Catalyzes the hydrolysis of L-arginine to urea and L-ornithine (Probable). The latter can be utilized in the urea cycle or as a precursor for the synthesis of both polyamines and proline (Probable). Possesses agmatinase activity. Catalyzes the formation of putrescine from agmatine (By similarity). H2O + L-arginine = L-ornithine + urea agmatine + H2O = putrescine + urea Binds 2 manganese ions per subunit. Nitrogen metabolism; urea cycle; L-ornithine and urea from L-arginine: step 1/1. Amine and polyamine biosynthesis; putrescine biosynthesis via agmatine pathway; putrescine from agmatine: step 1/1. Forms homohexamers. Belongs to the arginase family.