Catalyzes the conversion of allantoin (5-ureidohydantoin) to allantoic acid by hydrolytic cleavage of the five-member hydantoin ring. (S)-allantoin + H2O = allantoate + H(+) Binds 2 Zn(2+) ions per subunit. Can also use Ni(2+), Co(2+) or Mn(2+). Severely inhibited by copper, and competitively inhibited by dithiothreitol (DTT). Zinc concentrations above 2.5 mM and cobalt or nickel concentrations above 1 mM are inhibitors. Optimum pH is 7.5-8.0. The optimal activity is at pH 8.8 for zinc-containing form and pH 9.0 for cobalt-containing form. Optimum temperature is 40-45 degrees Celsius. Nitrogen metabolism; (S)-allantoin degradation; allantoate from (S)-allantoin: step 1/1. Homotetramer. By glyoxylate. Carboxylation allows a single lysine to coordinate two zinc ions. The zinc-containing form utilizes only the (S)-isomer of allantoin, whereas the cobalt-containing form prefers the (S)-isomer, but also hydrolyzes the (R)-isomer at about 1/10 the rate. Belongs to the metallo-dependent hydrolases superfamily. Allantoinase family.