Activation of Pepsinogen to Pepsin

Pepsinogen is converted into its active form pepsin in the presence of:

Correct Answer: Gastric HCl

Pepsinogen (inactive zymogen) is converted to pepsin (active protease) by gastric HCl secreted by the parietal cells of the stomach (abomasum in ruminants).

Mechanism

• Pepsinogen is secreted by chief cells (zymogenic cells) of the gastric mucosa.
• Gastric HCl lowers the pH to ~2.0, causing autocatalytic cleavage of a peptide fragment from pepsinogen → pepsin.
• Pepsin itself can also activate more pepsinogen (autocatalytic activation) once the initial pepsin is formed.
• Pepsin is a protease that begins protein digestion in the stomach, cleaving peptide bonds — it is most active at pH 2.0.

Comparison with Trypsin Activation

• Pepsinogen → Pepsin: activated by gastric HCl (in stomach/abomasum)
• Trypsinogen → Trypsin: activated by Enterokinase (enteropeptidase) (in small intestine)

Why Other Options Are Wrong

• Enterokinase — activates trypsinogen, not pepsinogen.
• Trypsin — works in the small intestine; does not activate pepsinogen.