Tannins metabolism

Tannins represent a class of plant secondary metabolites and are produced by plants in their intermediary metabolism. Tannins are considered to be a promising group of substances to decrease methane (CH ) emission from ruminants by dietary means. However, there is no common agreement whether tannins generally decrease CH formation in vivo or not and to which extent (Beauchemin et al., 2008). Therefore, research on this particular topic is needed to summarize and to quantify the tannin effects on CH production from ruminants and its associated variables. The objective of this study was to estimate the relationship between dietary tannin levels and CH emission from ruminant animals by compiling available literature data of respective experiments using a statistical meta-analysis approach. 

E) coumarins, (F) quinones, (G) flavonoids, (H) tannins, (I) alkaloids, (J) terpenoids and steroids and (K) miscellaneous and unknowns. Although many of these compounds are secondary products of plant metabolism, several are also degradation products which occur in the presence of microbial enzymes. 

In the meantime, all of the higher intermediates shown in Fig. 2 have been tested and numerous comprehensive surveys of this work have appeared (24, 81—83, 91, 93, 112, 113, 132). Some of these simultaneously describe the formation of secondary aromatic substances in wood, i.e. lignans, tannins, flavonoids, etc., which arise by essentially similar routes coupled with acetate metabolism. A few outstanding recent developments may bear repetition here. 

Finally there are several reports of metabolic or pathological effects consequent upon the consumption of tannins. These range in severity fron unusual metabolites appearing in the urine of animals... 

The information in Table I reveals differences between sane of the inportant types of tannin. Tannic acid is unique for its occurrence in all reports of diet-induced gut lesion and gastrointestinal damage (vertebrate or invertebrate) it is also the only tannin for which metabolism and excretion are reported. By contrast condensed tannins are not thought to leave the gut lumen. Thus,in comparison with tannic acid condensed tannins would not be espected to drain 1-carbon metalx>lism, notably methionine resources. However, the reports by Elkin (] ) and Ford ( ) of corrective methionine treatment for chicks fed a condensed-tannin diet, indicate that sane condensed tannin may enter the body or that methionine can react with tannin in the gut, so perhaps the position is once again not clear cut on tannin structure and its impact on methionine levels. The conclusion can, however, be made that despite its hydrolyzability, tannic acid does exert an allelochemical effect which is not abolished by hydrolysis. 

Hydrolyzable tannins are comparatively restricted in the human diet and there are no human metabolic data. Studies in rats have indicated that some 63% of a dose of 1 g/kg commercial tannic acid is excreted unchanged in the feces accompanied by small amounts of gallic acid, pyrogallol, and resorcinol. Plasma after enzymic hydrolysis was found to contain 4-O-methylgallic acid, pyrogallol, and resorcinol. Urine also contained a small amount of gallic acid after enzymic hydrolysis. The most notable observation from this study is the failure of the gut microflora to metabolize the galloylglucoses efficiently, at least at this substantial dose. The viability or composition of the gut microflora was not reported. ... 

Carbon metabolism differs fundamentally from nitrogen metabolism In that virtually all nitrogenous compounds can be recycled within the plant, whereas most of the structural components of the plant, principally cellulose and lignin, are not reusable. There are also putative defensive compounds In this category such as leaf-external resins (32) and possibly some condensed tannins. Thus the constraints on allocation have both an Immediate and future time frame. 

Maturing improves the taste and aroma of beer and the elimination of tannin, protein, and hop resins also has a beneficial effect. Some metabolic products of unpleasant taste are further converted or washed out by the carbon dioxide surplus. The time for lagering varies with different types of beer. For every type of beer there is an optimal lagering time, and longer lagering is usually detrimental to beer quality. The filled lagering tanks are subjected to the saturating pressure of carbon dioxide, usually 50—70 kPa (ca 0.5—0.7 atm), controlled by a safety valve. 

Haslam, E., and Cai, Y., 1994, Plant polyphenols (vegetable tannins) gallic acid metabolism, Nat. Prod. Rep. 11 41-65. 

Comprehensive information on the network of pathways responsible for the synthesis of numerous secondary metabolites can be found in Chapter 21. In addition, information on this aspect is also available in articles by Shimada et al. [2003], Toshiaki [2003], Tanner et al. [2003], Boatright et al. [2004], Hoffmann et al. [2004], Dixon et al. [2005], Niemetz and Gross [2005], Xie and Dixon [2005], and Ferrer et al. [2008], Nonetheless, the complete dissection of phenolic metabolic pathway is far from being complete. For example, recent reports underline that important questions still remain to be answered in the field of protoanthocyanidins and tannins [Xie and Dixon, 2005], and that the exact nature of the biosynthetic pathway(s) leading to lignin monomers has not been fully elucidated [Boudet, 2007]. 

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. 

Studies performed over the years have contributed to better understanding of the interactions between proteins and tannins, which are important not only due to their astringency but also because of their impact on food nutritional characteristics, on human health, and on plant metabolism. It is clear that protein-tannin interactions are influenced by several factors, among which polysaccharides could be important because they are also present in tannin-rich vegetables. Much remains to be studied in this field, particularly the specific phenomenon that occurs between proteins, tannins, and polysaccharides that leads to a decrease in aggregation, and further studies are needed involving other salivary proteins and digestive enzymes. 

The central vacuole is a relatively simple aqueous phase that can act as a storage reservoir for metabolites or toxic products. For example, the nocturnal storage of organic acids, such as malic acid, takes place in the central vacuoles of Crassulacean acid metabolism plants (mentioned in Chapter 8, Section 8.5A), and certain secondary chemical products, such as phenolics, alkaloids, tannins, glucosides, and flavonoids (e.g., antho-cyanins), often accumulate in central vacuoles. Compared with the central vacuole, the cytoplasm is a more complex phase containing many colloids and membrane-bounded organelles. Because the central vacuole contains few colloidal or other interfaces, any matric pressure in it is... 

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