Knowledge of the complete amino acid sequence of papain is a necessary prerequisite to an understanding of structure to function relationships and the mechanism of action of this proteolytic enzyme. Furthermore, the structure of papain is of added interest as a model of a proteolytic enzyme displaying a requirement for a free sulfhydryl group. It should be recalled that papain shows those properties typical of a "sulfhydryl enzyme."' Investigation of the structure of papain was initiated by us a number of years ago and this effort has involved the collaboration of a number of investigators.2The amino acid composition of papain was derived from recent analyses performed on the Spinco amino acid analyzer. The total number of amino acids is close to 200 which is appreciably higher than the 178 reported in 1954.3 It will still be necessary to await the completion of the amino acid sequence in order to deduce the exact composition of this protein; the precision of measurement with a protein of this size places a degree of uncertainty on the exact number of residues.Papain contains a single sulfhydryl group at the active center.4 This group is partly "masked" in the crystalline enzyme but is present in stoichiometric amounts after activation with mercaptans4 5 or sodium borohydride.5 The activity of

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