Figure 1.

Schematic depiction of polyamine metabolism and transport. Anabolic and catabolic enzymes in yellow circles or green rectangles, respectively, with key metabolic enzymes in bold. Pharmacological inducer and inhibitor are italicized in green and red, respectively. Ornithine decarboxylase (ODC) synthesizes putrescine from ornithine; putrescine further gets converted into spermidine by spermidine synthase (SRM) and further to the largest polyamine spermine by spermine synthase (SMS). During catabolism, spermine can be back converted into smaller polyamines, i.e., spermidine and putrescine, by the concerted actions of spermidine/spermine N1-acetyltransferase (SAT1) and polyamine oxidase (PAOX) with an intermediate acetylation step mediated by SAT1; acetylated polyamines are either exported out of the cells or catabolized by PAOX. Spermine oxidase (SMOX), another enzyme of polyamine catabolism, directly converts spermine into spermidine without an intermediate acetylation step. In addition to de novo synthesis, polyamines can also be transported. BENSPM: bis(ethyl)norspermine; DMFO: 2-difluoromethylornithine.