Department of Animal Sciences, University of Illinois, Urbana 61801.
Department of Animal Sciences, University of Illinois, Urbana 61801.
J Dairy Sci. 2020 Jul;103(7):6250-6257. doi: 10.3168/jds.2019-18121. Epub 2020 Apr 22.
The use of alternative sources of protein to substitute for milk proteins in milk replacers (MR) can increase the synthesis of endogenous proteins and therefore alter ileal or total-tract digestibility calculations. Mucin is the main component of gastrointestinal mucus and represents the greatest contribution to total endogenous protein. Mucin is difficult to isolate and has not been extensively studied in dairy calves. We explored 3 different procedures to analyze and estimate mucin protein (MUP) in ileal digesta of young dairy calves. Ileal digesta samples were collected from nine 30-d-old ileal-cannulated calves that were enrolled in a 3 × 3 replicated Latin square with 5-d periods. The 3 diets were a control whey protein-based MR (WPC), an isonitrogenous MR in which 50% of the protein was from enzyme-treated soybean meal (ESBM), and an N-free MR (NFREE). Mucin protein concentration and flow were analyzed by fractionation of the digesta and ethanol precipitation; this process served as the reference method. Alternative methods to estimate MUP consisted of using commercial enzymatic kits to analyze glucosamine (N-acetylglucosamine, GlcNAc) and galactosamine (N-acetylgalactosamine, GalNAc), 2 amino-sugars that are highly enriched in mucin. Before GlcNAc determination, samples were processed using 3 different procedures: sample clarification (GLCL), clarification plus hydrolysis (GLCH), and hydrolysis alone (GLHL). The MUP was estimated by regression of the GlcNAc and GalNAc values using previously validated equations. According with the bias and agreement analysis, none of the methods yielded MUP values similar to the reference method. However, GLHL showed a strong association with the reference method (ρ = 0.73). It allowed identifying the smaller MUP flows with NFREE compared with the other 2 diets and detecting the greater flow of ESBM than WPC, as observed with the reference method. Using the GlcNAc values from GLHL and the MUP measured with the reference method, we were able to establish a linear relationship between both methods (adjusted R = 0.75). We found that the GLHL method enabled detecting differences in MUP ileal flows between diets differing in protein level and source. Inferences about MUP secretions must be done cautiously because many dietary and physiological factors are involved. The adoption of practical techniques to determine MUP can help to increase our knowledge about gastrointestinal tract function and to improve the accuracy of MR digestibility calculations.
替代蛋白质源在代乳粉（MR）中替代牛奶蛋白的使用可以增加内源性蛋白质的合成，从而改变回肠或全肠道消化率的计算。粘蛋白是胃肠道粘液的主要成分，是总内源性蛋白质的最大贡献者。粘蛋白难以分离，在奶牛犊牛中尚未得到广泛研究。我们探索了 3 种不同的程序来分析和估计年轻奶牛犊牛回肠消化物中的粘蛋白（MUP）。从 9 头 30 天大的回肠套管犊牛中收集回肠消化物样本，这些犊牛参与了 3×3 重复拉丁方试验，每个试验期为 5 天。3 种日粮分别为对照乳清蛋白基 MR（WPC）、50%蛋白质来自酶处理豆粕的等氮 MR（ESBM）和无氮 MR（NFREE）。通过消化物的分级分离和乙醇沉淀来分析粘蛋白蛋白浓度和流量；该过程作为参考方法。估计 MUP 的替代方法包括使用商业酶试剂盒分析氨基葡萄糖（N-乙酰氨基葡萄糖，GlcNAc）和半乳糖胺（N-乙酰半乳糖胺，GalNAc），这 2 种氨基糖高度富含粘蛋白。在确定 GlcNAc 之前，样品经过 3 种不同的处理程序：样品澄清（GLCL）、澄清加水解（GLCH）和单独水解（GLHL）。通过使用先前验证的方程，根据 GlcNAc 和 GalNAc 值进行回归来估计 MUP。根据偏差和一致性分析，没有一种方法能得到与参考方法相似的 MUP 值。然而，GLHL 与参考方法具有很强的关联性（ρ=0.73）。它允许与其他 2 种日粮相比，用 NFREE 识别较小的 MUP 流量，并检测 ESBM 比 WPC 具有更大的流量，这与参考方法一致。使用 GLHL 的 GlcNAc 值和参考方法测量的 MUP，我们能够建立两者之间的线性关系（调整后的 R=0.75）。我们发现，GLHL 方法能够检测不同蛋白水平和来源的日粮对回肠 MUP 流量的影响。关于 MUP 分泌的推论必须谨慎进行，因为涉及许多饮食和生理因素。采用实用技术来确定 MUP 可以帮助我们增加对胃肠道功能的了解，并提高 MR 消化率计算的准确性。
