What is protein metabolism

   Amino acids absorbed in the intestine are used by hepatocytes for protein synthesis (plasma proteins ~ 20 g / d, can be increased to> 60 g / d) and deamination (production of ammonia / urea / glutamine; citrate cycle). Cells need glutamine for nucleotide synthesis and protein glycosylation, glutamate for the synthesis of other amino acids, α-ketoglutarate (citrate cycle) and glutathione. Glutamine is abundant in food and the leading amino acid in the blood plasma (~ 20% of the extracellular amino acid pool)
 
   Positive nitrogen balance (N-retention) in an anabolic metabolic situation (growth spurts, healing processes) lowers urea excretion (stimulated by insulin, somatotropin estrogens), negative nitrogen balance (N-loss) in a catabolic metabolic situation is accompanied by increased excretion (supported by catecholamines, glucagon, glucocortoids ). NH4+ is metabolized in the urea cycle (Krebs-Henseleit cycle) (6-20 g / d)
 
   The extracellular amino acid pool is ~ 0.5-0.9 mM / l, the liver stabilizes the concentration values ​​of the individual amino acids. Amino acids are transferred into cells via different transport systems, some of which are sodium-dependent. Glutamine (the most common amino acid in plasma) can be used quickly not only for anabolic processes (protein synthesis), but also for energy metabolism. Glutathione (GSH: glutamic acid, cysteine, glycine) has an antioxidant protective effect in cells (e.g. erythrocytes) and body fluids (bronchial secretions, aqueous humor)
 
   Plasma proteins are filtered by capillary action, migrate through the interstitium and back into the circulation via the lymphatic system. This exchange of plasma protein corresponds roughly to the amount present in the blood plasma (~ 0.2 kg) per day. Hypoproteinemia stimulates liver cells to divide and increase protein synthesis; the organ enlarges, the synthesis output can be tripled. When hungry, plasma proteins are also used as a source of amino acids (pinocytosis by macrophages, breakdown into amino acids, release to blood plasma, absorption in tissue). The ratio of total protein in the tissues to plasma protein pool is fairly constant 33: 1, even when hungry (protein reserve)