Inhibition of Insulin Specific Protease* by Proteinase Inhibitors from the Snail (Helix pomatia)**

The enzyme, insulin specific protease, described in the rat diaphragm [1] and partially purified from rat muscle [2] and liver [3] very rapidly cleaves insulin with considerable specificity [2, 3] at physiological concentrations, optimally at pH 7.5. The identification and partial purification of polypeptide inhibitors of the enzyme in the α1-globulin fraction of human plasma has recently been described [4].