Udenfriend reaction

When using the Udenfriend reaction for producing -OH, one has to keep in mind that OH reacts very fast with ascorbate (k = 7.9 x 109 dm3 mol-1 s"1 Red-path and Willson 1973) whereby not only the ASC radical but also adduct radicals are formed (Fessenden and Verma 1978 see also Schuler et al. 1974 Cabelli and Bielski 1983 Cabelli et al. 1984). 

It appears from the previous discussion that melanogenesis in vivo or in vitro is regulated by various factors. Hence, to study the dynamics of melanin formation (monooxygenase reaction, i.e. constructive metabolism) and breakdown (dioxygenase reaction, i.e. catabolism of melanin precursors), a non-enzymatic melanin synthesis from tyrosine and tryptophan, respectively, was devised by Roy et al. (227), using a prototype of a monooxygenase reaction, i.e. the Udenfriend reaction (Fe+ /EDTA/ ascorbic acid) (272). 

Synthesis of 2-hydroxy-3-methylcarbazole (83) has also been accomp-hshed from 3-methylcarbazole (62) in the Udenfriend reaction (722), which is a prototype of biological hydroxylation with mixed function oxidases and monoxygenases. 

Udenfriend et al. observed that aromatic compounds are hydroxyl-ated by a system consisting of ferrous ion, EDTA, ascorbic acid, and oxygend Aromatic and heteroaroinatic compounds are hydroxylated at the positions which are normally most reactive in electrophilic substitutions. For example, acetanilide gives rise exclusively to the o-and p-hydroxy isomers whereas quinoline gives the 3-hydroxy prod-uct. - The products of the reaction of this system w ith heterocyclic compounds are shown in Table XIII. 

It is notable that flavone and its derivatives are substituted predominantly in the phenyl group at positions 3 and 4 whereas reaction with Fenton s reagent gives mainly the S-hydroxy isomer." " Similarly, comparison of the distribution of products from coumarin obtained with Udenfriend s system with that given by Fenton s reagent re-... 

Breslin DT, Schuster GB (1996) Anthraquinone photonucleases mechanisms for GG-selective and nonselective cleavage of double-stranded DNA. J Am Chem Soc 118 2311-2319 BrodieBB, Axelrod J, Shore PA, Udenfriend S (1954) Ascorbic acid in aromatic hydroxylation. II. Products formed by reaction of substrates with ascorbic acid, ferrous ion, and oxygen. J Biol Chem 208 741-750... 

Certain fluorescent compounds, such as the coumarin derivative, scopoletin, can act as hydrogen donors in the oxidative reaction catalysed by horseradish peroxidase (Udenfriend, 1969), an enzyme that exhibits substrate specificity for hydrogen peroxide... 

The reaction sequences originally suggested for oxygen insertion in a substrate such as salicyclic acid in Udenfriend s System (19-22) in which molecular oxygen is the oxidant and an iron chelate such as Fe(II)-EDTA is the catalyst and ascorbic acid is the reductant, is indicated by Equations 14-18. In this reaction sequence oxygen insertion is considered to occur by direct reaction of the aromatic compound with HO2 and OH free radicals, and the ascorbate reductant merely serves the purpose of regenerating the Fe(II) chelate as indicated by Equations 16 and 17. 

Udenfriend et al developed a system for effecting aromatic hydroxylation consisting of ferrous sulfate, oxygen, ascorbic acid, and ethylenediaminetetraacetic acid (EDTA) in a phosphate buffer and noted that, in the cases tried, oxygen could be replaced by hydrogen peroxide in a nitrogen atmosphere. EOT A is not essential but greatly increases the reaction rate. Dihydroxymaleic acid and diethyl diketosuccinate were about as active as ascorbic acid alloxan and ninhydrin were less active. Examples ... 

This transformation, considered likely on the basis of experiments with the intact animal, was proved by the isotopic experiments of Moss and Schoenheimer (614), which also showed it to be an automatic process. Udenfriend and Cooper (886) found a highly specific enzyme in liver which carried out this reaction, possibly in two stages. The system required oxygen and DPN (diphosphopyridinenucleotide) and was considered responsible for the greater part of the normal metabolism of phenylalanine. 

Brodie, B.B. Alexrod, J. Shore, P. A. Udenfriend, S. Ascorbic Acid in Aromatic Hydroxylation ((. Products Formed by Reaction of Substrates with Ascorbic Acid, Ferrous Ion, and Oxygen ... 

Other oxidation reactions maybe useful in gaining a better understanding of biochemical mechanisms involved in the endogenous production of neurotransmitters (Arnow 1942) (Broodie 1954) (Dalgliesh) (La Du 1955) (Raper 1932) (Udenfriend 1954). 

Fluorescamine (Udenfriend et al., 1972 Weigele et al., 1972) was adapted for use with seawater samples by North (1975), Packard and Dortch (1975) and Zika (1977). It may be employed to make some distinction between free amino acids and peptides or proteins by changing the reaction pH from 9.0 to 7.0, respectively. The reaction takes place at room temperature with only low ammonia interference. On the other hand, fluorescamine is unstable in aqueous solutions and the relative fluorescence intensities for the various amino acids vary over a wide range (Zika, 1977). 

Unlike Udenfriend s system, which is active with several different metal ions (29), hydroxylation of aromatic compounds using NBNH and oxygen occurs only in the presence of ferric ions. Ceric or cupric sulfate are unable to activate the system, and more surprisingly, ferrous sulfate is ineffective. Ferrous ions under the reaction conditions (pH 4.2) are relatively stable and are oxidized only slowly to ferric ions by molecular oxygen thus, it would seem that only negligible amounts of ferric ions are formed during the reaction period, and hence no hydroxylation occurs. 

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