Tannin toxicity

Natural product-derived dispersants, such as tannins, lignins, and alginates, are still widely used as drilling mud thinners or in specialty applications where their low toxicity is a cmcial property, eg, in boilers producing steam for food applications. 

Griffiths D W (1991), Condensed tannins , in D Mello J P F, Duffus C M and Duffus J H, Toxic Substances in Crop Plants, The Royal Society of Chemistry, Cambridge, UK, 180-201. 

Herbivores that commonly feed on tannin-rich plants have evolved interesting methods to lessen the effect of ingested tannins on their digestive systems. For example, the salivary proteins of rabbits and other rodents are high in the amino acid proline, which has a very high affinity for tannins. Eating food high in tannins stimulates the secretion of these proteins and diminishes the toxic effect of the tannins. 

Tannins derive their name from their ability to tan (i.e. they combine with protein). They render plants less palatable and impair digestion by binding with the buccal mucosa, dietaiy proteins, and digestive enzymes of the animal. Tannins are thought to bind to proteins upon destruction of plant tissue by herbivores. This reduces the nutritive value of the plant to the herbivore. Some tannins such as oak gallotannins, are even toxic to livestock and rabbits (Meyer and Karasov, 1991). However, Martin and Martin (1983) have questioned the role of tannins as plant defense against herbivores. 

The flavonoid quercetin suppresses growth somewhat in prairie voles, M. ochrogaster, while tannic acid, a hydrolyzable tannin, has an even stronger effect (Lindroth and Batzli, 1984). Quercetin is toxic, but does not bind with protein and, therefore, does not affect protein digestibility. Tannic acid has some effect on protein digestibility but is also toxic. 

This review of literature on tannin degradation shows that our knowledge of this topic is only very slowly improving. Only a handful of laboratories are currently involved in this area. Of these, the Indian laboratories have made several interesting investigations recently, presumably because they are very active in leather manufacture and need to control the toxicity of their tannery effluents. 

Figure 4. Binding of a protein (hemoglobin) to several tannin extracts (tannic acid, sugar maple tannins, yellow birch tannins, quebracho tannins see 29) at various pH values. Ranges of microbial chitinase activity, NPV activity, and Bt toxicity are given. See text for discussion and references.

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