Quantitative analysis of yellow pigment in Takuan-zuke and their ABTS radical cation scavenging activity

2-[3-(2-Thioxopyrrolidin-3-ylidene)methyl]-tryptophan (TPMT) is the main yellow pigment in salted radish roots (takuan-zuke). In the present study, we investigated the content of 1-(2-thioxopyrrolidin-3yl)-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (TPCC) as a precursor of the yellow pigment and TPMT in commercial takuan-zuke. The total content of stereoisomers was 0.2-36.4 mg/100 g (2-364 ppm) for TPCC, and 0-6.6 mg/100 g (0-66 ppm) for TPMT. TPCC and TPMT showed radical scavenging activity against ABTS radical cation than the classical antioxidant (ascorbic acid and Trolox). The antioxidative activity showed no differences for TPCC stereoisomers, but showed differences for E/Z ratiocontrolled TPMT solutions.