TI - Steady-state kinetic studies and base-pair specificity of Ape1-NIR activity . AB - To further characterize the SUBstrate specificities of the Ape1 protein , the kinetic constants for the incision of THF*G and DHU*G and the influence of the base opposite were measured . Comparison of the kinetic constants ( Table 1 ) shows that under reaction conditions optimal for AP endonuclease activity ( 5 mM MgCl2 and pH 76 ) , the THF residue is the preferred SUBstrate for Ape1 ( KM 19 nM , kcat/KM 1600 min-1uM-1 ) . However , under reaction conditions optimal for NIR activity ( pH 68 and 05 mM MgCl2 ) , the kcat/KM values for the incision of THF*G , alphaA*T and DHU*G were in a similar range ( 45 , 16 and 19 min-1uM-1 ) . The apparent KM and kcat/KM values measured for Ape1 and hNth1 when acting upon DHU*G indicate that in vitro , the AP endonuclease is somewhat more efficient than the DNA glycosylase/AP lyase ( Table 1 ) . Thus , the analysis of kinetic constants suggests that Ape1 can efficiently back-up the DNA glycosylases to repair DHU residues . As shown in Supplementary Material , Figure 3 , Ape1 incised DHU , DHT , 5ohU and THF residues irrespective of the base opposite with similar efficiency . The apparently similar activity profiles of Ape1 towards different mismatches may reflect the fact that the enzyme recognizes a broad range of structures and is not sensitive to the variation in thermodynamic stability of the various mismatches .