Pyrrolizidine alkaloids

The pyrrolizidine nucleus is characteristic of this group of alkaloids. The a is either L-ornithine or L-arginine, and the ^ is a biogenic amine, the putrescine. Oxidative deamination by enzyme NAD+ converts two molecules of putrescine into the imine (รง). By the activity of NADH, imine is reduced to homospermi-dine (x). Then, the pyrrolizidine nucleus is formed via a chain of reactions such as oxidative deamination, Schift base formation, oxidative reaction again and the intramolecular Mannich reaction. Retronecine (A), which is simple pyrrolizidine alkaloid, commonly occurs in nature. The formation of the pyrrolizidine structure is presented in Figure 55. Retronecine and its P, the senecionine, necine, heliotrine, indicine-N-oxide, malaxine, monocrotaline and absulin, are typical representatives of this group of alkaloids32'40'262. Pyrrolizidine alkaloids are widely dispersed throughout the natural world. According to Robins262 these co2h nh2

L-ornithine

Putrescine

Putrescine

CO2H

CO2H

CO2H

L-arginine

NAD+

NAD+

CO2H

CO2H

Senecionine P Figure 55. Structural development of pyrrolizidine alkaloids.

(Imine)

Senecionine P Figure 55. Structural development of pyrrolizidine alkaloids.

alkaloids have been found in 15 different families, although three plant families (Boraginaceae, Asteraceae and Fabaceae) are the most important sources of these compounds32. In Fabaceae, the genus Crotalaria is particularly representative of pyrrolizidine alkaloids, and in Asteraceae, the genus Senecio40. The characteristics of these alkaloids are as follows: (1) they are accumulated in plants as N-oxides; (2) they are poisons; (3) some of them have a bioimpact (e.g., indicine-N-oxide).

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