Abstract
Peroxidase-catalyzed oxidation of 3-(3,4-dihydroxyphenyl)alanine (DOPA) and its S-cysteinyl derivatives (cysteinyldopas) in the presence of cysteine was studied by analyzing the products with chromatography on Dowex 50W. Products of the oxidation of DOPA were found to be 5- S- and 2- S-cysteinyldopa, 2,5- S, S-dicysteinyldopa, and three unknown compounds A 1, B, and C. 5- S- and 2- S-cysteinyldopa were also oxidized as easily as DOPA to give 2,5- S, S-dicysteinyldopa and similar patterns of the unknown compounds. Further oxidation of 2,5- S, S-dicysteinyldopa in the presence of cysteine yielded compounds A 1, B, and C, whereas in its absence compound B was not formed. From these results coupled with the spectral data, it is suggested that compounds A 1 and C are the two isomeric dihydrobenzothiazine derivatives of 2,5- S, S-dicysteinyldopa, while compound B is 2,5,6- S, S, S-tricysteinyldopa. These data suggest a possibility that peroxidase may play some role in the formation of cysteinyldopa and related metabolites in vivo.
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