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 Km 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 (Km 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.

Keywords: 3N-carbamoylglutamate; Excitatory amino acid transporters; Hyperammonemia; N-acetylglutamate; Oxaloacetate; Sodium-dependent dicarboxylate transport; Urea cycle disorders.