Reactive sites of the 21-kD protein inhibitor of serine proteinases from potato tubers

The effect of modifications of Met, Arg, and Lys residues on the inhibitory activity of a serine proteinase-inhibiting 21-kD protein from potato tuber s has been studied. The data indicate that the 21-kD protein has two indepe ndent reactive sites for human leukocyte elastase (or chymotrypsin) and try psin. It is concluded that the 21-kD inhibitor has Met and Arg residues in the P1 position of the reactive sites responsible for interactions with ela stase (or chymotrypsin) and trypsin. It is shown that the 21-kD protein is capable of farming a triple complex binding simultaneously one molecule of trypsin and one molecule of chymotrypsin.