The amino acid sequence at the N terminus of pepsin.

Attempts to apply the technique of Edman (l), for the stepwise degradation of peptides with the formation of phenylthiohydantoins (PTH), to the study of the N-terminal amino acid sequence of pepsin have resulted in only qualified success. One of the principal drawbacks of this technique arises from the fact that the reactions entailed are usually not complete. Thus, residual amounts of terminal amino acid, which are, theoretically at least, supposed to have been removed during the first degradative step, still remain attached to some of the protein molecules and appear as the products of subsequent degradations of the protein. This also occurs with the succeeding amino acids in the sequence. In the situation in which the succeeding amino acid residues yield different phenylthiohydantoins, an extensive portion of the sequence might be elucidated by this method. However, when succeeding or closely neighboring amino acid residues are the same, the number and arrangement of the identical residues cannot always be determined from qualitative data alone. By means of the reactions for the formation of PTH and the reaction with dinitrofluorobenzene (DNFB), it was found that the first two amino acids at the N terminus of pepsin were leucine and glycine. These amino acids were followed by what appeared to be more than one aspartyl residue. In order to determine the number of aspartyl residues and the nature of the subsequent amino acids, a quantitative study of the phenylthiohydantoins derived from the amino acids in the N-terminal sequence of pepsin was undertaken.