The Conversion of Tryptophan to Formylkynurenine and Kynurenine

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). 

The structure of the first intermediate in the normal biological degradation of tryptophan is therefore still uncertain. The difficulties in synthesizing any of the proposed intermediates are very considerable, and till such synthesis is accomplished conclusive evidence will not be available. The problem is made somewhat less urgent by the fact that, whatever the 

No cofactors have been reported in the tryptophan peroxidase-oxidase reaction, but the marked reduction in the conversion of tryptophan to nicotinic acid in thiamine deficiency has been found in all probability to be due to interference with the reaction at the tryptophan peroxidase-oxidase stage (173 c/. diagram 19). The evidence is still inadequate to show how enzyme function and vitamin are related. Biotin may also be concerned in the reaction (800 but see 175a). 

The enzyme called formylase by Knox and Mehler (490, 591) and ky-nurenine formamidase by Jakoby (437) is present in liver in a considerable excess relative to tryptophan peroxidase-oxidase (e.g., 491), and formylky-nurenine is therefore not normally found in tissues or excreted in urine (e.g., 171). Partially purified tryptophan peroxidase-oxidase, from which formylase activity has been removed, accumulates formylkynurenine, shown (591) to be identical with synthetic (947 or better, 172) material. Formylase occurs widely in bacteria, and has been partially purified from Neurospora (437). In both higher and lower organisms the enzyme shows considerable specificity. 

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