Bergman E N
Fed Proc. 1986 Jul;45(8):2277-82.
There are many complexities and theoretical aspects to consider for studies of amino acid absorption and metabolism by the gut, liver, and peripheral tissues. The experimental approach must vary depending on the amino acid. Whether to sample whole blood or plasma has to be considered carefully. Also, a specific blood vessel has to be chosen for taking samples. A jugular vein can be the poorest sampling site for many studies. The amounts of individual amino acids appearing in portal blood are different from amounts disappearing from the gut lumen. Some are absorbed in amounts equal to that disappearing but most are absorbed in lesser quantities because of intestinal metabolism. Further, the liver removes absorbed amino acids and synthesizes plasma proteins, urea, and glucose. Peripheral tissues, of course, exchange amino acids with protein for normal turnover but also use amino acids for oxidation and transamination. Alanine, glutamine, glycine, and arginine are important in transporting nitrogen out of peripheral tissues in a nontoxic form. Branched-chain amino acids are removed by both liver and peripheral tissues mainly for plasma protein and ketoacid formation, respectively. During fasting, however, muscle releases branched-chain amino acids while removal by liver is maintained.
对于肠道、肝脏和外周组织对氨基酸的吸收和代谢研究，有许多复杂因素和理论层面需要考虑。实验方法必须根据氨基酸的不同而有所变化。必须仔细考虑是采集全血还是血浆样本。此外，还必须选择特定的血管进行采样。对于许多研究而言，颈静脉可能是最差的采样部位。门静脉血中出现的单个氨基酸的量与从肠腔中消失的量不同。有些氨基酸的吸收量与消失量相等，但由于肠道代谢，大多数氨基酸的吸收量较少。此外，肝脏会去除吸收的氨基酸并合成血浆蛋白、尿素和葡萄糖。当然，外周组织会与蛋白质交换氨基酸以进行正常周转，但也会利用氨基酸进行氧化和转氨作用。丙氨酸、谷氨酰胺、甘氨酸和精氨酸在以无毒形式将氮转运出外周组织方面很重要。支链氨基酸分别主要被肝脏和外周组织去除，用于形成血浆蛋白和酮酸。然而，在禁食期间，肌肉会释放支链氨基酸，而肝脏对其的清除则保持不变。
