Chemical modification of bovine pancreatic deoxyribonuclease with phenylglyoxal--the involvement of Arg-9 and Arg-41 in substrate binding

The inactivation of bovine pancreatic DNase by phenylglyoxal exhibits pseudo-first-order and pH-dependent kinetics. At 13.2 mM phenylglyoxal and 25 degrees C, the half-life of DNase is 8 min at pH 8.0 and 2 h at pH 6.7. Calcium, which binds to DNase, does not protect against or facilitate the reaction of DNase with the reagent. However, due to DNA-DNase interaction the half-life of DNase is approx. doubled in the presence of 0.2% (w/v) DNA. Modified DNase has apparently lost its ability to interact with DNA since it elutes behind native DNase on a Sepharose 4B column developed with buffer containing DNA. Complete inactivation of the enzyme is achieved when approx. 4 of the 12 arginines in DNase are modified at pH 6.7. The identification of the radioactive peptides, isolated from the proteolytic digest of 7-14C]phenylglyoxal-treated DNase, showed the four modified arginines to be Arg-9, -27, -30 and -41. Based on the data from dual labeling experiments using a mixture of DNase modified (without DNA protection) by radioactive phenylglyoxal and DNase modified (with DNA protection) by cold phenylglyoxal, it is concluded that Arg-27 and Arg-30 are essentially un-protected by DNA while Arg-9 and Arg-41 are protected part of the time. This conclusion agrees with the proposed substrate binding site in the three-dimensional structure of DNase (Suck, D., Lahm, A. and Oefner, C. (1988) Nature 332, 464-468) where Arg-9 and Arg-41 are among the residues responsible for interaction with DNA.