Interaction of Excitatory Amino Acid Transporters 1 – 3 (EAAT1, EAAT2, EAAT3) with N-Carbamoylglutamate and N-Acetylglutamate

Abstract

Background/Aims: Inborn deficiency of the N-acetylglutamate synthase (NAGS) impairs the urea cycle and causes neurotoxic hyperammonemia. Oral administration of N-carbamoylglutamate (NCG), a synthetic analog of N-acetylglutamate (NAG), successfully decreases plasma ammonia levels in the affected children. Due to structural similarities to glutamate, NCG may be absorbed in the intestine and taken up into the liver by excitatory amino acid transporters (EAATs). Methods: Using Xenopus laevis oocytes expressing either human EAAT1, 2, or 3, or human sodium-dependent dicarboxylate transporter 3 (NaDC3), transport-associated currents of NAG, NCG, and related dicarboxylates were assayed. Results: L-aspartate and L-glutamate produced saturable inward currents with K_m values below 30 µM. Whereas NCG induced a small inward current only in EAAT3 expressing oocytes, NAG was accepted by all EAATs. With EAAT3, the NAG-induced current was sodium-dependent and saturable (K_m 409 µM). Oxaloacetate was found as an additional substrate of EAAT3. In NaDC3-expressing oocytes, all dicarboxylates induced much larger inward currents than did L-aspartate and L-glutamate. Conclusion: EAAT3 may contribute to intestinal absorption and hepatic uptake of NCG. With respect to transport of amino acids and dicarboxylates, EAAT3 and NaDC3 can complement each other.

Short communication: Effects of dietary N-carbamoylglutamate supplementation on the milk amino acid profile and mozzarella cheese quality in mid-lactating dairy cows

“…Chacher et al (2012) reported that degradation of NCG in the rumen was approximately 18%. A previous study showed that the NCG is absorbed via excitatory AA 3 transporters into the small intestine and liver and used for Arg synthesis (Burckhardt and Burckhardt, 2017). Based on the milk yield reported in our previous study (Gu et al, 2018), approximately 0.37% of dietary NCG is transferred to the milk.…”

“…A low NCG concentration (<1 μg/kg) was found in the cheese of cows from the NCG group. Burckhardt and Burckhardt (2017) reported that NCG is a watersoluble substance, which can explain why most of the NCG (~60-73%) in the milk was transferred to the whey during the cheese-making process and not stored in the cheese. In addition, the stretching and brining processes might have destroyed or extracted NCG from the curd and cheese.…”