The systematic name is S-adenosyl 3-(methylthio)propylamine:putrescine 3-aminopropyltransferase and it belongs to the group of aminopropyl transferases.

[1] With exception of the spermidine synthases from Thermotoga maritimum and from Escherichia coli, which accept different kinds of polyamines, all enzymes are highly specific for putrescine.

This is prevented by a conserved aspartatyl residue in the active site, which is thought to repel the carboxyl moiety of S-adenosyl methionine.

[3] The putrescine-N-methyl transferase whose substrates are putrescine and S-adenosyl methionine, and which is evolutionary related to the spermidine synthases, lacks this aspartyl residue.

[4] It is even possible to convert the spermidine synthase by some mutations to a functional putrescine-N-methyltransferase.