Polyamine biosynthesis, degradation, and transmembrane transport. The polyamines spermine and spermidine are synthesized from arginine. Arginase converts arginine to ornithine, and ornithine decarboxylase (ODC) catalyzes decarboxylation of ornithine to form putrescine, a polyamine precursor containing two amine groups. ODC, a rate-limiting enzyme with a short half-life, is inhibited by antizyme, and antizyme is inhibited by an antizyme inhibitor. S-adenosylmethionine decarboxylase (AdoMetDC) is the second rate-limiting enzyme in polyamine synthesis and is involved in the decarboxylation of S-adenosylmethionine. Spermidine synthetase and spermine synthase are constitutively expressed aminopropyltransferases that catalyze the transfer of the aminopropyl group from decarboxylated S-adenosylmethionine to putrescine and spermidine to form spermidine and spermine, respectively. Polyamine degradation is achieved by spermine/spermidine N1-acetyltransferase (SSAT) and N1-acetylpolyamine oxidase (APAO). In addition, spermine oxidase (SMO) specifically oxidizes spermine. Polyamines are transported across the membrane transmembrane by the polyamine transporter.