Flavoenzyme involved in polyamine back-conversion (PubMed:24105034). Catalyzes the oxidation of the secondary amino group of polyamines, such as spermine and its acetyl derivatives (PubMed:24105034). Substrate preference is thermospermine > spermine > norspermine > N(1)-acetylspermine (PubMed:24105034). No activity detected when putrescine or spermidine are used as substrates (PubMed:24105034). Plays an important role in the regulation of polyamine intracellular concentration (Probable). H2O + O2 + spermine = 3-aminopropanal + H2O2 + spermidine H2O + N(1)-acetylspermine + O2 = 3-acetamidopropanal + H2O2 + spermidine H2O + norspermine + O2 = 3-aminopropanal + H2O2 + norspermidine H2O + O2 + thermospermine = 3-aminopropanal + H2O2 + spermidine Binds 1 FAD per subunit. Optimum pH is 6.0 with thermospermine as substrate (PubMed:24105034). Optimum pH is 8.5 with spermine as substrate (PubMed:24105034). Optimum temperature is 30 degrees Celsius with thermospermine as substrate (PubMed:24105034). Optimum temperature is 35 degrees Celsius with spermine as substrate (PubMed:24105034). Amine and polyamine degradation; spermine degradation. Induced in roots by exogenous treatment with spermine and thermospermine. Belongs to the flavin monoamine oxidase family.