Soluble Free, Soluble Conjugated, and Insoluble Bound Phenolics in Tomato Seeds and Their Radical Scavenging and Antiproliferative Activities

The soluble free, soluble conjugated, and insoluble bound phenolic compounds in tomato seeds were extracted and analyzed using ultra-high-performance liquid chromatography-high-resolution mass spectrometry. Total phenolic content (TPC) and free radical scavenging activities along with the antiproliferative effects against the human colorectal cancer cell line (HCT-116) were also examined for the soluble free, soluble conjugated, and insoluble bound phenolic fractions. 13, 7, and 10 compounds were tentatively identified in the soluble free, soluble conjugated, and insoluble bound phenolic fractions, respectively, including indole-3-acetic acid derivatives, flavonoids, phenolic acid, and tyramine-derived hydroxycinnamic acid amines. The insoluble bound phenolic fraction was observed to have a greater TPC value and stronger free radical scavenging activities against ABTS^•+, DPPH^•, and peroxyl radicals and a stronger inhibitory effect against HCT-116 cells compared with the soluble free and the soluble conjugated fractions. Soluble free and insoluble bound fractions significantly inhibited the proliferation of the HCT-116 cell line, and no antiproliferative effects were observed with the soluble conjugated fraction under the experimental conditions. The results may provide a foundation for future application of tomato seeds as nutraceuticals in dietary supplements and functional foods.