Fig. 1.

Schematic diagram depicting the polyamine metabolic pathway. Spermidine (Spd) and spermine (Spm) are synthesized by the sequential addition of aminopropyl groups derived from decarboxylated S-adenosylmethionine (dcSAM) onto putrescine (Put) and Spd to form Spd and Spm, respectively. Catabolism of polyamines proceeds via the backconversion pathway or via the 1-step Spm oxidation by spermine oxidase (SMO). In the spermidine/spermine N-1-acetyltransferase (SSAT)/polyamine oxidase (PAO)-mediated polyamine backconversion pathway, Spm and Spd are first acetylated by SSAT, the rate-limiting enzyme of the pathway, and then oxidized by PAO, leading to Put and cytotoxic by-products (i.e., H₂O₂, acetyl-3-aminopropionaldehyde). SAM, S-adenosylmethionine; SAMDC, S-adenosylmethionine decarboxylase; MTA, 5-methylthioadenosine; ODC, ornithine decarboxylase.