In one aspect, the present invention provides an isolated mutant insulin degrading enzyme (IDE) having an amino acid sequence that is at least 90% identical to SEQ ID NO:1 over its entire length and comprises at least one amino acid substitution at any of amino acid residues 332, 339, 341, 359, 360, 361, 374, 429, 609, 898, 899 or 901 of the sequence. The mutant IDE has a differential activity relative to that of wild-type IDE. Also provided is a polynucleotide encoding the polypeptide of the invention
