Flavoenzyme involved in polyamine back-conversion (PubMed:18583528, PubMed:20532512, PubMed:21081665). Catalyzes the oxidation of the secondary amino group of polyamines, such as spermine, spermidine and their acetyl derivatives (PubMed:18583528, PubMed:20532512, PubMed:21081665). Substrate preference is spermidine >spermine >N(1)-acetylspermidine >N(1)-acetylspermine (PubMed:18583528). Plays an important role in the regulation of polyamine intracellular concentration (Probable). Involved in the production of hydrogen peroxide during pollen tube growth (PubMed:20626657). Hydrogen peroxide triggers the opening of the hyperpolarization-activated calcium permeable channels in pollen, and thus regulates pollen tube growth (PubMed:20626657).