Flavoenzyme involved in polyamine back-conversion (PubMed:21796433). Catalyzes the oxidation of the secondary amino group of polyamines, such as spermine (PubMed:21796433). Substrate preference is spermine > thermospermine > norspermine (PubMed:21796433). No activity detected when putrescine, spermidine or N(1)-acetylspermidine are used as substrates (PubMed:21796433). Plays an important role in the regulation of polyamine intracellular concentration (Probable). H2O + O2 + spermine = 3-aminopropanal + H2O2 + spermidine H2O + norspermine + O2 = 3-aminopropanal + H2O2 + norspermidine H2O + O2 + thermospermine = 3-aminopropanal + H2O2 + spermidine Binds 1 FAD per subunit. Amine and polyamine degradation; spermine degradation. Widely expressed. Belongs to the flavin monoamine oxidase family.