May be involved in the asymmetric conversion of racemic 5-substituted hydantoins to the corresponding L-amino acids. Catalyzes the racemization via enolization of D- and L-5-monosubstituted hydantoins. It shows a slight preference for hydantoins with short rather than long aliphatic side chains or those with aromatic rings. a D-5-monosubstituted hydantoin = a L-5-monosubstituted hydantoin Competitively inhibited by both D- and L-5-(methylthioethyl)hydantoin. Strongly inhibited by Cu(2+), Hg(2+), Pb(2+) and Zn(2+). The activity is twofold lower in the presence of Mn(2+), Co(2+) and Ni(2+). kcat is 6.42 sec(-1) for L-5-ethylhydantoin (L-EH) as substrate. kcat is 3.24 sec(-1) for D-5-isobutylhydantoin (D-IBH) as substrate. kcat is 2.29 sec(-1) for D-5-benzylhydantoin (D-BH) as substrate. kcat is 2.12 sec(-1) for L-5-isobutylhydantoin (L-IBH) as substrate. kcat is 1.94 sec(-1) for L-5-benzylhydantoin (L-BH) as substrate. Optimum pH is 8.5. The activity is gradually lost at temperatures of more than 30 degrees Celsius. Homotetramer. Belongs to the HyuE racemase family.