Effects of Exogenous Monosodium Glutamate on GABA Accumulation and Volatile/Nonvolatile Metabolites in Black Highland Barley

Abstract

γ-Aminobutyric acid (GABA) is a functional amino acid with multiple health benefits; however, the mechanism of its enrichment in black highland barley, particularly through precursor supplementation, has not been fully elucidated. In this study, the effects of exogenous monosodium glutamate (MSG) on GABA accumulation and metabolite regulation were investigated. Precursor types and germination conditions were optimized using a single-factor approach, and under the optimal MSG concentration, ultra-high performance liquid chromatography-mass spectrometry (UHPLC–MS), gas chromatography–mass spectrometry (GC–MS), and gas chromatography-ion mobility spectrometry (GC-IMS) were employed to profile amino acids, fatty acids, and volatile and nonvolatile metabolites. The results showed that MSG addition significantly enhanced GABA accumulation compared with glutamic acid (p < 0.05), reaching a maximum of 2.55 ± 0.12 mg·g⁻¹. The optimal MSG concentration ranged from 50 to 75 mmol·L⁻¹, indicating a concentration-dependent effect on GABA accumulation. A total of 1248 metabolites were identified, covering multiple classes including amino acids, fatty acids, and volatile compounds, indicating substantial metabolic variation during germination. Mechanistically, MSG was converted into glutamic acid under specific pH conditions and subsequently decarboxylated by glutamate decarboxylase, which was associated with enhanced endogenous GABA accumulation and activation of the GABA shunt. This process may contribute to the supply of carbon skeletons and energy to the tricarboxylic acid cycle, thereby promoting the synthesis of unsaturated fatty acids and amino acid-derived compounds, and ultimately increasing the levels of flavor-related metabolites. These findings provide new insights into the regulatory mechanism of GABA enrichment induced by MSG and offer a theoretical basis for improving GABA production and the high-value utilization of black highland barley.