Involved in the degradation of the plant hormone indole-3-acetic acid (IAA). Catalyzes the hydrolysis of the cyclic amide bond (lactam) of isatin (1H-indole-2,3-dione) to yield isatinate (2-(2-aminophenyl)-2-oxoacetate). H2O + isatin = H(+) + isatinate Binds 1 manganese ion per subunit. Inhibited by thioisatinate. kcat is 24 sec(-1) for isatin as substrate. Homodimer. Belongs to the Cyclase 1 superfamily.