Oxindolylalanine Hydroxytryptophan

It has been suggested that H2O2 might add to the double bond of the indolyl ring to give either 2-hydroxy- or 2,3-dihydroxytryptophan. Since 2-hydroxytryptophan is the tautomer of /3-3-oxindolylalanine, the arguments against the latter also apply to it. 

If it is the first intermediate, it can be speculated that the 2,3-dihydroxy-tryptophan, by dehydrogenation with simultaneous ring opening, could yield formylkynurenine. This is a hypothesis which remains to be proved. 

After the oxidation of tryptophan to kynurenine, the side chain can be cleaved to yield anthranilic acid, or if prior oxidation to 3-hydroxy-kynurenine takes place, the product is 3-hydroxyanthraniIic acid. Alternatively, probably after the removal of the a-amino group to form the a-keto acids corresponding to kynurenine and 3-hydroxykynurenine, ring closure of the side chain can occur to yield kynurenic and xanthurenic acids. These alternative reactions will be discussed in turn. 

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The nature of the intermediate A aroused widespread interest. The work already described suggested that it was a-hydroxytryptophan (more correctly described as oxindolylalanine, 157), which chemically is a plausible precursor of formylkynurenine (c/. review, 170). 

Another interesting study by Simat and Steinhart [44] focused on the oxidation of native Trp in peptides and proteins. Trp is one of the most readily oxidized residues in proteins and monitoring this degradation is important in a number of areas including stability of protein-based pharmaceuticals and other samples where long-tenn protein stability is of interest. Trp oxidation products may include kynurenine, 77-fonnylkynurenine, oxindolylalanine (Oia), and 5-hydroxytryptophan. Identification of these breakdown products in peptides and... 

In this article the term oxindole has been used instead of 2-hydroxy-indole , in accordance with Dalgliesh and coworkers 85), who reported that the absorption data suggest that the structure of the hydroxy derivative of tryptophan is best represented by a lactam structure the term hydroxytryptophan is therefore misleading and should be replaced by oxindolylalanine . 

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