Udenfriend System A Model for Mixed Function Oxidase

The Udenfriend System A Model for Mixed Function Oxidase 

A truly enormous number of different enzymes has been characterized as mixed function oxidases (MFOs) by Mason (162) and as monooxygenases by Hayaishi (107). They can catalyze a wide variety of oxidation reactions including hydroxylation of aromatic and aliphatic compounds, epoxidation of olefins, oxidative decarboxylation, lactoniza-tion etc. (104). According to Hamilton (105), MFO-catalyzed reactions proceed via an oxene mechanism and there is now ample evidence that the oxinoid species is a reduced form of the enzyme (or an enzyme with a reduced cofactor, e.g. metal ions) which reacts with oxygen and the substrate either in one step or more than one (273). 

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). 

The possible function of an EDTA ligand is to protect the nucleophilic oxygen in A from intermolecular reactions although the system operates also without it, even though more slowly (272, 273). 

In many cases two metal ions can perform the function of the reducing agent when oxygen is transferred to the substrate each metal ion is oxidized by one electron. This seems to be the explanation for many 

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