Epigenetic modification of murine Cu,Zn-superoxide dismutase

Binding of a single zinc ion to one subunit of copper-zinc superoxide dismutase apoprotein substantially influences the structure and stability of the entire homodimeric protein

The thermodynamics of zinc binding to metal-free (apo) human and bovine copper-zinc superoxide dismutases (SOD1) were measured using isothermal titration calorimetry. The apparent thermodynamics of zinc binding to the apoproteins were favorable (Ka > 108 M-1), with an observed stoichiometry of one zinc per homodimer. The change in heat capacity for the one-zinc binding event was large and negative (approximately -650 cal mol-1 K-1), suggestive of significant structural changes to the protein upon zinc binding. We further characterized the one-zinc derivative by circular dichroism and determined that this derivative had nearly the same secondary structure as the two-zinc derivative and that both are structurally distinct from the metal-free protein. In addition, we monitored the effect of zinc binding on hydrogen-deuterium exchange and accessibility of histidyl residues to modification by diethyl pyrocarbonate and observed that more than 50% protection was afforded by the binding of one zinc in both assays. Differential scanning calorimetry on the human SOD1 zinc derivatives also showed increased thermostability of the protein due to zinc binding. Further, the melting transitions observed for the one-zinc derivative closely resembled those of the two-zinc derivative. Finally, we observed that the quaternary structure of the protein is stabilized upon binding of one and two zinc ions in analytical ultracentrifugation experiments. Combined, these results suggest communication between the two monomers of SOD1 such that the binding of one zinc ion per homodimer has a more profound effect on the homodimeric protein structure than the binding of subsequent metal ions. The relevance of these findings to amyotrophic lateral sclerosis is discussed.

Classification of mouse submaxillary gland esteroproteases by their substrate specificities

The submaxillary gland esteroproteases were separated by two-dimensional polyacrylamide gel electrophoresis, and their substrate specificities were determined by histochemical staining procedures using cellulose acetate membranes. Twenty-one proteolytically active enzymes were classified into four groups based on substrate specificities on benzoyl-arginine-ethylester (BAEE), N-alpha-benzoyl-DL-arginine-p-nitroanilide (BAPNA), and poly-L-lysine. These types were further divided into eight subgroups by their sensitivity to inhibitors and androgen dependence. These results suggest that eight groups of esteroproteases are present in the submaxillary gland of male mice.