Udenfriend system

Most of the work reported with these complexes has been concerned with kinetic measurements and suggestions of possible mechanisms. The [Ru(HjO)(EDTA)] / aq. HjOj/ascorbate/dioxane system was used for the oxidation of cyclohexanol to cw-l,3-cyclohexanediol and regarded as a model for peroxidase systems kinetic data and rate laws were derived [773], Kinetic data were recorded for the following systems [Ru(Hj0)(EDTA)]702/aq. ascorbate/dioxane/30°C (an analogue of the Udenfriend system cyclohexanol oxidation) [731] [Ru(H20)(EDTA)]70j/water (alkanes and epoxidation of cyclic alkenes - [Ru (0)(EDTA)] may be involved) [774] [Ru(HjO)(EDTA)]702/water-dioxane (epoxidation of styrenes - a metallo-oxetane intermediate was postulated) [775] [Ru(HjO)(EDTA)]7aq. H O /dioxane (ascorbic acid to dehydroascorbic acid and of cyclohexanol to cyclohexanone)... 

Scheme 4. Proposed mechanism for Fe(II)-catalyzed oxygen insertion (Udenfriends system).

The Udenfriend system of 1954 was perhaps the first to be specifically presented as a model of a biological process. In this system, Fe(II) is the catalyst, EDTA the ligand, air is... 

Udenfriend suggested a simple non-enzymatic system as a possible model of monooxygenase [44]. It involves an iron(II) complex with EDTA and ascorbic acid and is capable of hydroxylating aromatic compounds. Later, Hamilton found that this system was able to hydroxylate cyclohexane (however, with very low yield) and epoxidize cyclohexene [45]. Afterwards, a number of similar systems were proposed. For example, Ullrich found two models which are more effective than the Udenfriend system. One of them includes a Sn(II) phosphate complex... 

The first model proposed for the drug monooxygenase stem in liver microsomes was described by Udenfnend et al. ) and since then referred to as the Udenfriend system . It consists of ferrous iron, EDTA and ascorbate which are incubated aerobically in an aqueous buffer of pH 5—8. Aromatic compounds were hydroxylated in this mixture to phenols and the substitution pattern pointed to an electrophilic mechanism. Breslow and Lukens argued that the Udenfriend system was not different from the Fenton stem since the autoxidation of ascorbate provides hydrogen peroxide, which by the reduction with ferrous ions forms hydroxyl radicals as hydroxylating agent ... 

Later, by studying carefully the product patterns in the Udenfriend system and in a system in which hydrogen peroxide was reduced by the Fe /EDTA complex in the presence of ascorbate, it was postulated by Staudinger and Ullrich that besides hydroxyl radicals a second hydroxylation mechanisms must be present, which was characterized by a random distribution of phenolic products in contrast to the electrophilic pattern observed with OH radicals. This mechanism, called the oxenoid mechanism, was found in all systems consisting of autoxidizing metal ions, but hydroxyl radicals were usually the predominant hydroxylating species. 

The Udenfriend System A Model for Mixed Function Oxidase... 

Many MFO-catalyzed reactions possess characteristic similarities to model reactions and since these enzymatic reactions occur very fast and the intermediates are too short lived, non-enzymatic model systems have been developed to study oxene mechanism. One specific example is the Udenfriend system (272) which involves the hydroxylation of aromatic compounds under physiological conditions by a mixture of ascorbate, Fe(n), EDTA, and O2. According to Hamilton (105), the intermediate complex A formed during the oxidation reaction (Fig. 7), is believed to be an oxinoid species which transfers electrons to the oxygen atom in the transition state and probably, at the same time, allows the transfer of singlet oxygen from complex A to substrate in one step so that diradical intermediates would not be necessary (705). 

One model reaction which has been closely scrutinised " was described by Udenfriend and his collaborators . These workers observed that aromatic compounds are hydroxylated in low yield by oxygen at neutral pH and ambient temperatures in presence of Fe (II) and ascorbic acid. Various species such as H2O2, HO and HOO have been ruled out as intermediates " in this reaction and Hamilton has suggested the mechanism shown in Figure 4.9 for oxidations utilising the Udenfriend system,... 

The first system to be developed involves in-vitro chemical activation, and is sometimes known as the Udenfriend system. This procedure entails exposing the test compounds to oxidising conditions in a solution of ascorbic acid, ferrous sulphate and ethylenediamine tetraacetatic acid (EDTA) under oxygen pressure at a pH of 5-5. Activation of dimethyl nitrosamine to a bacterial mutagen has been achieved in this manner. Although this procedure has been used successfully with other compounds, it is felt that other methods of activation, namely use of mammalian systems, may be more appropriate. 

Fig. 11. Action of catalase on tyrosine hydroxylation by the Udenfriend system. Total volume 6 ml., containing 40 /imoles of ascorbic acid, and 20 fxmoles of tyrosine. Temperature 20°C., pH 7, 30 minutes. On the abscissae milligrams of catalase added. On the ordinates imoles of dopa formed (open circles) and /moles of ascorbic acid oxidized (solid circles). From Zito and Kertesz (1962).

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