Thiourea: the antioxidant of choice for the purification of proteins from phenol-rich plant tissues.

Metabisulfite, diethyldithiocarbamate, and thiourea are potent phenoloxidase inhibitors commonly used during the extraction of plant proteins. Their effects on several amino acid derivatives and peptides are reported. Important and pH-dependent changes are induced by metabisulfite in the uv-absorption spectrum of N-acetyltryptophanamide but not of N-acetyltyrosinamide. Similar spectral changes are also induced in tryptophanyl-containing peptides. Neither diethyldithiocarbamate nor thiourea modified the spectral properties of tryptophan or tyrosine. Cysteinyl groups are rapidly modified by diethyldithiocarbamate, especially in the lower pH range, but not by metabisulfite or thiourea. Diethyldithiocarbamate as well as metabisulfite interfere with cysteinyl groups. It is concluded that thiourea is the most appropriate reagent for use as a phenoloxidase inhibitor.