Polyamine biosynthesis and metabolic transport pathways. (1) Polyamine synthesis: arginine is converted into ornithine and agmatine, which is then catalyzed and decomposed into putrescine and urea by ornithine decarboxylase (ODC) and agmatine (AGMAT). Next, putrescine is converted to spermidine and spermine. (2) Methionine salvage: S-adenosylmethionine (dcAdoMet) decarboxylation provides aminopropyl for the formation of spermidine and spermine, and its product 5′-methylthioadenosine (MTA) is recovered to methionine through a series of enzymatic reactions. Subsequently, methionine is catalyzed by methionine adenosine transferase 2 (MAT2) and S-adenosylmethionine decarboxylase (AdoMetDC) to generate dcAdoMet. (3) ODC-AZ axis: the activity of ornithine decarboxylase is regulated by antizyme (AZ) and antizyme inhibitor (AZI). (4) Polyamine catabolism: spermidine and spermine are decomposed by spermidine/sperm-N-acetyltransferase (SSAT) to produce N-acetylspermidine and N-acetylspermine, respectively.