Completion of the amino acid sequence of papain

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Abstract
Papain was inhibited with bromo[2−14C]acetic acid, the tertiary structure of the inhibited enzyme was unfolded and the disulphide bridges were reduced with mercaptoethanol and aminoethylated. Digestion with trypsin gave a radioactive peptide consisting of residues 18–58 inclusive and containing therefore the sequence of the thirteen unknown residues 29–41 in the primary sequence of papain. This peptide was digested with pepsin to give a radioactive peptide consisting of residues 18–47, which after digestion with 0·4m-hydrochloric acid gave a radioactive peptide consisting of residues 24–43 inclusive. Further digestion with 6m-hydrochloric acid gave peptides that were used to determine the sequence: Ser-Ala-Val-Val-Thr-Ile-Glx-Gly-Ile-Ile-Lys-Ile-Arg for the residues 29–41, so completing the amino acid sequence of papain.