Chemical modification of carboxypeptidase A crystals. Nitration of tyrosine-248.

Nitration of bovine carboxypeptidase A crystals with tetranitromethane increases esterase activity, decreases peptidase activity, and modifies about one tyrosyl residue. Modification of enzyme crystals avoids the polymerization that occurs when the enzyme is nitrated in solution. Two procedures have been employed to identify the tyrosyl residues nitrated. The first involves cyanogen bromide cleavage and isolation of the fragment containing residues 104-301. After solubilization by succinylation, this fragment is digested with chymotrypsin, the peptides are fractionated by gel filtration, and the nitrotyrosyl peptides are purified by affinity chromatography on an antinitrotyrosyl antibody-Sepharose conjugate followed by ion-exchange chromatography. In the second, the nitroenzyme is heat denatured, digested by chymotrypsin, and fractionated on the affinity and ion-exchange columns. By both methods, the major mitropeptides, representing between 60 and 80% of the nitrotyrosyl label, are uniquely compatible with that segment of the sequence of carboxypeptidase containing Tyr-248. A nearby cation, either the active site zinc ion or Arg-145, would seem to be an important factor in determining the selective nitration of this residue.