Selective loss of elastase inhibitory activity of alpha 1-proteinase inhibitor upon chemical modification of its tyrosyl residues.

Nitration or acetylation of tyrosine residues of human plasma alpha 1-proteinase inhibitor (alpha 1-PI) with tetranitromethane or N-acetylimidazole resulted in a selective loss of its inhibitory activity. While the chymotrypsin- and trypsin-inhibitory activities were unaffected, all of the elastase inhibiting activity was lost. Comparative amino acid analyses of the native inhibitor and the chemically modified derivatives revealed no differences in the amino acid compositions with the exception that in the case of the tetranitromethane-treated material there were losses in the tyrosine content which could be accounted for by the formation of 3-nitrotyrosine. Nitration of alpha 1-PI.trypsin, alpha 1-PI.chymotrypsin, and alpha 1-PI.elastase complexes with excess tetranitromethane indicated that complex formation protected the binding site of the inhibitor for elastase, but not for trypsin or chymotrypsin. Quantitative estimation of the number of nitrotyrosine residues in the alpha 1-PI isolated from the complexes has shown that one tyrosine residue was protected from nitration. Competition studies have also indicated that the elastase inhibitory site is different from that of trypsin or chymotrypsin, although the possibility of overlapping sites could not be disregarded.