Contrasting influences of glucuronidation and O-methylation of epicatechin on hydrogen peroxide-induced cell death in neurons and fibroblasts

The metabolic fate of dietary polyphenols in humans

Dietary polyphenols are widely considered to contribute to health benefits in humans. However, little is yet known concerning their bioactive forms in vivo and the mechanisms by which they may contribute toward disease prevention. Although many studies are focusing on the bioavailability of polyphenols through studying their uptake and the excretion of their conjugated forms, few are emphasizing the occurrence of metabolites in vivo formed via degradation by the enzymes of colonic bacteria and subsequent absorption. The purpose of this research was to investigate the relationship between biomarkers of the colonic biotransformation of ingested dietary polyphenols and the absorbed conjugated polyphenols. The results show that the majority of the in vivo forms derive from cleavage products of the action of colonic bacterial enzymes and subsequent metabolism in the liver. Those include the glucuronides of 3-hydroxyphenylacetic, homovanillic, vanillic and isoferulic acid as well as 3-(3-methoxy-4-hydroxyphenyl)-propionic, 3-(3-hydroxyphenyl)-propionic acid, and 3-hydroxyhippuric acid. In contrast, intact conjugated polyphenols themselves, such as the glucuronides of quercetin, naringenin and ferulic, p-coumaric, and sinapic acid were detected at much lower levels. The results suggest that consideration should be given to the cleavage products as having a putative role as physiologically relevant bioactive components in vivo.

Absorption and urinary excretion of procyanidin B2 [epicatechin-(4beta-8)-epicatechin] in rats

We evaluated the bioavailability and plasma antioxidative activity after administration of procyanidin B2 [epicatechin-(4beta-8)-epicatechin] in rats. After procyanidin B2 administration, procyanidin B2 is absorbed and excreted in urine, and a portion of the PB2 is degraded to (-)-epicatechin and to the metabolized conjugated and/or methylated (-)-epicatechin internally in the rat. Moreover, PB2 reduces the accumulation of lipid peroxide in plasma oxidized by copper ions.

Dietary flavonoids as potential neuroprotectants

There is an increasing awareness of the role of certain nutritional components, including dietary flavonoids found in fruit, vegetables and beverages, in the maintenance of health and prevention of chronic diseases. In this regard, recent studies highlight an exciting role with respect to their potential neuroprotective actions, in particular towards deficits commonly observed with aging, such as reduced performance of cognitive, memory and learning tasks. These neurological functions, and possible mechanisms involved in controlling them, can be influenced by supplementation of single dietary flavonoids, or as part of a flavonoid-rich preparation. With this, a renewed emphasis is aimed at further understanding their modes and sites of action. Moreover a common theme among many in vitro studies examining mechanisms of neuroprotection is the failure to include biologically relevant metabolites of the flavonoids known to enter the circulation, and thus most likely to be bioavailable to cells and tissues. This oversight will ultimately influence the mechanisms of action proposed to explain the neuroprotection observed in animals and human studies. As such, emerging findings suggest a variety of potential mechanisms of action of flavonoids and their bioavailable metabolites in cytoprotection against oxidative stress, which may be independent of conventional antioxidant reducing activities. Such mechanisms might involve their interaction with cell signalling cascades, their influence on gene expression and the down regulation of pathways leading to cell death.