Catalyzes the formation of putrescine from agmatine. Cannot use arginine. agmatine + H2O = putrescine + urea Binds 2 divalent metal cations per subunit (By similarity). Co(2+) is the most effection cation. Can also use Ca(2+), Mn(2+), Zn(2+) or Mg(2+) (PubMed:15752699). Inhibited by putrescine. Activity is not affected by arginine and ornithine. Optimum pH is 11.0. Optimum temperature is 100 degrees Celsius. Amine and polyamine biosynthesis; putrescine biosynthesis via agmatine pathway; putrescine from agmatine: step 1/1. Homotetramer. Belongs to the arginase family. Agmatinase subfamily.