The glutamine-alpha-ketoglutarate (AKG) metabolism and its nutritional implications

l-Glutamine is a nutritionally semi-essential amino acid for proper growth in most cells and tissues, and plays an important role in the determination and guarding of the normal metabolic processes of the cells. With the help of transport systems, extracellular l-glutamine crosses the plasma membrane and is converted into alpha-ketoglutarate (AKG) through two pathways, namely, the glutaminase (GLS) I and II pathway. Reversely, AKG can be converted into glutamine by glutamate dehydrogenase (GDH) and glutamine synthetase (GS), or be converted into CO2 via the tricarboxylic acid (TCA) cycle and provide energy for the cells. Different steps of glutamine metabolism (the glutamine-AKG axis) are regulated by several factors, rendering the glutamine-AKG axis a potential target to counteract cancer. Moreover, intracellular glutamine plays an important role in cellular homeostasis not only as a precursor for protein synthesis, but also for its nutritional roles in cell growth, lipid metabolism, insulin secretion, and so on. The main objective of this review is to highlight the metabolic pathways of glutamine to AKG, with special emphasis on nutritional and therapeutic use of glutamine-AKG axis to improve the health and well-being of animals and humans.