Histidine is an essential 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.
The measurement of histidine, along with 23 other amino acids, is included in the Amino Acids in Plasma and the Amino Acids in Urine tests.
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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 critical catalytic residue in many enzymes. It also performs essential anti-inflammatory, anti-oxidant, and anti-secretory functions within the body.
Histidine is one of the least abundant amino acids in human whole-body protein. The most abundant amino acids are proline (1328 g) and glycine (1247 g), both important in structural proteins. There is only 245 g of histidine, second only to tryptophan (88 g).
In addition to free and protein-bound histidine in the diet, histidine can be obtained from the proteolysis of endogenous protein and the hydrolysis of histidine-containing peptides. In addition to its role in protein synthesis, it can be converted to histamine or carnosine, and excess can be catabolized.
Appropriate dietary intake of histidine is crucial during development and throughout life. Deficiencies in histidine and genetic defects in histidine metabolism can pose problems across various body systems. Noteworthy metabolic products are histamine, urocanic acid, and muscle dipeptides such as carnosine and anserine. As a neurotransmitter, histamine modulates inflammatory response and 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, act as homeostatic regulators protecting tissues.
Genetic mutations have been reported in 3 enzymes of the histidine catabolic pathway in the liver: histidase, urocanase, and glutamate formiminotransferase. All three 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 reduced activity of the enzyme histidase. The initial characterization of the condition included mental retardation and speech impairment. Still, 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.
