McNabb W C, Waghorn G C, Barry T N, Shelton I D
Department of Animal Science, Massey University, Palmerston North, New Zealand.
Br J Nutr. 1993 Sep;70(2):647-61. doi: 10.1079/bjn19930155.
Two experiments were conducted with sheep fed on fresh Lotus pedunculatus containing 50-55 g condensed tannin (CT)/kg dry matter. Effects of CT were assessed by comparing control sheep (CT operating) with sheep receiving a continuous intraruminal infusion of polyethylene glycol (PEG) to bind and inactivate CT. Digestion of methionine and cystine was determined using a continuous intraruminal infusion of indigestible markers, whilst plasma irreversible loss (IRL) of methionine, cystine and inorganic sulphate was determined using 35S labelling. The proportion of microbial non-NH3-N (NAN) in whole rumen digesta NAN and the IRL of reducible S from the rumen were determined using a continuous intraruminal infusion of (NH4)2(35)SO4. The proportion of microbial NAN in whole rumen digesta NAN (0.44 v. 0.71) and the IRL of reducible S from the rumen (0.84 v. 2.49 g S/d) were lower in control than PEG sheep. PEG sheep lost 30% of ingested methionine and cystine across the rumen, whereas the control sheep lost no methionine and cystine across the rumen. Apparent absorption of methionine from the small intestine was 27% higher in control than PEG sheep, but both groups had a similar apparent absorption of cystine. The apparent digestibility of cystine in the small intestine was lower in control (0.42) than PEG (0.53) sheep, whereas the apparent digestibility of methionine was similar (0.78) for both groups. CT had no effect on plasma methionine IRL, but markedly increased the IRL of cystine (39.8 v. 22.4 mumol/min) and reduced the IRL of plasma inorganic sulphate (35.9 v. 50.2 mumol/min). A three-pool model comparing interconversions between the three plasma metabolites showed that CT increased the flow of cystine to body synthetic reactions (36.5 v. 17.3 mumol/min). This was due to trans-sulphuration of methionine to cystine being greater in control than in PEG sheep, whilst the oxidation of both methionine and cystine were reduced in control sheep. It was concluded that CT reduced the proteolysis of forage protein and the degradation of S amino acids to inorganic sulphide in the rumen, resulting in increased net absorption of methionine and increased utilization of cystine for body synthetic reactions in sheep with a high capacity for wool growth (and, hence, high cystine requirement).
用新鲜的含有50 - 55克缩合单宁（CT）/千克干物质的百脉根饲喂绵羊进行了两项实验。通过将对照绵羊（CT起作用）与持续瘤胃内输注聚乙二醇（PEG）以结合并使CT失活的绵羊进行比较，评估CT的效果。使用持续瘤胃内输注不可消化标记物来测定蛋氨酸和胱氨酸的消化率，同时使用³⁵S标记来测定蛋氨酸、胱氨酸和无机硫酸盐的血浆不可逆损失（IRL）。使用持续瘤胃内输注（NH₄）₂（³⁵）SO₄来测定整个瘤胃消化物中非氨氮（NAN）中微生物NAN的比例以及瘤胃中可还原硫的IRL。对照绵羊的整个瘤胃消化物NAN中微生物NAN的比例（0.44对0.71）和瘤胃中可还原硫的IRL（0.84对2.49克硫/天）低于PEG绵羊。PEG绵羊瘤胃中摄入的蛋氨酸和胱氨酸损失30%，而对照绵羊瘤胃中无蛋氨酸和胱氨酸损失。对照绵羊小肠中蛋氨酸的表观吸收率比PEG绵羊高27%，但两组胱氨酸的表观吸收率相似。对照绵羊小肠中胱氨酸的表观消化率（0.42）低于PEG绵羊（0.53），而两组蛋氨酸的表观消化率相似（0.78）。CT对血浆蛋氨酸IRL无影响，但显著增加了胱氨酸的IRL（39.8对22.4微摩尔/分钟）并降低了血浆无机硫酸盐的IRL（35.9对50.2微摩尔/分钟）。一个比较三种血浆代谢物之间相互转化的三池模型表明，CT增加了胱氨酸向身体合成反应的流量（36.5对17.3微摩尔/分钟）。这是因为对照绵羊中蛋氨酸向胱氨酸的转硫作用比PEG绵羊更强，而对照绵羊中蛋氨酸和胱氨酸的氧化作用均降低。得出的结论是，CT减少了瘤胃中饲草蛋白的蛋白水解以及含硫氨基酸向无机硫化物的降解，导致羊毛生长能力高（因此胱氨酸需求量高）的绵羊中蛋氨酸的净吸收增加以及胱氨酸用于身体合成反应的利用率增加。
