Occurrence of two different pathways in the activation of porcine pepsinogen to pepsin.

Activation of porcine pepsinogen at pH 2.0 was found to proceed simultaneously by two different pathways. One pathway is the direct conversion process of pepsinogen to pepsin, releasing the intact activation segment. The isolation of the released 44-residue segment was direct evidence of this one-step process. At pH 5.5 the segment bound tightly to pepsin to form a 1:1 pepsin-activation segment complex, which was chromatographically indistinguishable from pepsinogen. The other is a stepwise-activating or sequential pathway, in which pepsinogen is activated to pepsin through intermediate forms, releasing activation peptides stepwisely. These intermediate forms were isolated and characterized. The major intermediate form was shown to be generated by removal of the amino-terminal 16 residues from pepsinogen. The released peptide mixture was composed of two major peptides comprising residues 1-16 and 17-44, and hence the stepwise-activating process was deduced to be mainly a two-step process.