Isolation of an Activation Intermediate and Determination of the Amino Acid Sequence of the Activation Segment of Human Pepsinogen A1

Abstract

Upon activation of human pepsinogen A at pH 2.0 in the presence of pepstatin, an intermediate form was generated together with pepsin A. This activation intermediate could be separated from pepsinogen A and pepsin A by DE-32 cellulose chromatography at pH 5.5. It had a molecular weight intermediate between those of pepsinogen A and pepsin A, and contained about half the number of basic amino acid residues in pepsinogen A. It had phenylalanine as the amino(N)-terminal amino acid, and was deduced to be generated by release of the N-terminal 25 residue segment from pepsinogen A. Amino acid sequence determination of the N-terminal portions of pepsinogen A and the intermediate form enabled us to elucidate the entire amino acid sequence of the 47-residue activation peptide segment as follows: $Ile1$-Met-Tyr-Lys-Val-Pro-Leu-Ile-Arg- $Lys10$-Lys-Ser-Leu-Arg-Arg-Thr-Leu-Ser-Glu- $Arg20$-Gly-Leu-Leu-Lys-Asp-Phe-Leu-Lys-Lys- $His30$-Asn-Leu-Asn-Pro-Ala-Arg-Lys-Tyr-Phe- $Pro40$-Gln-Arg-Lys-Ala-Pro-Thr- $Leu47$⁠. On the other hand, upon activation of pepsinogen A at pH 2.0 in the absence of pepstatin, cleavage of the activation segment occurred at several additional bonds. In addition, upon activation both in the presence and in the absence of pepstatin, an additional activation intermediate, designated pepsin A', was formed in minor quantities. This form was identical with pepsin A, except that it had an additional Pro-Thr-Leu sequence preceding the N-terminal valine of pepsin A.