Department of Pediatrics, USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TX 77030, USA.
J Anim Sci. 2011 Dec;89(12):4093-9. doi: 10.2527/jas.2011-3944. Epub 2011 Aug 5.
Cysteine is a semi-indispensable AA in neonates and is synthesized from the indispensable AA, methionine, by transsulfuration. We previously showed that the gastrointestinal tract (GIT) is a metabolically important site of methionine transsulfuration to cysteine, yet the metabolic fate of dietary cysteine in the GIT has not been established. Cysteine use by gut epithelial cells may play an important role for maintenance of glutathione synthesis and cellular redox function. Our aim was to quantify the extent of gastrointestinal first-pass cysteine metabolism in young pigs. Four-week-old weanling pigs (n = 10) were fed a liquid milk-replacer diet and given an intragastric and intravenous [1-(13)C]cysteine infusion on 2 separate days in a crossover design. Arterial and portal blood samples were collected for cysteine isotopic enrichment by gas chromatography-mass spectrometry and for (13)CO(2) enrichment by isotope ratio mass spectrometry. Our results indicated that dietary cysteine is metabolized during its first-pass splanchnic metabolism, accounting for about 40% of dietary cysteine intake. We also showed that intestinal absorption was the major metabolic fate of dietary cysteine, representing about 75% of intake, indicating that the GIT utilizes 25% of the dietary cysteine intake. Thus, utilization by the GIT represents about one-half (approximately 53%) of the first-pass, splanchnic uptake of dietary cysteine. Moreover, a substantial proportion of dietary splanchnic cysteine metabolism was consumed by the GIT via nonoxidative pathways. We conclude that the gut utilizes 25% of the dietary cysteine intake and that synthesis of mucosal epithelial proteins, such as glutathione and mucin, are a major nonoxidative metabolic fate for cysteine.
半胱氨酸是新生儿必需氨基酸（AA），由必需氨基酸蛋氨酸通过转硫途径合成。我们之前的研究表明，胃肠道（GIT）是蛋氨酸向半胱氨酸转硫途径的重要代谢部位，但胃肠道中饮食半胱氨酸的代谢命运尚未确定。肠道上皮细胞对半胱氨酸的利用可能对半胱氨酸合成和细胞氧化还原功能的维持起着重要作用。我们的目的是定量研究幼猪胃肠道内的第一通过半胱氨酸代谢程度。将 4 周龄断奶仔猪（n = 10）饲养在液体牛奶替代饮食中，并在交叉设计的 2 天内分别进行胃内和静脉内[1-（13）C]半胱氨酸输注。采集动脉和门静脉血样，通过气相色谱-质谱法对半胱氨酸同位素丰度进行富集，通过同位素比质谱法对（13）CO2 丰度进行富集。我们的结果表明，饮食中的半胱氨酸在其第一次通过内脏代谢过程中被代谢，占饮食中半胱氨酸摄入量的 40%左右。我们还表明，肠道吸收是饮食中半胱氨酸的主要代谢命运，占摄入量的 75%左右，这表明 GIT 利用了饮食中半胱氨酸摄入量的 25%。因此，GIT 的利用代表了第一通过，内脏对半胱氨酸吸收的约一半（约 53%）。此外，饮食性内脏半胱氨酸代谢的很大一部分通过非氧化途径被 GIT 消耗。我们得出结论，肠道利用了饮食中半胱氨酸摄入量的 25%，而粘膜上皮蛋白（如谷胱甘肽和粘蛋白）的合成是半胱氨酸的主要非氧化代谢命运。
