based on redox potentials and species concentrations, Goldstein et al. (80) suggested an alternative mechanism (reactions (41) and (42)) to explain the unique toxicity of O2 compared to other biological reductants like glutathione and vitamin C. In the alternative system, the copper is oxidized rather than reduced as in reaction (36). The active species in this mechanism is trivalent copper. 

In a very recent review, Terroine (1960) discussed the way in which ascorbic acid might act in the sparing of vitamins. She leans heavily on the hypothesis that ascorbic acid seems to exercise its protective property as a non-specific pharmacodynamic redox agent, and she thinks that this applies particularly to thiamine and, with less certainty, to biotin. Thus, she says the oxidation cycle of the ternary chains in which thiamine participates is only a long succession of oxidation-reduction reactions. In the absence of thiamine, ascorbic acid might ensure the normal working of these operations by meeting the required redox potential. ... 

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