4-guanidinobutanoate + H2O = 4-aminobutanoate + urea D-arginine + H2O = D-ornithine + urea Binds 1 zinc ion per subunit. Inhibited by 5-aminovalerate, mercaptoacetate, 3-mercaptopropionate and 2-mercaptoethanol. Optimum pH is 7.5-9.5 with 4-guanidinobutyrate as substrate, and 9.5-10 with D-arginine or L-arginine as substrate. Homohexamer. Acts in decreasing order on 4-guanidinobutyrate, D-arginine, 3-guanidinopropionate, 5-guanidinovalerate and L-arginine. Belongs to the arginase family. Agmatinase subfamily.