Catalyzes the formation of putrescine from agmatine. agmatine + H2O = putrescine + urea Binds 2 Fe(2+) ions per subunit. Can also use Mn(2+), with lower efficiency. Inhibited by putrescine and by the substrate analog arginine. Inactive when the purified enzyme is incubated with dithiothreitol followed by excess iodoacetic acid or N-ethylmaleimide. kcat is 0.032 sec(-1). Amine and polyamine biosynthesis; putrescine biosynthesis via agmatine pathway; putrescine from agmatine: step 1/1. Forms multimers. Belongs to the arginase family. Agmatinase subfamily.