Histidine is a basic amino acid that has an imidazole ring as an important functional group. It is a semiessential amino acid that plays a central role in the organism. For children in the growth phase and people with renal insufficiency, the need for histidine is so high that it can be classified as an essential amino acid for this group of people.
What is histidine?
Histidine, along with arginine and lysine, is one of the few basic amino acids. At the same time, it is also an aromatic amino acid with its imidazole ring. It occurs in two optically active forms. The enantiomer L-histidine is the actual proteinogenic amino acid. D-histidine has no biological significance. In the following, the term histidine always refers to L-histidine only. Histidine contains a total of six carbon atoms. Furthermore, it has two nitrogen atoms in the imidazole ring. In addition to the obligatory nitrogen in the alpha-standing amino group, there are therefore three nitrogen atoms in the total molecule. Because of the imidazole ring, histidine reacts in a basic manner. Histidine exists in two tautomeric forms because the hydrogen atom bonded to nitrogen in the imidazole ring moves back and forth between the two nitrogen atoms. The peculiar structure of the imidazole ring and the resulting basicity confer buffering properties to histidine-containing proteins.
Function, action, and roles
Histidine assumes a central role in the organism. Because its isoelectric point is in the neutral region, histidine represents the only amino acid that can act as both a proton acceptor and a proton donor. Thus, in addition to basic properties, it also possesses acidic properties. Because of this, histidine can participate in many enzymatic reactions associated with proton transfer. Its central role is expressed in the so-called catalytic triad. The catalytic triad is a sequence of amino acids aspartic acid, histidine and serine frequently found in enzymes. This structural unit catalyzes protein degradation through the hydrolytic cleavage of peptide bonds in proteins. Furthermore, histidine serves as a starting material for the red blood pigment hemoglobin. Since it has good complexing properties, it forms complexes with iron within histidine-containing proteins. It is thus present in ferritin and ensures its iron storage capacity within the organism. Due to its ability to bind to heavy metals, it also has a detoxifying effect on the body. Histidine also serves as a precursor for the synthesis of histamine. In this way, it ensures the strengthening of the immune system, since histamine is involved in the defense against foreign substances. Histidine is also the starting material for the production of glutamate. It is of great importance for growth processes. It is therefore an essential amino acid for growing children and adolescents. It also supports wound healing and acts against inflammation.
Formation, occurrence, properties and optimal values
Histidine, as mentioned above, is actually a non-essential amino acid that can be produced by the body itself. However, the yield of synthesis is so low that it must also always be supplied with food to meet demand. That is why today we prefer to speak of semiessential amino acid. In growing children, however, external supply is essential because the growth process requires large amounts of histidine. The same applies in severe diseases such as renal insufficiency. Healthy adults, however, are supplied with sufficient histidine from their diet. A histidine deficiency is rare in Central Europe and is only to be expected in the case of an extremely unbalanced diet. Particularly high amounts of histidine are found in chicken meat, salmon, nuts, soybeans, unpeeled rice, peas, milk and chicken eggs. Since these products or components of these products are found in almost all foods, an adequate supply of histidine should generally be ensured. However, even with a lower intake of histidine, the body’s own production does not increase.
Diseases and disorders
Because of the central role of histidine, its deficiency or impaired breakdown of histidine breakdown products such as histamine can lead to various health complaints. In certain diseases, histidine concentrations found in the body are too low. This is the case in chronic renal insufficiency or rheumatoid arthritis.It was found that an increased intake of histidine has a positive effect on rheumatoid arthritis. As part of its central role in metabolism, it has an anti-inflammatory effect, so that the strong inflammatory processes are strongly limited. The need for histidine is also increased in cases of severe injuries and traumas. Too low a concentration leads to delayed wound healing. If too little histidine is supplied during the growth phase, growth disorders occur. Histidine is a good scavenger of radicals and therefore also has the potential to slow down degenerative processes. Furthermore, the positive effect of histidine on high blood pressure and infections has been recognized. Blood pressure can be lowered in the long term and the duration of a cold can be significantly reduced. There is a genetic disease in which the degradation of histidine is disturbed. However, the autosomal recessive disorder known as histidinemia does not usually cause symptoms, so treatment is usually not necessary. Under unusual conditions, however, central nervous symptoms may appear. In such cases, a low-histidine diet is recommended. Very high histidine concentrations have sometimes been found in patients with stress, anxiety disorders or schizophrenia. Overall, however, the increased concentration of a synthesis product of histidine, histamine, plays a greater role in disease processes. Histamine is a substance which, when degraded, causes a variety of symptoms such as autoimmune disorders, allergies, cardiovascular diseases or gastrointestinal disorders. For treatment, histamine-rich foods should be avoided. However, foods with a high histamine content also have a high histidine content at the same time.
