An epoxide-furanoid rearrangement of spinach neoxanthin occurs in the gastrointestinal tract of mice and in vitro: Formation and cytostatic activity of neochrome stereoisomers

Neoxanthin, a major carotenoid in green leafy vegetables, was reported to exhibit potent antiproliferative effect via apoptosis induction on human prostate cancer cells. However, the metabolic fate of dietary neoxanthin in mammals remains unknown. In the present study, we investigated the gastrointestinal metabolism of neoxanthin in mice and the in vitro digestion of spinach, and estimated the anti proliferative effect of neoxanthin metabolites on PC-3 human prostate cancer cells. Two hours after the oral administration to mice of purified neoxanthin, unchanged neoxanthin and stereoisomers of neochrome (8'-R/S) were detected in the plasma, liver, and small intestinal contents. To estimate the effect of intragastric acidity on the conversion of dietary neoxanthin into neochrome (epoxide-furanoid rearrangement), spinach was digested in vitro by incubating it with a pepsin-HCI solution at pH 2.0 or 3.0 (gastric phase) followed by a pancreatin-bile salt solution (intestinal phase). Spinach neoxanthin was largely converted into (R/S)-neochrome during the digestion when the gastric phase was set at pH 2.0, whereas the rearrangement was observed to a lesser extent at pH 3.0. (R/S)-neochrome dose-dependently inhibited the proliferation of PC-3 cells as well as neoxanthin at concentrations less than or equal to 20 mumol/L. Although neoxanthin induced evident apoptotic cell death, (R/S)-neochrome inhibited the cell proliferation without obvious apoptosis induction. These results indicate that dietary neoxanthin is partially converted into (R/S)-neochrome by intragastric acidity before intestinal absorption and that (R/S)-neochrome exhibits an anti proliferative effect on PC-3 cells by the induction of cytostasis.