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Keywords:

  • deoxyribonuclease;
  • cysteine;
  • disulfide;
  • site-directed mutagenesis;
  • thioredoxin;
  • thermal stability;
  • protein refolding
  • bp, bovine pancreatic;
  • br, bovine recombinant;
  • brDNase (F192C/A217C), the brDNase double mutant with changes of Phe192 to Cys192 and Ala217 to Cys217;
  • brDNase(C101A), the brDNase mutant with change of Cys101 to Ala101;
  • brDNase(C173A), the brDNase mutant with change of Cys173 to Ala173;
  • brDNase(C209A), the brDNase mutant with change of Cys209 to Ala209;
  • SDS, sodium dodecyl sulfate;
  • PAGE, polyacrylamide gel electrophoresis;
  • cm-Cys, S-carboxymethyl Cys

Abstract

We characterized the biochemical functions of the small nonessential (C101–C104) and the large essential (C173–C209) disulfides in bovine pancreatic (bp) DNase using alanine mutants [brDNase(C101A)] and [brDNase(C173A) and brDNase(C209A)], respectively. We also characterized the effects of an additional third disulfide [brDNase(F192C/A217C)]. Without the Ca2+ protection, bpDNase and brDNase(C101A) were readily inactivated by trypsin, whereas brDNase(F192C/A217C) remained active. With Ca2+, all forms of DNase, except for brDNase(C101A), were protected against trypsin. All forms of DNase, after being dissolved in 6 M guanidine-HCl, were fully reactivated by diluting into a Ca2+-containing buffer. However, when diluted into a Ca2+-free buffer, bpDNase and brDNase(C101A) remained inactive, but 60% of the bpDNase activity was restored with brDNase(F192C/A217C). When heated, bpDNase was inactivated at a transition temperature of 65°C, brDNase(C101A) at 60°C, and brDNase(F192C/A217C) at 73°C, indicating that the small disulfide, albeit not essential for activity, is important for the structural integrity, and that the introduction of a third disulfide can further stabilize the enzyme. When pellets of brDNase(C173A) and brDNase(C209A) in inclusion bodies were dissolved in 6 M guanidine-HCl and then diluted into a Ca2+-containing buffer, 10%–18% of the bpDNase activity was restored, suggesting that the “essential” disulfide is not absolutely crucial for enzymatic catalysis. Owing to the structure-based sequence alignment revealing homology between the “nonessential” disulfide of bpDNase and the active-site motif of thioredoxin, we measured 39% of the thioredoxin-like activity for bpDNase based on the rate of insulin precipitation (ΔA650nm/min). Thus, the disulfides in bpDNase not only play the role of stabilizing the protein molecule but also may engage in biological functions such as the disulfide/dithiol exchange reaction.