The action of pepsin on insulin and the plastein question.

N as determined by precipitation with 0 25N-trichloroacetic acid. In the second stage, the nonprotein N rises slowly to the 100% value. The biological action disappears at about the end of the first stage. Diffusion experiments showed that the 'protein-like' material left here had a molecular weight of only 4000, whilst that of the soluble fraction was about 800. The initial action of chymotrypsin thus involves the cleavage of about 10 or 11 peptides containing on the average 5 or 6 residues from each submolecule, leaving a fairly massive core. The latter is broken down in the second stage of the action. 3. The amino-N content of the 'core' indicates a probable chain length of 10-12 residues. The greater part of the cystine (from 75 to 85%) is in the core.