Graduate Thesis Or Dissertation
 

A study of antimutagenicity in yogurt

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  • The purpose of this study was to identify antimutagens in yogurt active against the experimental colon carcinogen, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Our initial experiments showed that acetone extracts of yogurt, or milk fermented by various lactic acid bacteria were antimutagenic against MNNG and 3,2'-dimethyl-4-aminobiphenyl (DMAB) in the Ames test (Salmonella typhimurium TA 100). Further experiments carried out with milk fermented by Lactobacillus delbrueckii ssp. bulgaricus 191R showed that the putative compounds were more soluble in DMSO than in water, and that extractability of activity against MNNG and DMAB varied with pH, suggesting the presence of ionizable groups. Subsequent experiments demonstrated the antimutagenicity of yogurt. An acetone extract of yogurt was found to be active against a range of mutagens and promutagens in the Ames test. Simulation of fermentation by addition of lactic acid, lactic acid bacteria, or both to milk did not increase antimutagenicity, suggesting that compounds responsible for the activity may be formed during fermentation. Conjugated linoleic acid (CLA), a known dairy anticarcinogen, did not inhibit MNNG or DMAB indicating that other antimutagens may be present in yogurt. Fractionation of the acetone extract by HPLC showed that anti- MNNG and anti-DMAB activities did not co-elute, indicating that different compounds were responsible for the two activities. Using the Ames test to direct purification, isolation of an anti-MNNG active compound was accomplished using silica gel, Sephadex LH-20 and C18 reversed phase medium pressure chromatographies. The antimutagen was identified as palmitic acid by: a) co-elution with authentic palmitic acid on GC and HPLC columns, and b) by comparison of mass and ¹³C-NMR spectra. Minor components of milk fat such as iso methyl branched fatty acids (isopalmitic acid, isomargaric acid, isomyrsitic acid, and isostearic acid) were found to be more active than their straight chain counterparts. Isopalmitic acid also inhibited 4-nitroquinoline-N-oxide (4NQO) and the P450-mediated activation of 7,12-dimethylbenz[a]anthracene (DMBA). The mechanism of antimutagenesis against MNNG has not been established.
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Déclaration de droits
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