[Modification of two tyrosine residues in aspergillopepsin A by p-nitrophenyldiazonium chloride]

p-Nitrophenyldiazonium chloride was found to modify the Tyr-75 and Tyr-189 residues in aspergillopepsin A. Incubation of the protein with a 45-fold molar excess of the reagent at pH 5,2 results in the attachment of 1,8 inhibitor residues, which leads to a 75% decrease of the enzyme activity towards haemoglobin. The pepsin inhibitor--pepstatin--protects one of the tyrosyl residues against modification. The chromatography on Sepharose-4B-Bacitracin showed that aspergillopepsin modified at pH 5.2, consists of molecules, which contain one and two incorporated inhibitor residues. The enzyme containing one inhibitor residue possesses 35% of activity of native protein. Apergillopepsin containing two inhibitor residues is practically inactive. It is concluded that at least one of the modified tyrosyl residues is closely related to the enzyme activity.