Polyamines biosynthesis, scheme

Precursors of /1-alanine 3 biosynthesis are uracil 2, L-aspartate 4, and polyamines which are subjected to degradation in distinct ways - whereas uracil is metabolized by hydrogenation followed by hydrolysis, polyamines like spermine 1 are oxidized and L-aspartate is decarboxylated (Scheme 1.6.1). 

Scheme 1.6.4. Biosynthesis of /-alanine by oxidation of polyamines according to Toyn [17. ...

Most pyrrolizidine alkaloids are esters of basic alcohols known as necine bases. The most frequently studied pyrrolizidine alkaloids are formed from the polyamines, putrescine and spermidine, and possess one of three common necine bases retronecine, heliotridine, and otonecine. Putrescine is utilized exclusively as a substrate in secondary metabolism, whereas spermidine is a universal cell-growth factor involved in many physiological processes in eukaryotes. Spermidine biosynthesis begins with the decarboxylation of SAM by SAM decarboxylase 165). The aminopropyl group is then transferred from decarboxylated SAM to putrescine by spermidine synthase to form spermidine (Scheme 5). Putrescine can be produced from ornithine by ODC. However, putrescine is derived from the arginine-agmatine pathway in pyrrolizidine alkaloid-producing plants due to the absence of ODC activity 166). 

---