Dietary polyphenols have been associated with a decreased risk of diseases such as coronary vascular disease (CVD). However, the health benefits of these compounds depend on their bioavailability. In this thesis, the phase II enzymes; uridine diphosphate glucuronosyl transferase (UGT), sulfotransferase (SULT) and catechol-O-methyltransferase (COMT), which affect bioavailability of polyphenols were investigated.
Quercetin and (-)-epicatechin are representatives of the flavonol and flavanol subclasses of polyphenols. Thus, co-consumption of these compounds may provide interesting information on the role of phase II enzymes on their metabolism and excretion. Quercetin and (-)-epicatechin are found abundantly in onion and dark chocolate, respectively. Prior to providing food samples to healthy volunteers, the content of quercetin in onion soup and also (-)-epicatechin in dark chocolate were quantified by HPLC-DAD and HPLC-DAD/FLD, respectively. As expected red onion contained higher total quercetin content than yellow onion. In addition, homogenisation was an efficient method for extracting quercetin glycosides compared to vortex and sonication. Thus, red onion was selected to prepare a soup and found that there were no significant differences in total quercetin content in red onion soup after preparation (61.7±15.60 mg/ 130 ml soup) and before microwaving (60.4±9.6 mg/ 130 ml soup) or before microwaving and after microwaving (59.9±8.2 mg/130 ml soup), p-value = 0.818 and 0.666, respectively. For dark chocolate, the content of (-)-epicatechin was also analysed and quantified in different percentages of cocoa from different brands. However, 70% cocoa dark chocolate was selected for the human study. The content of (-)-epicatechin of 70% dark chocolate was 92.2±4.4 mg/100g FW, as detected with FLD.
Then, COMT activity was investigated in vitro followed by a pilot study in vivo. For the in vitro study, (-)-epicatechin was more efficiently methylated compared to quercetin. In vivo, in urine, the amounts of methylated and the parent compound of quercetin and (-)-epicatechin after single consumption of red onion soup, 70% cocoa dark chocolate and co-consumption of both types of food were underestimated when assessed after enzymatic deconjugation. In addition, this thesis revealed that the limitation was due to some compounds in urine which inhibited sulfatase activity. Because of this, all main metabolites of quercetin and (-)-epicatechin found in urine without enzymatic hydrolysis were analysed.
Main conjugates of quercetin and (-)-epicatechin predicted to be found in urine were first synthesised using an enzymatic method prior to use as standards. Five quercetin conjugates and five (-)-epicatechin conjugates were successfully synthesised with a sufficient percentage yield. For the first time, 3-O-methyl-quercetin-di-glucuronides were also produced and detected after glucuronidation of 3-O-methyl-quercetin-glucuronide. In addition, methylated-(-)-epicatechin-glucuronides and methylated-(-)-epicatechin sulfates were originally synthesised by (-)-epicatechin-glucuronides and (-)-epicatechin sulfates which were the products of (-)-epicatechin glucuronidation and (-)-epicatechin sulfation reaction, using COMT from cytosol pig liver.
Then, the main metabolites of quercetin and (-)-epicatechin excreted in urine in 27 healthy volunteers were determined and compared between single consumption and co-consumption. The major metabolite of quercetin found most often in urine was quercetin glucuronide in single consumption of red onion soup and co-consumption of red onion and 70% dark chocolate. While, the methylated form of (-)-epicatechin was the most abundant form detected in urine after single consumption of 70% dark chocolate and co-consumption of both types of food. In addition, the % urinary excretion of quercetin was lower than quercetin in both single and co-consumption. Thus, different types of phase II enzymes (UGT, SULT and COMT) have a different affinity for substrates, quercetin and (-)-epicatechin, resulting in different types of the metabolic profiles in urine. However, at these low level of quercetin and (-)-epicatechin from red onion soup and 70% dark chocolate did not reveal a huge interact and had a large effect on their metabolism and excretion.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)