Histidine to histamine conversion

Histamine causes vasodilatation and bronchoconstriction. In the stomach it stimulates the secretion of hydrochloric acid (HCl). Histamine binds to H1 receptors in the stomach, stimulating the release of gastric acid. Pharmacological blockers of H1 receptors are used in the treatment of gastric reflux. H2 receptors are located on basophils; histamine stimulates their degranulation during the allergic responses. H2 receptor blockers are used to treat allergic conditions. Histamine is produced from Histidine by:

A. Deamination

B. Transamination

C. Carboxylation

D. Decarboxylation

E. Methylation

The correct answer is D- Decarboxylation.

Histamine is a biogenic amine. A biogenic amine is a potent signaling molecule formed from an amino acid by decarboxylation. Histamine, for example, is made from the common amino acid histidine by decarboxylation.

The reaction is catalyzed by histidine decarboxylase, (figure-1). The enzyme requires B6-P as a coenzyme. Bacteria also are capable of producing histamine using histidine decarboxylase enzyme unrelated to those found in animals. A non-infectious form of food born disease, occurs due to histamine production by bacteria in spoiled food, particularly fish. Fermented foods and beverages naturally contain small quantities of histamine due to a similar conversion performed by fermenting bacteria or yeasts.

Most histamine in the body is generated in granules in mast cells, basophils and eosinophils. Non-mast cell histamine is found in several tissues, including the brain, where it functions as a neurotransmitter. Another important site of histamine storage and release is the enterochromaffin-like (ECL) cell of the stomach.

histamine synthesis

Figure-1-The histidine to histamine conversion is catalyzed by histidine decarboxylase

Role in allergies and inflammation – Histamine release occurs when allergens bind to mast-cell-bound IgE antibodies. Upon first exposure to an allergen, the mast cells get sensitized by IgE and upon second exposure, the allergen cross links the IgE leading to degranulation and release of histamine(Figure-2)

Histamine release

Figure-2- Histamine release from mast cells.

Other Biogenic amines

The most common biogenic amines synthesized from their respective amino acids are

Arginine—Agmatine

Lysine—Cadaverine

Ornithine—Putrescine, Spermine, Spermidine

Phenylalanine—Phenyl ethylamine

Tryptophan—Tryptamine, Serotonin

Tyrosine–Tyramine

Glutamic acid- Gamma amino butyric acid (GABA)

As regards other options

A. Deamination- Deamination is the process of removal of alpha amino group of amino acids. It can be oxidative or non oxidative. Oxidative deamination reactions require FMN or FAD, whereas non oxidative reactions require B6-P as coenzymes.

B. Transamination- Transamination is the process of transfer of alpha amino group from a donor amino acid to an acceptor alpha keto acid for the formation of a new amino acid and a new keto acid. Vitamin B6-P acts as a coenzyme for transamination reactions.

C. Carboxylation- The introduction of carboxyl group (CO2)  in a molecule is carboxylation. It is an energy requiring process. Most of the carboxylases require Biotin as a coenzyme.

E. Methylation- These reactions occur either for the synthesis of important biological compounds or for the detoxification or degradation of endogenous or foreign toxic compounds. Methionine, Choline and Betaine are the important methyl group donors for methylation reactions.

 

 

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