Transformation of Flavonoids by Intestinal Microorganisms

Abstract

Fruit, vegetables and cereals contain a wealth of secondary plant metabolites which have been implicated in the promotion of health. To understand the mechanism of their action it is necessary to gain more information on their fate in the body following ingestion. A certain proportion of ingested secondary plant constituents may escape absorption in the small intestine and therefore undergo transformation by intestinal microorganisms or enterohepatic circulation. To study the transformation of secondary plant metabolites by bacteria, Eubacterium ramulus was isolated from human feces and incubated with selected flavonoids. E. ramulus is a strictly anaerobic bacterium which was found to be present in the gastrointestinal tract of most individuals investigated. E. ramulus cleaves the ring system of several flavonols and flavones giving rise to the corresponding hydroxyphenylacetic and hydroxyphenylpropionic acids, respectively, as well as acetate and butyrate. Degradation pathways were proposed based on the intermediates detected by high performance liquid chromatography (HPLC) and HPLC coupled with mass spectrometry (LC-MS) and the detection of enzymes that catalyze reactions such as taxifolin isomerization, phloretin hydrolysis and phloroglucinol reduction. The dearomatizing phloroglucinol reductase, presumably part of all flavonoid degradation pathways, was purified and characterized. The gene encoding phloretin hydrolase was cloned from a E. ramulus gene library taking advantage of a newly developed fluorescence test for activity screening. Moreover, a new intermediate was discovered and identified by MS and 1H and 13C NMR analysis as alphitonin. To investigate the degradational potential of E. ramulus under in vivo conditions, germfree rats were associated with E. ramulus. Following the intragastric application of quercetin-3-glucoside, urine and feces of gnotobiotic rats were analyzed for degradational products originating from quercetin-3-glucoside. In feces of rats monoassociated with E. ramulus, 3,4-dihydroxyphenylacetic acid was found, indicating that this organism is able to cleave quercetin under in vivo conditions. To investigate in which way the dietary flavonoid content affects the cell counts of E. ramulus in the human intestinal tract, twelve human subjects consumed a flavonoid-free diet for one week and at one point during this period a large dose of flavonoids. Fecal samples from both phases of the study were analyzed by in-situ hybridization for total bacterial counts and counts of E. ramulus. Total cell counts and the cell counts of E. ramulus decreased significantly during the flavonoid-free period, while there was an increase in the E. ramulus counts of up to 10-fold during the flavonoid-rich period indicating that dietary secondary plant metabolites may have an influence on the intestinal microflora. E. ramulus is also capable of converting the isoflavonoids genistein and daidzein to the products 2-(4-hydroxyphenyl)-propionic acid and O-desmethylangolensin, respectively.