Figure 3.

Schematic representation of polyamine catabolism and conversions. In the first step, the classical degradation pathway for spermine and spermidine involves N-acetylation via the corresponding N-acetyl transferase. N-acetylated PAs are oxidized by peroxisomal polyamine oxidase, yielding spermidine or putrescine, respectively. Alternately, spermine may be directly oxidized by spermine oxidase to N⁸-3-propyl-spermidine (upper left side), which spontaneously splits off acrolein and thus is converted to spermidine. This molecule, instead of N-acetylation, may be oxidized by diamine oxidase to N¹-3-propyl-putrescine (lower left side), which again under loss of acrolein forms putrescine. There are two side pathways. Spermidine may be attached to a lysine side chain of a nascent protein, which subsequently is hydroxylated to the functional elF5A-hypusine transcription factor (right side). In a separate pathway, N¹-3-propyl-putrescine, the product of the oxidation of N-acetyl-spermidine by diamine oxidase, is reduced to putreanine.