2-nitro-5-thiosulfobenzoic acid as a novel inhibitor specific for deoxyribonuclease I

Bovine pancreatic deoxyribonuclease I (bpDNase I) contains four cysteine residues forming two disulfide bonds. Though there are no free sulfhydryl groups, incubation of bpDNase I with 2-nitro-5-thiosulfobenzoic acid (NTSB) in the presence of Ca(2+) or Mg(2+) at pH 7.5 results in inactivation of the enzyme. Amino acid analysis shows that NTSB-treated bpDNase I still contains all 4 half-cystine residues. The only amino acid residues having reduced values are threonine and serine, indicating that these may be the reaction sites for NTSB. Plasmid scission assay and circular dichroism analysis reveal the structural integrity of the inactivated enzyme. Treatment of bpDNase I with NTSB does not result in fragmentation, as demonstrated by SDS-PAGE analysis. NTSB binds bpDNase I through covalent modification, since dialysis and gel filtration can not reverse the inactivation reaction. However, after dilution into an acid buffer of pH 4.7, the inactivated enzyme regains about 40% of its initial activity, suggesting a reversible inactivation by acid treatment. NTSB does not inactivate DNase II, ribonuclease, chymotrypsin and lysozyme, while it effectively inactivates rat parotid DNase I. These results strongly suggest that NTSB can be considered as a novel inhibitor specific for DNase I.