Inhibition of enzymatic browning and protection of sulfhydryl enzymes by thiol compounds.

In a reaction between (-)-epicatechin (EC) and 2-mercaptoethanol (2ME), catalyzed by partially purified polyphenol oxidase (PPO) extracted from the style of Rhododendron mucronatum, 2'-(2-hydroxyethylthio)-(-)-epicatechin (2'-HETEC), 5'-(2-hydroxyethylthio)-(-)-epicatechin (5'-HETEC), and 2',5'-bis(2-hydroxyethylthio)-(-)-epicatechin (2',5'-HETEC) were formed. The rate of formation of 2',5'-HETEC from 5'-HETEC was faster than that from 2'-HETEC. In the absence of 2ME, the concentration of EC decreased rapidly and the reaction mixture turned brown; 2'-, 5'-, and 2',5'-HETEC, especially 2'-substituted HETECs. reacted more slowly. These data indicate that 2ME acts both as an inhibitor of the polymerization of O-quinone, presumably by binding to it and as a reductant involved in the conversion of O-quinone to O-dihydroxyphenol, Inhibition of enzymatic browning by other thiol compounds such as cysteine and dithiothreitol was also investigated.