Glutamine Glutamic acid and Ornithine aKetoglutarate Figure

Glutamine is the most abundant amino acid in plasma and in tissue. In glutamine-consuming cells it is readily converted by the enzyme glutaminase to form ammonia and glutamic acid, which is the primary intermediate in almost all routes of glutamine degradation. In the presence of ammonia this process can occur in reverse, catalyzed by the enzyme glutamine synthetase. In contrast to glutamic acid, glutamine can easily pass through the cellular membrane, thus exporting waste nitrogen out of the cell and serving as an inter-organ nitrogen carrier. In the kidney glutamine donates NH3, which is the acceptor for protons released from carbonic acid, to form NH4 and thus facilitates the formation of HCO3, which is essential in plasma pH regulation.

Following conversion to glutamic acid and subsequently a-ketoglutarate, glutamine may supplement intermediates of the citrate cycle. In this manner glutamine serves as the preferred fuel for rapidly dividing cells of, for example, the immune system cells and intestinal mucosa. In the brain glutamic acid is the most abundant excitatory neurotransmit-ter and the precursor for gamma-aminobutyric acid, which is an important inhibitory neurotransmitter. Glutamine is a direct precursor for purine and pyr-imidine and therefore is involved in RNA and DNA synthesis and cell proliferation. In addition it is a constituent of the tripeptide glutathione, which is the principal intracellular antioxidant in eukaryotes (see also sections on cysteine and glycine).

Glutamine

Purines, pyrimidines -(ammonia detoxification,

(RNA, DNA synthesis) nttrogen transport)

Ammonia

(renal bicarbonate production)

Cysteine

Alpha-ketoglutarate 4 Glutamic acid ( Glutathione

(gluconeogenesis) / (excitatory ^ (antioxidant)

neurotransmission) Glycine

Ornithine

(proline, arginine precursor)

Gamma-aminobutyric acid

(inhibitory neurotransmission)

Figure 3 Specific functions of glutamine and glutamine degradation products.

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