Fig. 1. Polyamine catabolic pathways.

The two intersecting PA catabolic pathways are depicted. The first pathway involves SSAT that produces N1-acetylated PAs, which are either excreted from the cell through their specific transport systems or oxidized by PAOX, resulting in Put or Spd, H₂O₂ and 3Ac-AP. The second pathway involves a one-step reaction where Spm is directly oxidized by hSMOX to produce Spd, H₂O₂ and 3-AP. SPDS (spermidine synthase) and SPMS (spermine synthase) are the two enzymes that, through the addition of the aminopropyl group to putrescine and spermidine, generate spermidine and spermine, respectively.