Institute of Animal Science, Physiology and Hygiene Unit, University of Bonn, 53115 Bonn, Germany.
Department of Clinical Science, Faculty of Veterinary Medicine, University of Tabriz, 516616471 Tabriz, Iran.
J Dairy Sci. 2019 Apr;102(4):3556-3568. doi: 10.3168/jds.2018-14463. Epub 2019 Feb 1.
Branched-chain α-keto acid dehydrogenase (BCKDH) complex catalyzes the irreversible oxidative decarboxylation of branched-chain α-keto acids. This reaction is considered as the rate-limiting step in the overall branched-chain amino acid (BCAA) catabolic pathway in mammals. For characterizing the potential enzymatic involvement of liver, skeletal muscle, adipose tissue (AT), and mammary gland (MG) in BCAA metabolism during early lactation, tissue and blood samples were examined on d 1, 42, and 105 after parturition from 25 primiparous Holstein cows. Serum BCAA profiles were analyzed and the mRNA and protein abundance as well as the activity in the different tissues were assessed for the BCAA catabolic enzymes, partly for the branched-chain aminotransferase and completely for BCKDH. Total BCAA concentration in serum was lowest on d 1 after parturition and increased thereafter to a steady level for the duration of the experiment. Pronounced differences between the tissues were observed at all molecular levels. The mRNA abundance of the mitochondrial isoform of branched-chain aminotransferase (BCATm) was greatest in AT as compared with the other tissues studied, indicating that AT might be an important contributor in the initiation of BCAA catabolism in dairy cows. From the different subunits of the BCKDH E1 component, only the mRNA for the β polypeptide (BCKDHB), not for the α polypeptide (BCKDHA), was elevated in liver. The BCKDHA mRNA abundance was similar across all tissues except muscle, which tended to lower values. Highest BCKDHA protein abundance was observed in both liver and MG, whereas BCKDHB protein was detectable in these tissues but could not be quantified. Adipose tissue and muscle only displayed abundance of the α subunit, with muscle having the lowest BCKDHA protein of all tissues. We found similarities in protein abundance for both BCKDH E1 subunits in liver and MG; however, the corresponding overall BCKDH enzyme activity was 7-fold greater in liver compared with MG, allowing for hepatic oxidation of BCAA transamination products. Reduced BCKDH activity in MG associated with no measurable activity in AT and muscle may favor sparing of BCAA for the synthesis of the different milk components, including nonessential AA. Deviating from previously published data on BCAA net fluxes and isotopic tracer studies in ruminants, our observed results might in part be due to complex counter-regulatory mechanisms during early lactation.
支链α-酮酸脱氢酶（BCKDH）复合物催化支链α-酮酸的不可逆氧化脱羧。在哺乳动物中，该反应被认为是整个支链氨基酸（BCAA）分解代谢途径的限速步骤。为了研究哺乳期早期肝脏、骨骼肌、脂肪组织（AT）和乳腺（MG）在 BCAA 代谢中的潜在酶参与，在产后第 1、42 和 105 天，从 25 头初产荷斯坦奶牛中采集组织和血液样本。分析血清 BCAA 谱，并评估不同组织中 BCAA 分解代谢酶的 mRNA 和蛋白质丰度以及活性，部分为支链氨基酸转氨酶，完全为 BCKDH。产后第 1 天血清总 BCAA 浓度最低，此后直至实验结束时逐渐升高。在所有分子水平上，组织之间都存在明显差异。线粒体同工型支链氨基酸转氨酶（BCATm）的 mRNA 丰度在 AT 中最高，与研究的其他组织相比，表明 AT 可能是奶牛 BCAA 分解代谢起始的重要贡献者。在 BCKDH E1 成分的不同亚基中，只有β多肽（BCKDHB）的 mRNA 升高，而不是α多肽（BCKDHA）。除肌肉外，所有组织的 BCKDHA mRNA 丰度相似，肌肉趋于低值。在肝脏和 MG 中观察到最高的 BCKDHA 蛋白丰度，而在这些组织中可检测到 BCKDHB 蛋白，但无法定量。脂肪组织和肌肉仅显示α亚基的丰度，所有组织中肌肉的 BCKDHA 蛋白最低。我们在肝脏和 MG 中发现了 BCKDH E1 亚基的蛋白丰度相似；然而，与 MG 相比，肝脏中的整体 BCKDH 酶活性高 7 倍，允许 BCAA 转氨基产物的肝氧化。MG 中 BCKDH 活性降低与 AT 和肌肉中无可测量活性相关，可能有利于 BCAA 用于不同乳成分的合成，包括非必需 AA。与反刍动物中先前发表的关于 BCAA 净通量和同位素示踪研究的数据不同，我们观察到的结果可能部分归因于哺乳期早期的复杂代偿机制。
