Inhibitory effect of cysteine and glutathione on phenoloxidase from kuruma prawn (Penaeus japonicus)

The inhibitory effect of cysteine and glutathione on phenoloxidase (PO) from kuruma prawn were investigated. Cysteine and glutathione inhibited the oxidation of 3-(3,4-dihydroxylphenyl)-l-alanine (l-DOPA) catalyzed by kuruma prawn PO. Those thiol compounds showed competitive inhibition with Ki values of 0.45–0.46 mM. The inactivated PO could be partially recovered by the addition of copper acetate (0.01–0.2 mM). Almost complete restoration of PO activity was achieved with the addition of N-ethylmaleimide (NEM) in the range 0.05–0.2 mM, suggesting the importance of sulfhydryl groups of cysteine and glutathione for their inhibitory activity. Additionally, both cysteine and glutathione prevented the colour development by trapping the colour intermediates or reducing o-quinone to colourless compounds.