The Complete Amino Acid Sequence of Papain

The active cysteine of papain was labeled with W-iodoacetate and the cystine residues were reduced and coupled with unlabeled iodoacetate. The heptacarboxymethyl papain was then maleylated and hydrolyzed with trypsin. Key peptides were isolated from this hydrolysate which have permitted completion of the amino acid sequence of the protein. Thus, an earlier tentative and incomplete version of this sequence has been corrected and shown to be in accord with the studies of Drenth ei al. (DRENTH, J., JANSONIUS, J. N., KOEKOEK, R., SWEN, H. M., AND WOLTHERS, B. G., Nature, 218, 929 (1968)) by x-ray crystallographic methods.