Previous structural and mutational studies of bovine pancreatic deoxyribonuclease I (bpDNasc I) have demonstrated that the active site His 134 and His252 played critical roles in catalysis. In our present study, mutations of these two His residues to Gin, Ala or Gly reduced the DNase activity by a factor of four to five orders of magnitude. When imidazole or primary amines were added exogenously to the Ala or Gly mutants, the residual DNase activities were substantially increased by 60-120-fold. The rescue with imidlazole was pH-and concentration-dependent. The pH-activity profiles showed nearly bell-shaped curves, with the maximum activity enhancement for H134A at pH 6.0 and that for H252A at pH 7.5. These findings indicated that the protonated form of imidazole was responsible for the rescue in H134A, and the unprotonated form was for that in H252A, prompting us to assign unambiguously the roles for His 134 as a general acid, and His252 as a general base, in bpDNase I catalysis. (c) 2006 Elsevier Inc. All rights reserved.