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Histamine

BP-S-5-MEDICINAL CHEMISTRY
2 min read

Overview:

  • Chemical Nature: Histamine is an amine compound (hist + amine) released by tissues during allergic and inflammatory reactions, as well as in response to injury.

  • Function: Acts as a local response against parasite infections and protects cells from foreign substances.

Structure:

  • Histamine consists of an amine group (NH₂) attached to an imidazole ring.

Structure of histamine
Structure of histamine

Biosynthesis:

  • Precursor: Histidine

  • Enzyme: L-histidine decarboxylase catalyzes the decarboxylation of histidine to produce histamine.

Conversion of histidine to histamine by histidine decarboxylase
Conversion of histidine to histamine by histidine decarboxylase

Storage:

  • Primary Sources: Basophils and mast cells (types of white blood cells) store most of the body's histamine in granules.

  • Other Locations:

    1. Brain's hypothalamus (acts as a neurotransmitter)

    2. Enterochromaffin-like (ECL) cells of the stomach

    3. High concentrations in skin, mucosal cells of the lungs, intestine, urinary tract, and cerebrospinal fluid.

Mechanism of Release:

Histamine is released from mast cells through a process involving:

  1. Sensitization: Exposure to an allergen leads to the production of Immunoglobulin E (IgE) antibodies.

  2. Binding: IgE antibodies attach to receptors on the surface of mast cells.

  3. Activation: Upon re-exposure, the allergen cross-links the bound IgE antibodies.

  4. Degranulation: This triggers mast cells to release histamine and other inflammatory mediators such as prostaglandins, kinins, and platelet-activating factor.

Common Antigens

  • Antigens that can trigger histamine release include proteins and polysaccharides from sources like dust, pollen grains, food substances, venoms, toxins, detergents, and various chemicals.

Physiological Effects:

  • Hypersensitivity Reactions: Itching, sneezing, watery eyes, runny nose.

  • Smooth Muscle Contraction: Bronchoconstriction (asthma) and ileum contraction.

  • Vasodilation: Causes hypotension.

  • Gastric Secretion: Increases HCl secretion, leading to acidity.

  • Inflammation: Causes pain and allergic reactions.

Catabolism or Degradation of Histamine

Histamine is inactivated by enzymes that alter its structure, preventing it from binding to its receptors:

  • Diamine Oxidase (DAO): Metabolizes extracellular histamine.

  • Histamine-N-Methyltransferase (HNMT): Metabolizes intracellular histamine.


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