A polypeptide having at least 70% homology, in particular 80%, 90% or 95% homology to the polypeptide of SEQ ID NO:2 representing the wild-type of the protein insect metalloproteinase inhibitor IMPIα and having at least one mutation at position 35, 36 and/or 39 of the amino acid sequence of IMPIα and the polypeptide having an IC50 value to thermolysine of less than the IC50 value of IMPIα wherein the nonpolar amino acid isoleucine at position 35 of IMPIα is replaced either by a nonpolar amino acid selected from the group consisting of leucine, methionine and phenylalanine or by polar amino acid selected from the group consisting of cysteine, asparagine, glutamine, histidine, lysine and arginine; and/or the nonpolar amino acid isoleucine at position 36 of IMPIα is replaced either by a nonpolar amino acid selected from the group consisting of valine, phenylalanine and tryptophan or by polar amino acid selected from the group consisting of tyrosine, serine, threonine, asparagine, glutamine, histidine, lysine and arginine; and/or the polar amino acid position 39 of IMPIα is replaced either by the nonpolar amino acid valine or by the polar amino acids histidine or lysine.