Brodsky Irwin G, Suzara Dennis, Hornberger Troy A, Goldspink Paul, Yarasheski Kevin E, Smith Samuel, Kukowski Jayme, Esser Karyn, Bedno Sheryl
Department of Medicine, University of Illinois at Chicago, Chicago, IL 60612, USA.
J Nutr. 2004 Feb;134(2):328-34. doi: 10.1093/jn/134.2.328.
The synthesis of muscle protein is restrained during dietary protein restriction. This is widely understood to vary quantitatively with the degree of nutritional deprivation, but there has been little discussion of qualitative changes in muscle protein deriving from dietary protein restriction. We studied 14 healthy subjects in a 2-sample study. Subjects were randomly assigned to a diet providing an ample, American-style protein intake (1.67 g. kg fat-free mass(-1). d(-1)) or a diet approximating the mean minimum adult protein requirement (0.71 g. kg fat-free mass(-1). d(-1)). We found that consumption of an isoenergetic diet at the mean adult minimum protein requirement for 4 wk produced an 81% lower fractional synthesis rate of myosin heavy chain (MHC) proteins in vastus lateralis muscle than did consumption of an ample protein diet (P = 0.05). Protein deprivation altered the skeletal muscle myosin composition such that the proportion of the total myosin content represented by fast-twitch MHC IIx was 51% lower than with ample intake (P = 0.013). The steady state content of MHC IIx messenger RNA (mRNA) did not differ in subjects consuming the minimum requirement of protein, suggesting that the reduced proportion of MHC IIx arises from posttranscriptional events. A 68% lower rate of 3-methylhistidine excretion with protein restriction (P < 0.01) suggests that myofibrillar protein degradation was lower. We conclude that dietary amino acid scarcity produces a change in myosin isoform distribution via posttranscriptional mechanisms. The relative contribution of inhibited myosin synthesis and inhibited degradation to the altered myosin isoform composition remains unknown. This has implications for the mechanisms by which amino acids govern muscle protein composition in vivo, and further exploration is required.
在饮食蛋白质限制期间，肌肉蛋白质的合成受到抑制。人们普遍认为，这种抑制在数量上会随着营养剥夺程度的不同而变化，但对于饮食蛋白质限制导致的肌肉蛋白质质量变化却鲜有讨论。我们在一项双样本研究中对14名健康受试者进行了研究。受试者被随机分配到两种饮食组，一组摄入充足的美式蛋白质（1.67 g·kg去脂体重⁻¹·d⁻¹），另一组摄入接近成人最低蛋白质平均需求量的饮食（0.71 g·kg去脂体重⁻¹·d⁻¹）。我们发现，以成人最低蛋白质平均需求量摄入等能量饮食4周后，股外侧肌中肌球蛋白重链（MHC）蛋白的合成率比摄入充足蛋白质饮食时低81%（P = 0.05）。蛋白质缺乏改变了骨骼肌肌球蛋白的组成，使得快肌MHC IIx在总肌球蛋白含量中所占比例比充足摄入时低51%（P = 0.013）。摄入最低蛋白质需求量的受试者中，MHC IIx信使核糖核酸（mRNA）的稳态含量并无差异，这表明MHC IIx比例降低是由转录后事件引起的。蛋白质限制时，3 - 甲基组氨酸排泄率降低68%（P < 0.01），这表明肌原纤维蛋白降解减少。我们得出结论，饮食中氨基酸缺乏通过转录后机制导致肌球蛋白异构体分布发生变化。肌球蛋白合成受抑制和降解受抑制对肌球蛋白异构体组成改变的相对贡献尚不清楚。这对氨基酸在体内调控肌肉蛋白质组成的机制具有重要意义，需要进一步探索。
