Comparative action of dopachrome tautomerase and metal ions on the rearrangement of dopachrome.

A vis-a-vis comparison between the effects of dopachrome tautomerase (DCT) and metal ions, e.g., cupric ions, on the kinetics and mode of rearrangement of dopachrome has been carried out under appropriate analytical conditions. The enzyme-promoted reaction is highly stereospecific for L-dopachrome, is unaffected by metal chelators and has an optimal pH around 6.8. By contrast, the kinetics of dopachrome rearrangement catalysed by cupric ions are not dependent on the stereochemistry of the substrate, are affected by EDTA and are not influenced by the pH of the medium in the range between 5-7.5. Both cupric ions and DCT catalyse the rearrangement of dopachrome to give 5,6-dihydroxyindole-2-carboxylic acid (DICA) rather than 5,6-dihydroxyindole (DI). However, at comparable activity, the ratio of formation DICA/DI is significantly higher in the enzyme-catalysed than in the metal-catalysed reaction. These results provide an improved background to look into the mode of action of DCT and metal ions, enabling a clear cut differentiation between the effects of the two factors when both are present in biological extracts.