Histamine: Overview, Structure, Biosynthesis, Storage, Mechanism of Release, Physiological Effects & Catabolism or Degradation of Histamine

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.

Biosynthesis:

• Precursor: Histidine

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

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.