Raguso C A, Ajami A M, Gleason R, Young V R
School of Science, Massachusetts Institute of Technology, Cambridge 02139, USA.
Am J Clin Nutr. 1997 Aug;66(2):283-92. doi: 10.1093/ajcn/66.2.283.
There is evidence based on nitrogen balance that dietary cystine spares, from approximately 16% to 89%, the total methionine requirement. In a previous study we did not detect, by tracer techniques, a sparing effect of cystine when the diet provided methionine at a limiting intake (requirement level: 13 mg.kg-1.d-1). One reason could be that we used an intravenous infusion of the tracer, which may not, therefore, have labeled the carbon dioxide derived from the splanchnic oxidation of dietary methionine. The aim of this study was to compare methionine metabolism and oxidation in eight healthy adults given for 6 d each of three different diets: 13 mg (87.0 mumol) methionine.kg-1.d-1 and no cystine (diet A); 5 mg (33.5 mumol) methionine.kg-1.d-1 and no cystine (diet B); and 5 mg (33.5 mumol) methionine.kg-1.d-1 and 6.5 mg (52.4 mumol) cystine.kg-1.d-1 (diet C). On day 7, tracers ([1-13C, methyl-2H3]methionine and [2H2]cysteine) were administered orally at 30-min intervals for 8 h. Blood and breath samples were obtained for analysis during 3-h fasting and consecutive 5-h feeding periods. During fasting, methionine oxidation and methionine methyl (Qm) and carboxyl (Qc) fluxes and plasma concentrations were not affected by the amount of sulfur amino acids in the three diets. In the fed state methionine oxidation was significantly lower during diets B (3.0 +/- 0.5 mumol.kg-1.h-1) and C (2.8 +/- 0.6 mumol.kg-1.h-1) than during diet A (4.1 +/- 0.9 mumol.kg-1.h-1); there were no significant differences between diets B and C. Qm and Qc decreased with decreased methionine intake but no effect was observed by adding cystine. Cysteine flux (Qcys) was not affected by diet composition but it was lower during feeding than during fasting. In conclusion, replacing approximately 60% of the total requirement for methionine with cystine over a short diet period did not result in a detectable sparing of methionine oxidation.
有基于氮平衡的证据表明，膳食中的胱氨酸可节省约16%至89%的总蛋氨酸需求量。在之前的一项研究中，当饮食提供的蛋氨酸处于限制摄入量（需求量水平：13mg·kg⁻¹·d⁻¹）时，我们通过示踪技术未检测到胱氨酸的节省效应。一个原因可能是我们使用了示踪剂的静脉输注，因此可能未标记来自膳食蛋氨酸内脏氧化产生的二氧化碳。本研究的目的是比较八名健康成年人在分别给予三种不同饮食各6天的情况下蛋氨酸的代谢和氧化情况：13mg（87.0μmol）蛋氨酸·kg⁻¹·d⁻¹且无胱氨酸（饮食A）；5mg（33.5μmol）蛋氨酸·kg⁻¹·d⁻¹且无胱氨酸（饮食B）；以及5mg（33.5μmol）蛋氨酸·kg⁻¹·d⁻¹和6.5mg（52.4μmol）胱氨酸·kg⁻¹·d⁻¹（饮食C）。在第7天，每隔30分钟口服给予示踪剂（[1-¹³C,甲基-²H₃]蛋氨酸和[²H₂]半胱氨酸），持续8小时。在3小时禁食期和连续5小时进食期采集血液和呼气样本进行分析。在禁食期间，三种饮食中硫氨基酸的量对蛋氨酸氧化、蛋氨酸甲基（Qm）和羧基（Qc）通量以及血浆浓度没有影响。在进食状态下，饮食B（3.0±0.5μmol·kg⁻¹·h⁻¹）和饮食C（2.8±0.6μmol·kg⁻¹·h⁻¹）期间的蛋氨酸氧化显著低于饮食A（4.1±0.9μmol·kg⁻¹·h⁻¹）；饮食B和C之间没有显著差异。Qm和Qc随着蛋氨酸摄入量的减少而降低，但添加胱氨酸未观察到影响。半胱氨酸通量（Qcys）不受饮食组成的影响，但在进食期间低于禁食期间。总之，在短时间饮食期间用胱氨酸替代约60%的总蛋氨酸需求量并未导致可检测到的蛋氨酸氧化节省。
