3- Methyl-5,6-dihydroxyindole, formation

The enzyme assay contained 0.1 mM 5,6-dihydroxyindole-2-carboxylic acid in 30 mM Tris-HCl buffer (pH 7.8), 2.5 mM MgCl2,1 mM 5-adenosylmethio-nine (as methyl donor), 0.5% Triton X-100,0.5 mM EGTA, and 5 mAf dithio-threitol in a total volume of 250 / L. The reaction was stopped with 25 fiL of 4 M perchloric add. Following centrifugation, the supemate was analyzed by HPLC. Product formation was linear with time for 60 minutes. 

More recently the oxidation of tyrosine methyl ester with potassium nitrosodisulphonate (Fremy s salt), was shown to give 2-carbomethoxy-5,6-dihydroxyindole, via the intermediate formation of dopachrome methyl ester [81a]. It was also shown that tyrosine containing peptides are oxidised by Fremy s salt in a similar manner to that previously described for their enzymatic oxidation [cf. 29]. 

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