Of flavonoids

The leaves of Camellia sinensis are similar to most plants in general morphology and contain all the standard enzymes and stmctures associated with plant cell growth and photosynthesis (10—12). Unique to tea plants are large quantities of flavonoids and methylxanthines, compounds which impart the unique flavor and functional properties of tea. The general composition of fresh tea leaves is presented ia Table 1. 

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

Theaflavins. One of the more well-defined groups of flavonoid polymers that forms duriag black tea manufacturiag is that of the theaflavins (14). Exhibiting a bright orange-red color in solution, these are important contributors of brightness, a desirable visual attribute used by professional tasters to describe the appearance of tea infusions. 

There are numerous synthetic and natural compounds called antioxidants which regulate or block oxidative reactions by quenching free radicals or by preventing free-radical formation. Vitamins A, C, and E and the mineral selenium are common antioxidants occurring naturally in foods (104,105). A broad range of flavonoid or phenoHc compounds have been found to be functional antioxidants in numerous test systems (106—108). The antioxidant properties of tea flavonoids have been characterized using models of chemical and biological oxidation reactions. 

Fig. 1 Separation of flavonoids fluorescence scanning curve of rutin (1), hyperoside (2), quercitrin (3) and quercetin (4).

Fig. 1 Chromatography of flavonoids. 1. Extract. Solidaginis 2. Rutin — chlorogenic acid — isoquercitrin — quercitrin 3. Extract. Hyperici 4. Hyp>erosid — quercetin-3-arabinosid — hypericin — quercetin 5. Extract. Betulae...

T.R. Seshadri, Interconversions of Flavonoid Compounds, in "The Chemistry of Flavonoid Compounds" T.A. Geissman, Ed. Pergamon Press p. 156, Oxford, (1962)... 

The antioxidant properties of flavonoids are attributable to the ring whose radical has the lower reduction potential. Conjugation between the 2-aryl and the fused benzene rings is very inefficient <96JCS(P2)2497>. 

Fig. 4. Comparison of the two signal-reaction chains leading either to the UV light-induced formation of flavonoids or to the elicitor-induced formation of furanocoumarins and related compounds with antimicrobial activity. From Hahlbrock et al. (1985). PR -proteins are pathogenesis-related proteins.

Perhaps the most unusual observation in this study, other than the unique pigment profile in S. angulatus, is the simple and identical flavonoid profile in the Kenyan, Madagascaran, and Canary Islands specimens. A close relationship between the two varieties from East Africa is not difficult to appreciate. The occurrence of this profile in specimens from the Canary Islands, however, points to a closer relationship than the distance between these areas might suggest. There is no way to know, at least from the data presented, whether this represents a case of convergence of flavonoid biosynthetic capacities involving unrelated species, whether it points to a relationship based... 

Phlox Carolina L., a tall, perennial phlox native to southeastern United States, was shown to exhibit a substantial degree of flavonoid variation by Levy and Levin (1975). In a subsequent paper. Levy and Lujii (1978) described attempts to establish geographical patterns in the occurrence of leaf fiavonoids. Seventy-three collections were made representing populations in Georgia, Alabama (a total of 64 populations). 

See D. M. Smith, 1980, for a study of flavonoid profiles of the varieties.) The overall flavonoid profile of P. triangularis is fully in accord with the uifique status of the species. A detailed discussion of the chemistry of this system, which is beyond the scope of the present treatment, can be found in a paper by Wollenweber and Dietz (1980). An example of the complexity of flavonoid biosynthesis in this species can be found in a description of biflavonoids present in the farinose exudate (linuma et al., 1994). 

Structural type. The possible function of flavonoids as antiherbivore defense com-ponnds was discnssed, bnt Mears (1980b) found no correlation between complexity of flavonoid profile and latitnde, as might be the case if complexity of profile increases as one goes from temperate to more tropical climates with concomitant increase in insect predators. Althongh the nnmber of samples stndied was not large, there was a relationship between latitnde and complexity of pigment profiles in taxa restricted to calcareons snbstrates. No driving force for this apparent relationship is evident. 

An early study of flavonoids in Hypolaenafastigiata had shown the presence of novel and potentially useful compounds, including the newly reported 8-hydroxyluteolin [316] (see Fig. 4.7 for structures 316-320), which took its name, hypolaetin, from the source genus (Harborne and Clifford, 1969). A more recent survey of the family... 

Table 4.3 Distribution of flavonoids in Restionaceae in Australia and Africa (from Williams etal., 1998)...

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