Bovine Pancreatic Deoxyribonucleases A and C

Peptide maps obtained with tryptic-chymotryptic hydrolysates of bovine pancreatic DNases A, B, and C show a high degree of similarity. The map for DNase C, however, shows one peptide to have moved to a significantly less basic position. The peptide in question was isolated from hydrolysates of A and C and its sequence determined in each instance. The results show that DNase C contains a proline residue at the position occupied by histidine-118 in the 257-residue sequence of DNase A. The results confirm the conclusion, initially based on the amino acid compositions of DNases A and C, that the two forms differ by a residue change that could arise from a single base change in the relevant gene. Histidine-118 in DNase A is thus not a residue essential to the activity of the enzyme.