Natural histamines

Twelve histamines have been so far identified in living animal tissues. The two reactive sites of the histamine molecule involved in the production of these derivatives are represented by the imino nitrogen of the nucleus and the amino group of the lateral chain. The ring imino nitrogen may be methylated, to form 3-methylhistamine, or conjugated with ribose to give origin to histamine riboside, ribotide and dinucleotide. The amino group of thelateral chain, in its turn, may be methylated (N'-methyl-histamine, N',N'-dimethylhistamine), conjugated with aliphatic acids (N'-imidazoleproprionylhistamine, N'-urocanylhistamine) or amino acids (beta-alanylhistamine or carcinine), and finally linked, through a methyl group, to the imidazole nucleus to give origin to imidazo-c-pyridine derivatives or cyclized histamines. The biosynthesis of the above natural histamines and their physiological significance are discussed.