Flavoenzyme involved in polyamine back-conversion (PubMed:18703589, PubMed:20532512, PubMed:21081665, PubMed:26925084). Catalyzes the oxidation of the secondary amino group of polyamines, such as spermine and spermidine (PubMed:18703589, PubMed:20532512, PubMed:21081665, PubMed:26925084). Substrate preference is spermine > spermidine (PubMed:18703589, PubMed:21081665). No activity detected when putrescine or N(1)-acetylspermine are used as substrates (PubMed:18703589). Plays an important role in the regulation of polyamine intracellular concentration (Probable). H2O + O2 + spermine = 3-aminopropanal + H2O2 + spermidine H2O + O2 + spermidine = 3-aminopropanal + H2O2 + putrescine Binds 1 FAD per subunit. Optimum pH is 7.5-8.5. Amine and polyamine degradation; spermine degradation. Amine and polyamine degradation; spermidine degradation. Highly expressed in roots, flowers and greening cotelydons. Lower expression in other tissues. By abscisic acid, salicylic acid, wounding and flagellin 22, a pathogen elicitor. No visible phenotype under normal growth conditions. Belongs to the flavin monoamine oxidase family.