A-Tocopherylquinone

Antioxidants are compounds that inhibit autoxidation reactions by rapidly reacting with radical intermediates to form less-reactive radicals that are unable to continue the chain reaction. The chain reaction is effectively stopped, since the damaging radical becomes bound to the antioxidant. Thus, vitamin E (a-tocopherol) is used commercially to retard rancidity in fatty materials in food manufacturing. Its antioxidant effect is likely to arise by reaction with peroxyl radicals. These remove a hydrogen atom from the phenol group, generating a resonance-stabilized radical that does not propagate the radical reaction. Instead, it mops up further peroxyl radicals. In due course, the tocopheryl peroxide is hydrolysed to a-tocopherylquinone. 

When 6-hydroxychromans act as antioxidants, they are themselves oxidized to one or more of several compounds depending on the conditions. Amongst the commonest products from a-tocopherol are a-tocopherylquinone (702), a-tocored (703), the 5-benzoyloxy-methyl derivative (704) and the spiro dimer (705). 

The rate constant for the bimolecular decay of the a-tocopheroxyl radical is only 3.5x 102M-1 s . Therefore, its half-life is several hours in chloroform at ambient temperature. This implies that vitamin E free radical can react with a second peroxyl radical. In biological membranes, a-tocopherylquinone is generally believed to be the major end-product, but the mechanism of its production remains controversial. It may arise either from the decomposition of a-tocopherone, or from dismutation of a-tocopheroxyl radicals. However, the steady-state concentration of a-tocopherylquinone is usually too low to be measurable ex vivo when tissue homogenization and extraction are performed in the presence of pyrogallol and butylated hydroxytoluene, respectively,... 

This is a specific reagent for the oxidation of o-tocopherol to a-tocopherylquinone. Addition of an aqueous solution of gold ehloride to a solution of a-tocopherol in... 

Marcus and Hawley used a chronoamperometric method to study the kinetics and mechanism of the ring opening of 7-hydroxy-2,2,5,7,8-pentamethyl-6-chromanone (III IV, Figure 16), a model of the hemiketal intermediate in the oxidation of a-tocopherol to a-tocopherylquinone. In... 

Marcus and Hawley have also studied the electrochemical reduction of a-tocopherylquinone and 2,3j5-trimethyl-6-(3 -methyl-3 -hydroxy-butyl)quinone (2,3>5-TMHQ, 15) in nonaqueous solvents. The electrochemical... 

A cyclic voltammogram of 2,3,5-TMHQ in acetonitrile is shown in Figure 18. Peak Ic is a reversible e reduction of a-tocopherylquinone or its model quinone (Q) to an anion radical (Q ) [Eq. 11a)] and peak lie the further reversible e reduction of the anion radical to a dianion [Q", Eq. (11b)]. 

This behavior is typical of simple quinones in aprotic solution. " However, the electrochemical behavior of 2,3,5-TMHQ and a-tocopherylquinone in acetonitrile is altered considerably by the addition of a weak acid such as ethyl malonate (Figure 19). Thus, the peak Ic process remains unchanged but reduction peak lie broadens and shifts towards more positive potentials. In addition peak Ila, corresponding to electrooxidation of the quinone dianion... 

In the presence of a stronger proton donor such as benzenethiol a cyclic voltammogram of 2,3,5-TMHQ or a-tocopherylquinone shows some profound changes (Figure 20). Thus, peak Ic is moved to more positive potentials and peak lie and la disappear. A scheme consistent with these observations is shown in Eq. (13). 

The electrochemical behavior of a-tocopherylquinone and related model compounds points out a number of interesting facts. For example, in aprotic media in the absence of protons or proton donors essentially all biological quinones, including a-tocopherylquinone, are electrochemically reduced in stepwise e processes giving first an anion radical and then a dianion. Both these processes are electrochemically reversible. However, addition of protons or a proton donor to the a-tocopherylquinone or other bioquinones in an... 

A reaction of special interest found in our laboratories is the reductive cyclization of trimethylphytylbenzoquinone into a-tocopherol by means of a series of acidic reagents, e.g., boron trifluoride in petroleum ether (Fig. 12). This transformation which probably involves a hydride transfer reaction, can also be achieved with a-tocopherylquinone (Issidorides, 19. il) and... 

The tocopherylquinones absorb much more intensely than the tocopherols. For a-tocopherylquinone, the value Eitm = 425 was found (Kofler, 1947), but the absorption maxima are at shorter wavelengths (260 and 269 m ) so that considerable irrelevant absorption must be anticipated. The absorption maxima of /3-, y-, and -tocopherylquinone are at 255 and 263 m/i (Eggitt and Norris, 1956). 

The potencies of various tocopherols, K vitamins, ubiquinones, and similar substances in the prevention of respiratory decline have been assayed by supplementation of graded dose levels to the whole homogenate system. For each dose level, averages of at least four experiments were established. From the dose-response curves, the 50 % effective dose (EDbo) was derived. The EDso for df-a-tocopherol was found to be 1.1 ng per 3 ml of medium containing 50 mg of homogenized liver (Table VII). A comparison of the potencies of a-, /3-, 7-, and 5-tocopherols gave the same relative order of activity as the resorption sterility assay for vitamin E. df-a-To-copheryl acetate demonstrated only 3 % of the activity of the free vitamin. a-Tocopherylquinone was about 10% as active as tocopherol itself. The Simon metabolite (Simon et al., 1956) (Fig. 4) showed 40% of the activity of di-a-tocopherol, in spite of the fact that it was supplied in the quinone form. 

Vitamin Vitamin Kg Vitamin Kg Plastoquinones Ubiquinones, Coenzyme Q a-Tocopherylquinone... 

Tocopherols and tocotrienols are abundant in seeds, grains, and vegetable oils (46), but only the tocopherols are absorbed by humans to an appreciable extent (47). a-Tocopherol is the predominant form in plasma and is biologically the most active homolog. Although its exact biochemical function in humans is controversial, vitamin E is known to protect membrane lipids from peroxidation by scavenging free radicals and other oxidative species (48). A number of oxidized metabolites, e.g. a-tocopherylquinone have been suggested (49). 

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