Free radicals flavin

He/minthosporium (15). The mode of action is considered to be inhibition of the enzyme NADPH-cytochrome C reductase, which results in the generation of free radicals and/or peroxide derivatives of flavin which oxidize adjacent unsaturated fatty acids to dismpt membrane integrity (16) (see Enzyme inhibitors). 

Ehrenberg, A., Eriksson, L.E.G., and Hyde, J.S. 1968. Electron-nuclear double resonance from flavin free radicals in NADPH dehydrogenase ( old yellow enzyme ). Biochimica et Biophysica Acta 167 482-484. 

At the initial step electrons are transferred from NADPH to the oxidized FAD, reducing it to FADH2. Disproportionation between flavins leads to the formation of two free radicals FADH and FMNH. Electron transfer from FMNH to the heme results in the reduction of Fe3+ to Fe2+, and the reduced heme becomes able to bind 02 to form the intermediate... 

FIGURE 32-7 Sources of free radical formation which may contribute to injury during ischemia-reperfusion. Nitric oxide synthase, the mitochondrial electron-transport chain and metabolism of arachidonic acid are among the likely contributors. CaM, calcium/calmodulin FAD, flavin adenine dinucleotide FMN, flavin mononucleotide HtT, tetrahydrobiopterin HETES, hydroxyeicosatetraenoic acids L, lipid alkoxyl radical LOO, lipid peroxyl radical NO, nitric oxide 0 "2, superoxide radical. 

In the reduction or oxidation of quinone/ quinol systems, free radicals also appear as intermediate steps, but these are less reactive than flavin radicals. Vitamin E, another qui-none-type redox system (see p.l04), even functions as a radical scavenger, by delocalizing unpaired electrons so effectively that they can no longer react with other molecules. 

Otvos, J. D., Krum, D. P., and Masters, B. S. (1986). Localization of the free radical on the flavin mononucleotide of the air-stable semiquinone state of NADPH-cytochrome P-450 reductase using 3IP NMR spectroscopy. Biochemistry 25, 7220-7228. 

FMN consists of the structure above the dashed line on the FAD (oxidized form). The flavin nucleotides accept two hydrogen atoms (two electrons and two protons), both of which appear in the flavin ring system. When FAD or FMN accepts only one hydrogen atom, the semiquinone, a stable free radical, forms... 

Three facts account for the need of cells for both the flavin and pyridine nucleotide coenzymes (1) Flavins are usually stronger oxidizing agents than is NAD+. This property fits them for a role in the electron transport chains of mitochondria where a sequence of increasingly more powerful oxidants is needed and makes them ideal oxidants in a variety of other dehydrogenations. (2) Flavins can be reduced either by one- or two-electron processes. This enables them to participate in oxidation reactions involving free radicals and in reactions with metal ions. (3) Reduced flavins... 

However, these experiments may not have established a mechanism for natural flavoprotein catalysis because the properties of 5-deazaflavins resemble those of NAD+ more than of flavins.239 Their oxidation-reduction potentials are low, they do not form stable free radicals, and their reduced forms don t react readily with 02. Nevertheless, for an acyl-CoA dehydrogenase the rate of reaction of the deazaflavin is almost as fast as that of natural FAD.238 For these enzymes a hydride ion transfer from the (3 CH (reaction type D of Table 15-1) is made easy by removal of the a-H of the acyl-CoA to form an enolate anion intermediate. 

A long-known characteristic of D-amino acid oxidase is its tendency to form charge-transfer complexes with amines, complexes in which a nonbonding electron has been transferred partially to the flavin. Complete electron transfer would yield a flavin radical and a substrate radical which could be intermediates in a free radical mechanism, as discussed in the next section.256... 

A number of scientists in the late 1960s through the mid-1970s pointed out that many sources in biological systems produce oxygen free radicals. For example, some oxidative enzymes which contain flavin as a prosthetic group proceed by a radical mechanism. When illuminated,... 

Unusual features of riboflavin as recorded by some researchers include (1) High levels in liver inhibit tumor formation by azo compounds in animals (2) free radicals are formed by light or dehydrogenation flavine semiquinone dihydroflavin+ (3) free vitamin is found only in retina, urine, milk, and semen (4) substitution of adenine by other purines and pyrimidines destroys activity of flavin adenine dinucleotide (FAD) ... 

Evidence is presented in support of free-radical mechanisms for the oxidation of ionizable carbon acids by oxidized flavin. Activation of molecular oxygen by reduced flavin is shown to occur through the formation of a 4a-hydroperoxyflavin that may, dependent upon conditions and substrate, transfer one or two oxygen atoms. Examples of all reactions are provided in the text. The present state of knowledge concerning the chemiluminescent oxidations of aldehydes by 4a-hydroperoxyfla-vin is considered. 

The high concentration of polyunsaturated fatty acids in cellular and subcellular membranes makes them particularly susceptible to free radical damage. In addition, mitochondrial membranes contain flavins as a part of their basic structure, potentially contributing C>2 resulting in free radical damage. The process of uncontrolled lipid peroxidation can result in the loss of essential polyunsaturated fatty acids, and the formation of toxic hydroperoxides and other secondary products. The loss of essential fatty acids may then result in loss of membrane integrity and loss of function. Extensive oxidation can also lead to rupture of... 

One way of circumventing this activation energy barrier involves a free radical pathway in which a singlet molecule reacts with 302 to form two doublets (free radicals) in a spin-allowed process (Fig. 4.1, Reaction (1)). This process is, however, highly endothermic (up to 50 kcal mol-1) and is observed at moderate temperatures only with very reactive molecules that afford resonance stabilized radicals, e.g. reduced flavins (Fig. 4.1, Reaction (2)). It is no coincidence, therefore,... 

Although charge transfer in macromolecules is of fundamental interest and has been studied piecemeal, often with model systems, this field is in its infancy. DNA and RNA complexes have been discussed in Section III. D. The oxidized and reduced forms of flavin mononucleotide produce charge-transfer complexes as precursors of the semiquinone free radical... 

In sharp contrast to the reductive half-reaction, where the free oxidized flavin is totally inert in the presence of physiological substrates, reduced model flavins are appreciably reactive (nonenzymatically) with O2 and other electron acceptors. However, the O2 reactivity of reduced flavin is complicated for two perhaps related reasons (61). First, the reaction is autocatalytic owing to the formation of 2F (from F and FH2) which in its anionic state is extremely reactive with Oo. Second, the superoxide radical is an important kinetic intermediate in O2 reduction (59). Neither of these features is observed with the reduced flavoprotein oxidases. 

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