Chemical modification of human α1-proteinase inhibitor by tetranitromethane. Structure-function relationship

Abstract

Nitration of tyrosine residues of α 1-proteinase inhibitor (α 1-PI) by tetranitromethane yielded a product that maintained its inhibitory activity against trypsin but lost most of its inhibitory activity against elastase. Chemical analysis of the product showed that four out of the six tyrosine residues in α 1-PI had been nitrated to various degrees: Tyr-38 and Tyr-297 were not nitrated, whereas Tyr-138, Tyr-160, Tyr-187 and Tyr-244 were nitrated to extents in the range 40-80%. We interpreted these data to mean that modification of these tyrosine residues decreased the association constant between α 1-PI and the proteinases and that the decrease differs from one proteinase to the other. When either α 1-PI-trypsin or α 1-PI-elastase complex was nitrated, nitration took place only to a very slight extent at these latter four tyrosine residues. On the other hand, Tyr-38 and Tyr-297 underwent nitration to about 20%. We concluded that Tyr-138, Tyr-160, Tyr-187 and Tyr-244 were located on the surface of α 1-PI that interacts with either trypsin or elastase in the formation of complexes, and were therefore protected from nitration.