The amino acid histidine (abbreviation His in the three-letter code and H in the one-letter code) is a proteinogenic amino acid (used to form proteins) with a basic imidazole ring in the side chain. Histidine can therefore be categorized as both a basic and aromatic amino acid. Only the L-configuration of the amino acid has a biological effect in the human body.
Histidine is now also considered essential (necessary for life) for adult humans. The human organism cannot synthesize histidine itself to a sufficient extent and a long-term histidine-free diet leads to deficiency symptoms. For infants, histidine is essential in any case.
The synthesis of histidine in metabolism occurs through several steps involving 8 different enzymes from phosphoribosyl pyrophosphate (PRPP) and ATP.
Proteins from the diet are cleaved into tripeptides and dipeptides (protein chains consisting of 3 and 2 amino acids, respectively) and free amino acids before absorption (uptake via the intestine). This cleavage by specific enzymes (exo- and endopeptidases) already starts in the stomach and continues in the small intestine.
In the brush border membrane of the mucosa cells (cells of the intestinal mucosa), special transport systems exist for the absorption of amino acids. Free amino acids are taken up by an active Na+-dependent transporter, while tri- and dipeptides are taken up by H+-coupled transport into the enterocytes (cells of the small intestinal epithelium). The proteins of the exfoliated cells of the small intestinal mucosa themselves are also broken down into their individual amino acids and reabsorbed. In the enterocytes, the tri- and dipeptides are hydrolyzed to free amino acids (cleaved by reaction with water) and transported to the liver.
The human body has a total protein inventory of approximately 10 to 11 kg. The pool of free amino acids in blood plasma is about 100 g. Less than 1% of the protein stock from the liver, kidney and small intestinal mucosa is so-called labile protein and can be broken down without affecting body function. Human body protein is in a dynamic state of build-up and degradation (protein turnover) and quickly adapts to the metabolic situation. The degradation and remodeling of the body’s own protein structures, in addition to the amino acids supplied through the diet, contributes significantly to the maintenance of the amino acid pool. The reutilization rate (recycling rate) from proteolysis (breakdown of proteins) of endogenous proteins can be as high as 90%.
Protein turnover in the body depends on nutritional status and availability of free amino acids. For example, an intake of 100 g of dietary protein results in a turnover of approx. 250 to 300 g of body protein, whereby individual amino acids are released and used, for example, for the daily renewal of intestinal mucosa cells, muscle metabolism or the formation and breakdown of plasma proteins.
The breakdown products of protein metabolism (protein metabolism) are nitrogen compounds such as urea, ammonia, uric acid and creatinine and are excreted in the urine. With normal protein intake, 80 to 85% of the total nitrogen is excreted as urea via the kidneys. This would correspond to about 80 g of protein per day.
Unabsorbed dietary protein and protein secreted (excreted) into the intestinal lumen are excreted in the feces (stool). This amount is equivalent to approximately 10 g of protein per day.