Imidazoleacetic riboside

Heterocyclic substances other than purines and pyrimidines are found in nucleotides and nucleosides. Some examples are nicotinamide in diphos-phopyridine nucleotide (DPN) and triphosphopyridine nucleotide (TPN), 5,6-dimethyl benzimidazole in vitamin Bu, and imidazoleacetic acid in imidazoleacetic riboside (S). 

Imidazoleacetic acid riboside Figure 4.11 Histamine metabolism. 

Histamine is degraded to imidazoleacetic acid, which either yields aspartic acid derivatives (Fig. 242) or is transformed to the riboside. 

Histamine possesses hormone activity in animals (E 3.1) and acts as a neurotransmitter (E 3.2). It is a plant feeding deterrent present, for instance, in nettle toxin (E 5.5.3). Also the alkaloid pilocarpine has strong physiological activity in vertebrates and deters potential predators (E 5.5.3). It is used as a cholinergic drug in medicine (F 2). Urocanic acid, hydantoin propionic acid, imidazoleacetic acid riboside, 1-methylhistidine, 1-methylimidazoleacetic acid and other imidazole derivatives are involved in the removal of L-histidine and histamine residues from the organism of animals. They are found in the urine. Urocanic acid is also a constituent of sweat (E 1). It is involved in the protection of skin to UV radiation (E 2.2). 

---