Smith Gordon I, Villareal Dennis T, Mittendorfer Bettina
Washington University School of Medicine, St. Louis, MO 63110, USA.
Am J Physiol Endocrinol Metab. 2007 Sep;293(3):E666-71. doi: 10.1152/ajpendo.00185.2007. Epub 2007 May 29.
The goal of this study was to discover whether using different tracers affects the measured rate of muscle protein synthesis in human muscle. We therefore measured the mixed muscle protein fractional synthesis rate (FSR) in the quadriceps of older adults during basal, postabsorptive conditions and mixed meal feeding (70 mg protein x kg fat-free mass(-1) x h(-1) x 2.5 h) by simultaneous intravenous infusions of [5,5,5-(2)H(3)]leucine and either [ring-(13)C(6)]phenylalanine or [ring-(2)H(5)]phenylalanine and analysis of muscle tissue samples by gas chromatography-mass spectrometry. Both the basal FSR and the FSR during feeding were approximately 20% greater (P < 0.001) when calculated from the leucine labeling in muscle tissue fluid and proteins (fasted: 0.063 +/- 0.005%/h; fed: 0.080 +/- 0.007%/h) than when calculated from the phenylalanine enrichment data (0.051 +/- 0.004 and 0.066 +/- 0.005%/h, respectively). The feeding-induced increase in the FSR ( approximately 20%; P = 0.011) was not different with leucine and phenylalanine tracers (P = 0.69). Furthermore, the difference between the leucine- and phenylalanine-derived FSRs was independent of the phenylalanine isotopomer used (P = 0.92). We conclude that when using stable isotope-labeled tracers and the classic precursor product model to measure the rate of muscle protein synthesis, absolute rates of muscle protein FSR differ significantly depending on the tracer amino acid used; however, the anabolic response to feeding is independent of the tracer used. Thus different precursor amino acid tracers cannot be used interchangeably for the evaluation of muscle protein synthesis, and data from studies using different tracer amino acids can be compared qualitatively but not quantitatively.
本研究的目的是探究使用不同示踪剂是否会影响人体肌肉中测得的肌肉蛋白质合成速率。因此，我们通过同时静脉输注[5,5,5-(2)H(3)]亮氨酸和[环-(13)C(6)]苯丙氨酸或[环-(2)H(5)]苯丙氨酸，并采用气相色谱-质谱法分析肌肉组织样本，测量了老年人在基础状态、空腹状态和混合餐喂养（70毫克蛋白质×千克去脂体重(-1)×小时(-1)×2.5小时）期间股四头肌的混合肌肉蛋白质分数合成率（FSR）。与根据苯丙氨酸富集数据计算得出的结果（分别为0.051±0.004%/小时和0.066±0.005%/小时）相比，当根据肌肉组织液和蛋白质中的亮氨酸标记计算时，基础FSR和喂养期间的FSR均高出约20%（P<0.001）（空腹：0.063±0.005%/小时；喂养：0.080±0.007%/小时）。亮氨酸和苯丙氨酸示踪剂引起的喂养诱导FSR增加（约20%；P = 0.011）并无差异（P = 0.69）。此外，亮氨酸和苯丙氨酸衍生的FSR之间的差异与所使用的苯丙氨酸同位素异构体无关（P = 0.92）。我们得出结论，当使用稳定同位素标记的示踪剂和经典的前体-产物模型来测量肌肉蛋白质合成速率时，肌肉蛋白质FSR的绝对速率会因所使用的确示踪氨基酸而有显著差异；然而，对喂养的合成代谢反应与所使用的示踪剂无关。因此，不同的前体氨基酸示踪剂不能互换用于评估肌肉蛋白质合成，并且使用不同示踪氨基酸的研究数据可以进行定性比较，但不能进行定量比较。
