Catechols bind iron, and this property is exploited in catecholic siderophores (Crosa and Walsh 2002) and, by contrast, in strategies to prevent bacterial growth by restricting the supply of iron (Scalbert 1991; Mila et al. 1996). Further, since free Fe2+ ions participate in the Fenton reaction with H2O2, which produces highly
Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK e-mail: [email protected]
M. Lyte and P.P.E. Freestone (eds.), Microbial Endocrinology, Interkingdom Signaling in Infectious Disease and Health,
DOI 10.1007/978-1-4419-5576-0_4, © Springer Science+Business Media, LLC 2010
reactive hydroxyl radicals, the sequestration of free iron by catechols and other iron-chelating compounds can restrict the potential for free-radical generation and protect biological molecules from oxidative damage (Rice-Evans et al. 1995; Khokhar and Apenten 2003). Catechols also possess an autoxidation activity that oxidises Fe2+ to Fe3+, although conversely, reduction of Fe3+ to Fe2+ can also occur, particularly at low pH (Moran et al. 1997; Chvatalova et al. 2008). Catechols with an additional neighbouring hydroxyl group, such as gallic acid (3,4,5-trihydroxy-benzoic acid), appear to bind iron and also to oxidise Fe2+ to Fe3+ somewhat less effectively than simple catechols such as protocatechuic acid (3,4-dihydroxybenzoic acid) (Khokhar and Apenten 2003; Andjelkovic et al. 2006; Chvatalova et al. 2008). As discussed later, a study of the effects of tea catechins revealed that only compounds possessing a 3,4-dihydroxyphenyl group in the B-ring stimulated bacterial growth under iron-restrictive conditions; compounds with an additional neighbouring hydroxyl group in the 5-position failed to stimulate growth (Freestone et al. 2007c). In this chapter, the term catechol is used to denote compounds with the odihydroxyphenyl, as distinct from a trihydroxyphenyl, grouping.
Catechols in the diet are most often plant-derived. The large majority are eventual products of the central phenylpropanoid pathway from phenylalanine via cinnamic acid. In plants, phenylpropanoid-pathway derivatives fulfil diverse functions in defence, signalling, protection against ultraviolet light and insect attraction (Parr and Bolwell 2000; Boudet 2007). Those present in the largest amounts are likely to serve a broad-spectrum defence function as anti-feedants or as antimicrobial agents. Catechols are rarely a specific focus of attention, being more often considered nonspecifically within other secondary-product classes, especially the hydroxycin-namic acids and the various sub-classes of the flavonoids. Many plant polyphenolic substances exhibit a broad-spectrum antibacterial activity and catechols are not necessarily among the most potent (Taguri et al. 2006); however, studies of sidero-phore mutants of Erwinia chrysanthemi demonstrate that plant polyphenols containing catechol groups can act to prevent bacterial growth by sequestering Fe3+, although sequestration of other metal ions, notably Cu2+ and Zn2+, might also occur (Mila et al. 1996). As discussed further, plant catechols include, in particular, cate-cholic representatives of the benzoic and cinnamic acids and their derivatives, notably protocatechuic acid (3,4-dihydroxybenzoic acid), caffeic acid (3,4-dihydroxy-trans-cinnamic acid) and the chlorogenic acids (principally 5-O-caffeoylquinic acid); oleuropein, a hydroxytyrosol ester found in olives; catecholic flavonols, notably quercetin; catecholic flavanols, for example (-)-epicatechin and (-)-epicatechin gallate; many anthocyanins, which are glycosides of anthocyanidins such as the catechol, cyanidin, and which are widespread as blue, purple and red plant pigments; and finally, many proanthocyanidins or condensed tannins, formally polymerised flavanols. Flavonols, flavones and anthocyanins, though not flavanols, are all generally not found free in plants but are typically found as O-glycosides. Figure 4.1 shows the structures of the principal flavonoid sub-classes.
Many of these compounds attract considerable interest on account of their reported effects in relation to a range of cancers, inflammatory conditions and cardiovascular diseases, which to varying extents (and not in every case) may be a
result of their radical-scavenging and antioxidant properties (Ross and Kassum 2002; Cooper et al. 2005; Evans et al. 2006; Hodgson and Croft 2006; Prior et al. 2006; Rahman 2006; Schroeter et al. 2006; Espin et al. 2007; Khan et al. 2008; Loke et al. 2008). One consequence of this is a concern to acquire reliable data on dietary intakes. Extensive information on the contents of phenylpropanoid metabolites in fruits, vegetables and beverages has been compiled (Hollman and Arts 2000; Tomas-Barberan and Clifford 2000; Clifford 2000a, b; Santos-Buelga and Scalbert 2000; Manach et al. 2004). Levels can vary widely on account of varietal characteristics, cultural conditions, developmental stage, position on the plant and storage - and very often within an individual harvested fruit or vegetable (Manach et al. 2004).
Catecholic benzoic and cinnamic acids are amongst the simplest catecholic compounds found in plant foods. They and their derivatives are widespread and may sometimes be present in appreciable amounts. Thus, blackberry fruits may contain ca. 0.07-0.2 g of protocatechuic acid kg-1 fresh weight (Tomas-Barberan and Clifford 2000) and potato tubers may contain around 1.2 g kg-1 fresh weight of chlorogenic acid (principally 5-0-caffeoylquinic acid), though much is likely to be lost during cooking (Clifford 2000a). Coffee also contains large amounts of chlo-rogenic acid and Clifford (2000a) has estimated that 200 ml of instant coffee brew (2% w/v) may provide 50-150 mg of the compound (equivalent to ca. 25-75 mg of caffeic acid).
The occurrence of oleuropein, a secoiridoid glucoside ester of the catechol, hydroxytyrosol, is restricted to olives, where in young fruits it may account for 14% of dry matter, although in processed fruit and in olive oil, levels are lower than this, partly on account of hydrolysis, including hydrolysis to hydroxytyrosol. Olives also contain verbascoside, a compound containing two catecholic residues, hydroxyty-rosol and caffeic acid (Soler-Rivas et al. 2000).
In most fruits, vegetables and beverages the levels of flavonols, flavones and flavanols are below about 0.015 g kg-1 fresh weight, although there are conspicuous exceptions (Hollman and Arts 2000). Thus, the catecholic flavonol, quercetin, occurs in the form of glycosides in onions at 0.35 g kg-1 fresh weight and in kale at around 0.11 g kg-1 fresh weight (and in each case possibly considerably more, depending upon variety and cultural conditions); the flavone, luteolin, may reach 0.2 g kg-1 fresh weight in celery leaves; levels of the catecholic flavanols, (-)-epicatechin and (-)-epicatechin gallate, can reach ca. 20-150 |g ml-1 in brewed tea; and levels of (+)-catechin and (-)-epicatechin can reach 100-200 |g ml-1 in some red wines. Cocoa and chocolate are also rich sources of flavanols (Schroeter et al. 2006). Levels of anthocyanins are greatest in those fruits and vegetables that are highly pigmented; for example, blueberries contain ca. 0.8-4.2 g of anthocyanins kg-1 fresh weight and aubergines may contain 7.5 g kg-1 fresh weight (Clifford 2000b). Not surprisingly, red wines, and port wine in particular, contain appreciable levels of anthocyanins, within the range 140-1,100 |g ml-1 (Clifford 2000b). Proanthocyanidins or condensed tannins are abundant in many common or staple foods or beverages (Santos-Buelga and Scalbert 2000), notably black tea, in which two groups of these compounds, the theaflavins and thearubigins, are derived from the flavanols of green tea during processing. Levels of proanthocyanidins of ca. 3-10 g kg-1 dry weight in lentils, of up to 7.4 g kg-1 dry weight in faba beans and of up to 39 g kg-1 dry weight in sorghum, have been determined. High concentrations of proanthocyanidins are often present in red wine and apple juices, and particularly in cider, where levels are reported to range between 2,300 and 3,700 |g ml-1 (Santos-Buelga and Scalbert 2000).
In addition to the catechols discussed above, catecholamines also occur in many plants, and there is evidence for their involvement in defence against pathogens, in responses to plant growth substances and in carbohydrate metabolism, but details of the mechanisms involved still remain uncertain (Kulma and Szopa 2007). Where determined, levels of catecholamines have been found to be low (below 1 mg kg-1 fresh weight), except for bananas, plantains and avocados. Thus, levels in excess of 40 mg kg-1 of dopamine were found in the fruit pulp of red banana and yellow banana, whilst the peel of Cavendish banana contained 100 mg kg-1 (Kulma and Szopa 2007). These values are similar to those of, for example, protocatechuic acid in blackberry fruits (Tomas-Barberan and Clifford 2000). Fruit pulp of Fuerte avocado contained smaller amounts of catecholamines: 4 mg kg-1 of dopamine and <3.5 mg kg-1 of norepinephrine. The relatively high content of dopamine in banana pulp is of particular interest in view of the ability of banana pulp to promote the growth of Gram-negative bacteria in iron-restricted medium, as reported by Lyte (1997).
Dietary catechols also arise from non-plant sources. In particular, tyramine arises in cheeses and other fermented foods as a result of the bacterial decarboxyla-tion of tyrosine (Santos 1996). Concentrations exceeding 0.1 g kg-1 can be present in matured cheeses (Komprda et al. 2008). Micromolar concentrations of tyramine have been found to increase the adherence of Escherichia coli O157:H7 to murine intestinal mucosa (Lyte 2004b).
Was this article helpful?