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Histidine is an important source of carbon atoms in the synthesis of purines, one of the two groups of nitrogen bases that make up DNA and RNA. It is also the direct precursor of histamine, one of the proteins involved in the immune response. Histidine is needed to help grow and repair body tissues and to maintain the myelin sheaths that protect nerve cells. It also helps manufacture red and white blood cells and helps to protect the body from heavy metal toxicity. Histamine stimulates the secretion of the digestive enzyme gastrin and acts as a catalytic site in certain enzymes. Measurement of histidine is included in the Amino Acids, Plasma and Amino Acids, Urine tests along with 23 other amino acids.

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Histidine (His/H) is one of the nine essential amino acids humans must get from their diet and is present in most protein-rich foods such as meat, fish, eggs, soy, whole grains, beans, and nuts. Histidine’s imidazole side chain is unique amongst amino acids, giving rise to its aromaticity and amphoteric properties at physiologic pH. This property makes it a key catalytic residue in many enzymes. It also performs important anti-inflammatory, anti-oxidant, and anti-secretory functions within the body.

Histidine is one of the least abundant amino acids in whole body protein in humans. The most abundant amino acids are proline (1328 g) and glycine (1247 g), both important in structural proteins, whereas there is only 245 g histidine, second only to tryptophan at 88 g.

In addition to free and protein-bound histidine in the diet, histidine can be obtained from the proteolysis of endogenous protein and from the hydrolysis of histidine-containing peptides in the diet. Besides its role in protein synthesis, it can be converted to histamine or to carnosine and excess can be catabolized.

Appropriate dietary intake of histidine is crucial, both during development and throughout life. Deficiencies in histidine, as well as genetic defects in histidine metabolism, can pose problems across various systems of the body. Noteworthy metabolic products are histamine, urocanic acid, and muscle dipeptides such as carnosine and anserine. As a neurotransmitter, histamine is crucial to modulating inflammatory response as well as gastric acid regulation. Urocanic acid (urocanate) is vital to epidermal barrier formation in the skin. It also has links to UV light absorption and immunosuppression. Finally, muscle dipeptides, like carnosine and anserine, play roles as homeostatic regulators that protect tissues.

Genetic mutations have been reported in 3 enzymes of the histidine catabolic pathway in the liver: histidase, urocanase, and glutamate formiminotransferase. All 3 disorders are relatively benign, although many of the patients have mental retardation which may be independent of the enzyme defects.

Histidinemia is the most frequent inborn metabolic error in Japan. It is characterized by increased concentrations of histidine in the blood and urine and decreased concentrations of urocanate in blood and skin. It results from decreased activity of the enzyme histidase. The initial characterization of the condition included mental retardation and speech impairment, but it is now apparent that there are diverse phenotypes of this disease, ranging from a benign phenotype in the majority of cases to mental retardation, in the minority of subjects.

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