Branched-chain amino acids (BCAA) are major components of milk protein and important precursors for nonessential AA. Thus, the BCAA transport and break-down play a key role in the metabolic adaptation to the high nutrient demands in lactation. However, in monogastrics, increased BCAA levels have been linked with obesity and certain metabolic disorders such as impaired insulin sensitivity. Our objective was to study the effect of over-conditioning at calving on plasma BCAA levels as well as the tissue abundance of the most relevant BCAA transporters and degrading enzymes in dairy cows during late pregnancy and early lactation. Thirty-eight Holstein cows were allocated 15 wk antepartum to either a normal- (NBCS) or over-conditioned (HBCS) group, receiving 6.8 or 7.2 MJ of NEL/kg of DM, respectively, during late lactation to reach the targeted differences in body condition score (BCS) and back fat thickness (BFT; NBCS: BCS <3.5, BFT <1.2 cm; HBCS: BCS >3.75, BFT >1.4 cm) until dry-off. During the dry period and next lactation, cows were fed the same diets, whereby differences in BCS and BFT were maintained: prepartum means were 3.16 ± 0.06 and 1.03 ± 0.07 cm (NBCS) vs. 3.77 ± 0.08 and 1.89 ± 0.11 cm (HBCS), postpartum means were 2.89 ± 0.06 and 0.81 ± 0.05 cm (NBCS) vs. 3.30 ± 0.06 and 1.38 ± 0.08 cm (HBCS). Blood and biopsies from liver, semitendinosus muscle, and subcutaneous adipose tissue (scAT) were sampled at d 49 antepartum, 3, 21, and 84 postpartum. Free BCAA were analyzed and the mRNA abundance of solute carrier family 1 member 5 (SLC1A5), SLC7A5, and SLC38A2 as well as branched-chain aminotransferase 2 (BCAT2), branched-chain α-keto acid dehydrogenase E1α (BCKDHA), and branched-chain α-keto acid dehydrogenase E1β (BCKDHB) as well as the protein abundance of BCKDHA were assessed. Concentrations of all BCAA changed with time, most markedly in HBCS cows, with a nadir around calving. Apart from Ile, neither individual nor total BCAA differed between groups. The HBCS group had greater BCKDHA mRNA as well as higher prepartum BCKDHA protein abundance in scAT than NBCS cows, pointing to a greater oxidative capacity for the irreversible degradation of BCAA transamination products in scAT of over-conditioned cows. Prepartum hepatic BCKDHA protein abundance was lower in HBCS than in NBCS cows. In both groups, SLC1A5, SLC7A5, and BCAT2 mRNA were most abundant in scAT, whereas SLC38A2 was higher in scAT and muscle compared with liver, and BCKDHA and BCKDHB mRNA were greatest in liver and muscle, respectively. Our results indicate that scAT may be a major site of BCAA uptake and initial catabolism, with the former, however, being independent of BCS and time relative to calving in dairy cows.

中文翻译：

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支链氨基酸：处于高或正常身体条件下的奶牛脂肪组织，骨骼肌和肝脏中其转运蛋白和代谢酶的含量。

支链氨基酸（BCAA）是牛奶蛋白的主要成分，是非必需氨基酸的重要前体。因此，BCAA的运输和分解在泌乳过程中对高营养需求的代谢适应中起着关键作用。但是，在单胃中，BCAA水平升高与肥胖症和某些代谢紊乱（例如胰岛素敏感性受损）有关。我们的目的是研究产犊时过度调节对血浆BCAA水平以及最重要的BCAA转运蛋白的组织丰度和奶牛在怀孕后期和早期哺乳期降解酶的影响。将38头荷斯坦奶牛在产前15周分配到正常（NBCS）或过度适应（HBCS）组，分别接受6.8或7.2 MJ的NEL / kg的DM，在哺乳后期达到身体状况评分（BCS）和背部脂肪厚度（BFT; NBCS：BCS <3.5，BFT <1.2 cm; HBCS：BCS> 3.75，BFT> 1.4 cm）的目标差异，直至干燥。在干燥期和下一次泌乳期，给母牛喂相同的日粮，从而保持BCS和BFT的差异：产前平均值为3.16±0.06和1.03±0.07 cm（NBCS），而平均值为3.77±0.08和1.89±0.11 cm（HBCS） ），产后平均值为2.89±0.06和0.81±0.05厘米（NBCS），而3.30±0.06和1.38±0.08厘米（HBCS）。在产前49天，产后3天，21天和84天对来自肝脏，半腱肌和皮下脂肪组织（scAT）的血液和活检样本进行采样。分析了游离BCAA，并分析了溶质载体家族1成员5（SLC1A5），SLC7A5和SLC38A2以及支链氨基转移酶2（BCAT2）的mRNA丰度，评估了支链α-酮酸脱氢酶E1α（BCKDHA）和支链α-酮酸脱氢酶E1β（BCKDHB）以及BCKDHA的蛋白质丰度。所有BCAA的浓度均随时间变化，其中以HBCS奶牛最为明显，产犊时最低点。除Ile外，各组的个人BCAA和总BCAA均无差异。与NBCS奶牛相比，scCS中的HBCS组具有更高的BCKDHA mRNA含量和更高的产前BCKDHA蛋白丰度，这表明过度调节奶牛的scAT中不可逆降解BCAA转氨产物的氧化能力更高。HBCS的产前肝BCKDHA蛋白丰度低于NBCS奶牛。在两组中，与肝脏相比，scAT中SLC1A5，SLC7A5和BCAT2 mRNA最丰富，而scAT和肌肉中SLC38A2较高，BCKDHA和BCKDHB mRNA分别在肝脏和肌肉中最大。我们的结果表明，scAT可能是BCAA摄取和初始分解代谢的主要场所，但是前者与BCS和泌乳牛的产犊时间无关。