Beyond Mass Action: Regulation of Systemic Glutamine and Nitrogen Homeostasis

Abstract

Elevated serum ammonia (NH3) levels have lasting adverse effects on multiple organ systems, particularly the brain and the liver, resulting in neurological conditions such as hepatic encephalopathy. L-glutamine is a nonessential, glucogenic amino acid that serves as the primary nitrogen carrier responsible for maintaining nitrogen homeostasis in the human body. The amino acid transports excess ammonia from the tissues to the liver for waste disposal via the urea cycle, and it contributes to biosynthesis of purines, pyrimidines, and other amino acids required for tissue functioning. Investigating glutamine metabolism provides an avenue for identifying the mechanisms by which nitrogen is handled and regulated at the systemic level. By integrating stable isotope tracing and liquid chromatography-mass spectrometry with mouse models, this study aims to quantitatively profile systemic glutamine levels in response to exogenously delivered stable isotope-labeled glutamine as well as other nutrients. To accomplish this, I performed non-perturbative and perturbative infusions, oral gavage, and passive feedings of stable isotope-labeled nutrients in mice, with subsequent metabolomic profiling of extracted tissue and serum for analysis. Following enteral and intravenous U-13C515N2-glutamine delivery, I observed a dose-dependent reduction in endogenous synthesis of glutamine that was independent of tissue uptake or consumption. Moreover, this suppressive effect was attenuated in liver-specific glutamine synthetase knockout mice, highlighting the contribution of hepatic enzyme activity to systemic glutamine regulation. Our findings also provide evidence of an age-associated impairment of nitrogen handling through the urea cycle that was relevant to glutamine metabolism. This study defines how physiological and nutritional contexts modulate the active regulation of systemic glutamine levels at the level of production and nitrogen disposal, with perturbations at either level disrupting nitrogen homeostasis.