Research on the putative health benefits has indicated that kefir, a traditional fermented milk, might have antimutagenic and antitumor properties. The major objective of the present thesis was to isolate and identify antitumor compounds in cow's milk kefir and investigate the possible mechanisms involved. High speed centrifugation (HSC), molecular weight cut-off filtration (MWCO), size exclusion high performance liquid chromatography (SEC-HPLC) and reverse phase-HPLC (RP-HPLC) were utilized for fractionation of kefir and a cell culture model was developed to screen for the antiproliferative effects of the kefir fractions. The antiproliferative effects of bacteria-free extracts from different fermentation stages of kefir production, as well as bacteria-free extracts from milk and yogurt were compared. The results showed that extracts from an early stage of fermentation (i.e., kefir mother culture) and the final commercial kefir product both exerted dose-dependent inhibition effects on human mammary tumor MCF-7 cells, yogurt extracts showed less potent antiproliferative effects, while pasteurized milk extracts showed no antiproliferative effects. No antiproliferative effects of the kefir extracts were observed on human mammary epithelial cells (HMEC) whereas the yogurt extracts showed antiproliferative action in HMEC cells at a high dose. A fraction of the kefir mother culture isolated by HSC, MWCO and RP-HPLC contained components that inhibited MCF-7 cell growth and had no effect on HMEC cells. Characterization of the bioactive fraction using mass spectrometry (MS) indicated that the main components in the fraction are likely fragments of kefiran and/or ceramide containing compounds such as gangliosides. The growth inhibitory effect may be mainly caused by the induction of TNF-alpha in MCF-7 cells. Whole extracts of kefir depleted glutathione (GSH) in MCF-7 cells, while the SEC-HPLC Fraction 7 and the RP-HPLC Fraction 30 induced GSH produc

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