Plasma glutamate concentrations are constant despite dynamic changes in diets. Most likely, virtually all the dietary glutamate is metabolized in the gut. The present study investigated permeability and metabolism of dietary glutamate in a Caco-2 intestinal epithelial cell layer model by tracing the fate of [U-¹³C] or [¹⁵N]glutamate added to the apical medium. For comparison, several other labelled essential and non-essential amino acids were tested as well. Almost all the labelled glutamate in the apical medium (98% and 96% at 24 h of the culture, respectively) was incorporated in the cell layer, while it barely appeared at the basolateral side, indicating an almost complete utilization of glutamate. Indeed, the ¹³C was incorporated into alanine, proline, ornithine, and glutamine, and the ¹⁵N was incorporated into alanine, glutamine, ornithine, proline, branched chain amino acids and also found as ammonia indicative of oxidation. In contrast, substantial apical-to-basolateral transport of amino acids (8–85% of uptake) other than glutamate and aspartate was evident in studies using amino acid tracers labelled with ¹³C, ¹⁵N or D. These results suggest that the intestinal epithelial cell monolayer utilizes dietary glutamate which adds to maintaining glutamate homeostasis in the body.

尽管饮食发生动态变化，但血浆谷氨酸浓度是恒定的。最有可能的是，几乎所有膳食谷氨酸盐都在肠道中代谢。本研究通过追踪添加到顶端培养基中的 [U- ¹³ C] 或 [ ¹⁵ N] 谷氨酸的命运，研究了 Caco-2 肠上皮细胞层模型中膳食谷氨酸的渗透性和代谢。为了进行比较，还测试了其他几种标记的必需和非必需氨基酸。顶端培养基中几乎所有标记的谷氨酸盐（分别在培养 24 小时时为 98% 和 96%）都被掺入到细胞层中，而它几乎没有出现在基底外侧，表明谷氨酸盐的利用几乎完全。确实，¹³C 结合到丙氨酸、脯氨酸、鸟氨酸和谷氨酰胺中，而¹⁵ N 结合到丙氨酸、谷氨酰胺、鸟氨酸、脯氨酸、支链氨基酸中，也被发现为指示氧化的氨。相比之下，在使用¹³ C、¹⁵ N 或 D标记的氨基酸示踪剂的研究中，除了谷氨酸和天冬氨酸之外，氨基酸（摄取的 8-85%）的大量顶端到基底外侧转运是明显的。这些结果表明肠道上皮细胞单层利用膳食谷氨酸盐来维持体内谷氨酸盐的稳态。