what substance converts the inactive pepsinogen to its active form, pepsin?
Parietal Cells and HCl. Once the inactive pepsinogen is secreted by the chief cells, it's inside the stomach. And it needs to be converted to its active form, pepsin, before it can work to break down the proteins in your stomach. To do this, we need hydrochloric acid (HCl), which is a very strong acid. The presence of hydrochloric acid creates an acidic environment in your stomach that is needed to convert pepsinogen to pepsin.