Hydroquinone, biological

In experimental animals and in vitro, DHBs show a variety of biological effects including binding of metaboHtes to various proteins. Clastogenic effects have been observed in vitro and in some in vivo studies with the three compounds. No reproductive effects have been shown by conventional studies with either hydroquinone, catechol, or resorcinol (122). Hydroquinone has been shown to induce nephrotoxicity and kidney tumors at very high doses in some strains of rat (123) catechol induces glandular stomach tumors at very high dose (124). Repeated dermal appHcation of resorcinol did not induce cancer formation (125). 

For over 35 years, the quinone methide species has been invoked as a reactive intermediate in bioreductive alkylation and in other biological processes.8 29 Generally, there is only circumstantial evidence that the quinone methide species forms in solution. Conceivably, the O-protonated quinone methide (i.e., the hydroquinone carbocation) could be the electrophilic species. If so, bioreductive alkylation may simply be an SN1 reaction. Also, there are questions concerning the mechanism of quinone methide... 

To reach the reductive step of the azo bond cleavage, due to the reaction between reduced electron carriers (flavins or hydroquinones) and azo dyes, either the reduced electron carrier or the azo compound should pass the cell plasma membrane barrier. Highly polar azo dyes, such as sulfonated compounds, cannot pass the plasma membrane barrier, as sulfonic acid substitution of the azo dye structure apparently blocks effective dye permeation [28], The removal of the block to the dye permeation by treatment with toluene of Bacillus cereus cells induced a significant increase of the uptake of sulfonated azo dyes and of their reduction rate [29]. Moreover, cell extracts usually show to be more active in anaerobic reduction of azo dyes than whole cells. Therefore, intracellular reductases activities are not the best way to reach sulfonated azo dyes reduction the biological systems in which the transport of redox mediators or of azo dye through the plasma membrane is not required are preferable to achieve their degradation [13]. 

A species must fulfil five simple tests for it to be considered as a suitable choice of mediator (see Table 5.1). The most common mediators for biological systems are methyl viologen, benzyl viologen and hydroquinone. Each of these mediators obey all five of the criteria presented in this table. 

Biological reductive dissolution by Shewanella putrifaciens of Fe oxides in material from four Atlantic pleistocene sediments (ca. 1.5-41 g/kg Fe oxides) was compared with that of the synthetic analogues (ferrihydrite, goethite, hematite) (Zachara et al. 1998). In the presence of AQDS as an electron shuttle, the percentage of bio-reduc-tion of the three oxides was increased from 13.3 %(fh) 9.2%(gt) and 0.6%(hm) to 94.6% 32.8% and 9.9% with part of the Fe formed being precipitated as vivianite and siderite, but not as magnetite. The quinone was reduced to hydroquinone which in turn, and in agreement with thermodynamics, reduced the Fe as it had much better access to the oxide surface than did the bacteria themselves. 

The ongoing research into the structure and mechanism of flavoenzymes has been the subject of several recent excellent reviews The proceedings of six symposia held at intervals over the past 16 years provide an overall perspective on the progress of flavoenzyme research over this time period. The intent of this article will be to focus directly on the chemical and physical properties of the semiquinone form of flavin coenzymes to the extent that current knowledge permits, from the point of view of both model system studies and from existing knowledge of their properties in flavoenzyme systems. For an in-depth treatment of flavin and flavoenzyme redox properties in which the oxidized and hydroquinone forme as well as the semiquinone form are discussed as related to their biological function, the reader is refered to the article by F. Muller in this volume. 

Li, Y. Tmsh, M.A. (1993b) Oxidation of hydroquinone by copper chemical mechanism and biological effects. Arch. Biochem. Biophys., 300, 346-355... 

Additionally, during the search for biologically active sponge metabolites belonging to the sesterterpenoid class, a sulfated sesterterpene hydroquinone, halisulphate, Fig. (20), was isolated from the dark brown sponge Halichotidriidae sp. 

An assay for the determination of vitamin K3 (2-methyl-1,4-naphthoquinone) by a combination of both constant-potential and constant-current cou-lometry has been reported [13]. The assay requires the two-electron reduction of the compound to the corresponding hydroquinone at a mercury pool electrode (E = -0.60 V vs. SCE) in acetate buffer, pH 5.9, followed by the coulometric titration of the reduction product with electrogenerated Ce(VI). This method is preferable to the standard method requiring preliminary reduction to the hydroquinone by zinc dust in acid medium, followed by titration with standard Ce(IV) solution. It is capable of low-level determination (1-2 mg) of this vitamin in pharmaceuticals, biological fluids, and foods. 

Phenolic substances are for the most part readily oxidized at a graphite electrode. The oxidation potentials for phenols vary widely with structure, and some (hydroquinones and catechols) are far more readily oxidized than others (cresols). Many compounds of biological interest (catecholamines, pharmaceuticals, plant phenolics) [32] and industrial interest (antioxidants, antimicrobials, agricultural chemicals) [33] are phenolic, and trace determination based on LCEC is now quite popular. 

The sesquiterpene quinones and hydroquinones display a wide range of biological activities.4 Some of the compounds are cytu-toxic, whereas others show antimicrobial characteristics. Avarol (2) has been the subject of investigation as a consequence of its ability to inhibit reverse transcriptase, but no clinical value (as an anti-AIDS agent, for example) has yet been established.3... 

The oxidation of carbohydrate involves the making and breaking of C-H bonds. This is a two electron process, and the most efficient way of achieving the oxidation will use a two electron oxidant. In many biological systems one of the key redox steps involves the two-electron conversion of a quinone to a hydroquinone (Fig. 10-6). 

The importance of non - covalent interactions in biological systems has motivated much of the current interest in supramolecular assemblies [1]. A classical example of a supermolecule has been provided by the rotaxanes [2,3], in which a molecular rotor is threaded by a threaded by a linear axle . Another examples have been previously included as cyclic crown ethers threaded by polymers, paraquat -hydroquinone complexes [4] and cyclodextrin complexes [5,6],... 

Apart from these systems, which involve a metal on which is adsorbed a modifier (see above), there is another kind of experiment, although one from which data are as yet less available. One can make up a surface of a metal covered with a biolipid membrane (90% lipid and 10% proteins). There is evidence that some of the proteins in these ensembles are themselves the origin of electrons that can exchange with small redox molecules (e.g., the quinone-hydroquinone system) in solution. Such evidence (though scarce) is significant, for there are no metal underlayers in real biological systems, yet interfacial electron transfer seems to be common there. 

The bombardier beetle] squirts a lethal mixture of hydroquinone and hydrogen peroxide into the face of its enemy. These two chemicals, when mixed together literally explode. So in order to store them inside its body, the bombardier beetle has evolved a chemical inhibitor to make them harmless. At the moment the beetle squirts the liquid out of its tail, an antiinhibitor is added to make the mixture explosive once again. The chain of events that could have led to the evolution of such a complex, coordinated and subtle process is beyond biological explanation on a simple step-by-step basis. The slightest alteration in the chemical balance would result immediately in a race of exploded beetles.17... 

Unlike irreducible complexity (where we can enumerate discrete parts), minimal function is sometimes hard to define. If one revolution per hour is insufficient for an outboard motor how about a hundred Or a thousand Nonetheless, minimal function is critical in the evolution of biological structures. For example, what is the minimum amount of hydroquinone that a predator can taste How much of a rise in the temperature of the solution will it notice If the predator didn t notice a tiny bit of hydroquinone or a small change in temperature, then our Dawkins-esque tale of the bombardier beetle s evolution can be filed alongside the story of the cow jumping over the moon. Irreducibly complex systems are nasty roadblocks for Darwinian evolution the need for minimal function greatly exacerbates the dilemma. 

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