Catalyzes the hydrolysis of N-formyl-L-kynurenine to L-kynurenine, the second step in the kynurenine pathway of tryptophan degradation. H2O + N-formyl-L-kynurenine = formate + H(+) + L-kynurenine Binds 2 zinc ions per subunit. kcat is 50.56 sec(-1) for N-formyl-L-kynurenine as substrate. Amino-acid degradation; L-tryptophan degradation via kynurenine pathway; L-kynurenine from L-tryptophan: step 2/2. Homodimer. Belongs to the Cyclase 1 superfamily. KynB family.