Proanthocyanidins, metabolism

Some inconsistencies occur in the distribution of dihydroflavonols, flavan-3,4-diols, and proanthocyanidins in plants outside the Leguminosae that have caused doubts about the central role of flavan-3,4-diols in proanthocyanidin metabolism (134-136). One concern is the fact that no 5,7-dihydroxy-substituted... 

The two principal classes of proanthocyanidins found (10) in plant tissues are the procyanidins (1, R e H) and the prodeTphin-idins (1, R s OH). Proanthocyanidins of mixed anthocyanidin character (1, R = H or OH) have been noted. In any tissue where proanthocyan din synthesis occurs there is invariably found a range of molecular species - from the monomeric flavan-3-ols (catechins, gallocatechins) to the polymeric forms (1) and biosynthetic work (11) suggests a very close relationship between the metabolism of the parent f1avan-3-o1 and the synthesis of proanthocyanidins, Figure 4. 

Comparative Aspects of Polyphenol Metabolism - Proanthocyanidins and the complex esters of gallic and hexahydroxydiphenic acid show many structural similarities as plant metaijol i tes. The shape and size of the ester (5) is thus very similar to that of a proanthocyanidin hexamer (1, n = 4). The most striking feature of both structures however s the manner in which free phenolic groups are distributed over the surface of the molecule providing a structure with the inbuilt capacity for multidentate attachment to other species by hydrogen bonding. 

Studies have shown that phenylpropanoid metabolism can be stimulated by ozone. The activity of PAL increased in soybean [91], Scots pine (Pinus sylvestris L.) [92], and parsley (Petroselinum crispum L.) [93] soon after treatment with 150-200 nmol O3 mol 1. Rapid increases in transcript levels for PAL in response to ozone have been observed in parsley [93], Arabidopsis thaliana L. Heynhold [94] and tobacco (Nicoticma tabacum L.) [95]. Transcript levels for 4-coumarate CoA ligase (4CL), the last enzyme in the general phenylpropanoid pathway, increased commensurately with PAL transcripts in ozone-treated parsley seedlings [93]. Phenolic compunds reported to accumulate in leaf tissue in response to ozone include hydroxycinnamic acids, salicylic acid, stilbenes, flavonoids, furanocoumarins, acetophenones, and proanthocyanidins [85, 92, 93, 96, 97]. 

Phenolic compounds and flavonoids are a unique category of plant phytochemicals especially in terms of their vast po ential health-benefiting properties. They represent the most abundant and the most widely represented class of plant natural products. A substantial amount of research has been carried out over the past two decades yet large information gaps still exist. For example, the inventory of these compounds is still incomplete, although there is continuous effort to provide new structures. In addition the dissection of the metabolic pathways for certain phenolic compounds remains to be resolved. Recent reports underline that important questions that still need to be answered in the field of proanthocyanidin and tannin biosynthesis [Xie and Dixon, 2005], and even the exact nature of the biosynthetic pathway(s) leading to lignin monomers is not fully elucidated. 

Sharma SB, Dixon RA. 2006. Metabolic engineering of proanthocyanidins by ectopic expression of transcription factors in Arabidopsis thaliana. Plant J 44 62-75. 

Anthocyanins are a proanthocyanidin-type of flavonoid distributed in various fruits. These anthocyanins are the most important visible plant pigments in the natural kingdom. Anthocyanins have been clinically used in many folklore medicines worldwide for the treatment of age-related diseases and other disease. This review presents the functionality of anthocyanins in relation to their chemistry, synthetic pathway, antioxidant activity, antitumor activity (including apoptosis-inducing activity), pharmacodynamics (absorption, metabolism, distribution, and excretion) and toxicity, and discusses their possible use as food and dietary supplements and usage in potential nutraceuticals. 

Metabolically, anthocyanins are built up from the dihydroflavonols by means of a reduction of C4, catalyzed by the dihydroflavonol reductase, which leads to the flavan-2,3-trans-3,4-cis-diols, which are intermediates of proanthocyanidins and anthocyanidins. However, despite all the data in this direction, it has not been possible to obtain in vitro the transformation of leucoanthocyanidins in anthocyanins [33],... 

Polyphenols include flavonoids, proanthocyanidins, stilbenes, microbial metabolites of lignan, and hydroxycinnamates (Fig. 2). Flavonoid metabolism, while still far from being fully understood, has been the most widely studied and will therefore form the basis of this chapter. Six main subclasses of flavonoids are widely consumed by humans flavonols, flavones, flavanones, isoflavonoids, flavanols (catechins), and anthocyanins these posses the generic structure shown in Fig. 3. These classes differ in the degree of saturation and the nature and position of reactive groups on their three rings examples of substitution patterns for selected flavonoids are given in Table 1. 

Havanols are a wide group of polyphenols that include flavan-3-ols (e.g., catechin and proanthocyanidins), flavan-4-ols, and flavan-3,4-diols. They arise from plant secondary metabolism through condensation of phenylalanine derived from the shikimate pathway with malonyl-CoA obtained from citrate that is produced by the tricarboxylic acid cycle, leading to the formation of the key precursor in the flavonoids biosynthesis the naringenin chalcone. The exact nature of the molecular species that undergo polymerization and the mechanism of assembly in proanthocyanidins are still unknown. From a structural point of view, flavanols... 

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