Spermine biosynthesis in animals starts with decarboxylation of ornithine by the enzyme Ornithine decarboxylase in the presence of PLP.
This decarboxylation gives putrescine.
Thereafter the enzyme spermidine synthase effects two N-alkylation by decarboxy-S-Adenosyl methionine.
The intermediate is spermidine.

Plants employ additional routes to spermine.
In one pathway L-glutamine is the precursor to L-ornithine, after which the synthesis of spermine from L-ornithine follows the same pathway as in animals.

Another pathway in plants starts with decarboxylation of L-arginine to produce agmatine.
The imine functional group in agmatine then is hydrolysed by agmatine deiminase, releasing ammonia, converting the guanidine group into a urea.
The resulting N-carbamoylputrescine is acted on by a hydrolase to split off urea group, leaving putrescine.
After that the putrescine follows the same pathway to completing the synthesis of spermine.
Spermine biosynthesis in animals starts with decarboxylation of ornithine by the enzyme Ornithine decarboxylase in the presence of PLP. This decarboxylation gives putrescine. Thereafter the enzyme spermidine synthase effects two N-alkylation by decarboxy-S-Adenosyl methionine. The intermediate is spermidine.
Plants employ additional routes to spermine. In one pathway L-glutamine is the precursor to L-ornithine, after which the synthesis of spermine from L-ornithine follows the same pathway as in animals.
Another pathway in plants starts with decarboxylation of L-arginine to produce agmatine. The imine functional group in agmatine then is hydrolysed by agmatine deiminase, releasing ammonia, converting the guanidine group into a urea. The resulting N-carbamoylputrescine is acted on by a hydrolase to split off urea group, leaving putrescine. After that the putrescine follows the same pathway to completing the synthesis of spermine.
Spermine