S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = H(+) + S-methyl-5'-thioadenosine + spermine Amine and polyamine biosynthesis; spermine biosynthesis; spermine from spermidine: step 1/1. Present with 3870 molecules/cell in log phase SD medium. Belongs to the spermidine/spermine synthase family.