Proanthocyanidins in plants

Feucht, W. and Treutter, D. (1999). The role of flavan-3-ols and proanthocyanidins in plant defense , in Inderjit, K. and Dakshini M.M F.C.L., Principles and Practices of Plant Ecology, CRC Press, London, 307-338. 

The most abundant type of proanthocyanidins in plants are the procyani-dins, which consist exclusively of (epi)catechin units. The less common proanthocyanidins containing (epi)afzelechin (Fig. 1.11) and (epi)gallocatechin (Fig. 1.10) subunits are called propelargonidins and prodelphinidins, respectively [Balentine et al., 1997]. 

J. P. Steynberg E. V. Brandt M. J. H. Hoffman R. W. Hemingway D. Ferreira, Conformations of Proanthocyanidins. in Plant Polyphenols Synthesis, Properties, Significance (Basic Life Sciences - Volume 59) R. W. Hemingway, P. E. Laks, Eds. Plenum Press New York, 1992, p 501. 

Hellstrom JK, Manila PH (2008) HPLC determination of extractable and unextractable proanthocyanidins in plant materials. J Agrie Food Chem 56 7617-7624... 

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 presence of proanthocyanidins in plant tissue may be readily detected by hydrolysis with hot alcoholic acid (108) to produce the anthocyanidin pigment (e.g. cyanidin) that may be positively identified by paper chromatography or cellulose TLC in Forestal solvent (H0Ac H20 HCl, 30 10 3 v/v/v), or by a colorimetric method using vanillin and concentrated hydrochloric acid (17). The latter is very sensitive. 

Proanthocyanidins are an important group of di- to oligomeric flavonoids in plants. Four proanthocyanidins (procyanidin B3, prodelphinidin B4, ECG-(4 8)-ECG and GC-(4 8)-EGCG) were determined quantitatively in tea. The amounts in fresh tea leaves were between 1 and 2 g/kg per compound (Nakabayashi, 1991). The occurrence of proanthocyanidins may serve as a criterion for the differentiation between fermented and non-fermented teas (Kiehne et al, 1997). 

The B-type procyanidins include a mixture of oligomers and polymers composed of flavan-3-ol units linked mainly through C4 C8 and/or C4 C6 bonds, and represent the dominant class of natural proanthocyanidins. Among the dimers, procyanidins Bl, B2, B3 and B4 (Fig. 2a) are the most frequently occurring in plant tissues. Procyanidin B5 (EC-(4j6 6)-EC), B6 (catechin-(4o 6)-catechin), B7 (EC-(4/3 6)-catechin) and B8 (catechin-(4q 6)-EC) are also widespread (Eig. 2b) [17-19]. 

The A-type proanthocyanidins are characterized by a second ether linkage between an A-ring hydroxyl group of the lower unit and C-2 of the upper unit. Since they are less frequently isolated from plants than the B-types, they have been considered unusual structures [18,19]. The first identified A-type proanthocyanidin was procyanidin A2 isolated from the shells of fruit of Aes-culus hippocastanum. Since then, many more A-type proanthocyanidins have been found in plants, including dimers, trimers, tetramers, pentamers and ethers [18,21]. 

Although both catechins and proanthocyanidins are currently found in plant tissue composition, as reported above, their isolation requires sometime time-consuming procedures and purity and reproducibility are often suspect. 

First, the qualitative and quantitative variability of the amount of catechins and proanthocyanidins present in plant extracts used for different studies is probably the most significant. This might be due to the use of different procedures of extraction, quantification and structural elucidation. In most cases, even the lack of rigorous phytochemical characterization and quantification of active compoimd(s) constitutes a severe limitation on the rehabihty of the results. The lack of commercially available pure standards (particularly for some proanthocyanidins) represents an additional problem that has so far hampered the execution of rigorous SAR studies. This hmitation means that although a munber of in vitro or in vivo studies have been carried out by using more or less pure standards of catechins or with plant extracts containing both catechins and proanthocyanidins, only a handful of authors have... 

Peel GJ, Dixon RA (2007) Detection and quantification of engineered proanthocyanidins in transgenic plants. Nat Prod Commun 2 1009-1014... 

Tanner, G.J. et al., Proanthocyanidin biosynthesis in plants. Purification of legume leucoantho-cyanidin reductase and molecular cloning of its cDNA. J. Biol. Chem., 278, 31647, 2003. 

Kitamura, S., Shikazono, N., and Tanaka, A., TRANSPARENT TESTA 19 is involved in the accumulation of both anthocyanins and proanthocyanidins in Arabidopsis. Plant J., 37, 104, 2004. 

Matsui, K., Tanaka, H., and Ohme-Takagi, M., Suppression of the biosynthesis of proanthocyanidin in Arabidopsis by a chimeric PAPl repressor. Plant Biotech. J., 2, 487, 2004. 

Flavonoid levels in plants can be affected by their nutritional status. Low nitrate concentrations, for example, induce flavonoid accumulation, which then serve as chemoattractants to nitrogen-fixing bacteria. Nodulation then restores the nitrogen economy of the plant.In L. pedunculatus, proanthocyanidins are not formed under high-nitrogen conditions, presumably because the requirement for nodulation is not so critical. 

Hemingway, R.W., Biflavonoids and proanthocyanidins, in Natural Products of Woody Plants 1 Chemicals Extraneous to the Lignocellulosic Cell Wall, Rowe, J.W., Ed., Springer Verlag, New York, 1989, chap. 7.6. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

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. 

For example, extract the proanthocyanidin mixture with chloroform to remove chlorophyll, carotenoids, and waxy material. Use ethyl acetate if substantial amounts of flavan-3-ol monomers are present. Tissues that would benefit from a chloroform extraction include leafy tissues that contain chlorophyll (i.e., tea leaves) and seeds that contain oils (i.e., grape seeds). Ethyl acetate would be useful in plants such as apples, berries, grapes, and teas (i.e., tissues known to contain significant amounts offlavan-3-ol monomers). 

In plants accumulating anthocyanins, flavonols, and proanthocyanidins, naringenin is stereospecifically hydroxylated at position 3 of the C-ring (C3) by the 2-oxoglutarate-dependent dioxygenase flavanone 3-hydroxylase (F3H, EC 1.14.11.9) to yield the 3-hydroxy-trans-flavanone (syn. dihydroflavonol) dihy-drokaempferol [Springob et al., 2003] (Fig.21 2). Dihydroquercetin (3, 4, 5,5, 7-... 

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