Reed J D
Department of Meat and Animal Science, University of Wisconsin-Madison 53706, USA.
J Anim Sci. 1995 May;73(5):1516-28. doi: 10.2527/1995.7351516x.
Proanthocyanidins (PA) (condensed tannins) and hydrolyzable tannins (HT) are the two major classes of tannins. Proanthocyanidins are flavonoid polymers. Hydrolyzable tannins are polymers of gallic or ellagic acid esterified to a core molecule, commonly glucose or a polyphenol such as catechin. Proanthocyanidins are the most common type of tannin found in forage legumes. Problems in the analysis of tannins are that sample processing and drying decrease extraction and reactivity, suitable standards are unavailable, and quantitative analytical methods are poorly correlated with enzyme inhibition, protein precipitation, and nutritional effects. Hydrolyzable tannins are potentially toxic to ruminants. Pyrogallol, a hepatotoxin and nephrotoxin, is a product of HT degradation by ruminal microbes. Proanthocyanidins are considered to be non-toxic because they are not absorbed, but they are associated with lesions of the gut mucosa. Research on tannins in forage legumes has determined their effects on protein digestion and metabolism but more research on tannin structure in relation to digestion of specific proteins is needed. The widely accepted explanation for positive effects of PA on protein digestion and metabolism is that PA-protein complexes escape ruminal degradation and the protein is available in the lower tract. This proposed mechanism may be incorrect because PA also complex carbohydrates, endogenous proteins, and microbial products and the degradability of PA-protein complexes by ruminal microbes has not been adequately studied. Several alternative hypotheses (to escape protein) that explain the effect of PA on protein digestion and metabolism in ruminants are also consistent with experimental results on forage legumes. These include increased microbial protein synthesis, increased use of endogenous nitrogen in the rumen, and increased secretion of salivary glycoproteins. Research on manipulating the content and type of PA in forage legumes is justified because they are associated with non-bloating legumes, lower soluble non-protein nitrogen in silage, and improved efficiency of protein utilization. Research on the biosynthesis, molecular genetics, and cell biology of PA in forage legumes needs to be integrated with research on toxicology and nutrition.
原花青素（PA）（缩合单宁）和水解单宁（HT）是单宁的两大主要类别。原花青素是类黄酮聚合物。水解单宁是没食子酸或鞣花酸与核心分子（通常是葡萄糖或多酚，如儿茶素）酯化形成的聚合物。原花青素是饲料豆科植物中最常见的单宁类型。单宁分析存在的问题包括：样品处理和干燥会降低提取率和反应活性，缺乏合适的标准品，定量分析方法与酶抑制、蛋白质沉淀及营养效应的相关性较差。水解单宁对反刍动物可能具有潜在毒性。焦性没食子酸是一种肝毒素和肾毒素，是瘤胃微生物对水解单宁降解的产物。原花青素被认为无毒，因为它们不会被吸收，但会导致肠道黏膜损伤。关于饲料豆科植物中单宁的研究已确定了它们对蛋白质消化和代谢的影响，但仍需要更多关于单宁结构与特定蛋白质消化关系的研究。原花青素对蛋白质消化和代谢产生积极影响的广泛接受的解释是，原花青素 - 蛋白质复合物能逃过瘤胃降解，蛋白质可在消化道下段被利用。这一提出的机制可能不正确，因为原花青素还能与碳水化合物、内源性蛋白质和微生物产物结合，且瘤胃微生物对原花青素 - 蛋白质复合物的降解能力尚未得到充分研究。其他几个解释原花青素对反刍动物蛋白质消化和代谢影响的假设（替代“逃过蛋白质”的假设）也与饲料豆科植物的实验结果一致。这些假设包括增加微生物蛋白质合成、增加瘤胃内源性氮的利用以及增加唾液糖蛋白的分泌。对饲料豆科植物中原花青素的含量和类型进行调控的研究是合理的，因为它们与非胀气豆科植物、青贮饲料中较低的可溶性非蛋白氮以及提高蛋白质利用效率有关。饲料豆科植物中原花青素的生物合成、分子遗传学和细胞生物学研究需要与毒理学和营养学研究相结合。
