Histamines and Anti-histaminic Drugs-Part-I: Introduction

In this week, I would like to give a brief introduction about an essential autocoid, i.e- histamine, various characteristics associated with it, role played by it and few histamine antagonists used commonly.

1)Histamines—
1.1) Definition and Category--
Histamines come under the category of autocoids (self healing elements of the body derived from two words-‘autos’ meaning self and ‘akos’ meaning healing). Histamines are derived primarily from cells and tissues, as indicated by the word ‘histos’ meaning tissues.
Histamine is chemically β-imidazolylethylamine. It may be represented by the chemical structure as drawn below:

Histamine (β-imidazolylethylamine)

1.2) Biosynthesis and Metabolism—
Biosynthesis—
Histamine is derived from Histidine moiety, an imidazole ring containing amino acid. Histidine is decarboxylated by an enzyme, decarboxylase, which eliminates a CO2 group from it thereby forming a histamine molecule.

Metabolism—
Histamine is metabolized in two ways—

1) Formation of N-methylhistamine
Histamine is methylated by an enzyme N-methyl Transferase, which transfers a methyl group at the N-position of histamine, i.e.- at NH2 position to form –NH-CH3 group.

2) Fomation of Imidazole acetic acid
An enzyme named as Diamine Oxidase oxidizes the –NH2 group of histamine to form –COOH group forming Imidazole acetic acid.

1.3) Release of Histamine—
• Histamine is positively charged and is held within the granules in cytoplasm of cells like mast cells etc. bounded by negatively charged proteins and heparin (anti-coagulant).
• However, during exocytosis of granules, there occurs an interchange in position of Na+ ions and histamine.
• Now, the position of histamine is occupied by Na+ ions and histamine is released free.