Which mucin is not precipitated by acetic acid during mucin testing?

• A. Gastric mucin
• B. Intestinal mucin
• C. Colonic mucin
• D. Bronchial mucin

Answer: (A)

In mucin testing, acetic acid is used to probe how different mucins respond to an acidic environment, which reflects their glycosylation and charge. Gastric mucin tends to be more neutral with fewer acidic residues, so it remains soluble in acetic acid and does not precipitate. The mucins from intestinal, colonic, and bronchial tissues contain higher levels of acidic groups (sialylated or sulfated residues) that promote intermolecular interactions and cause them to precipitate when exposed to acetic acid. So the mucin that does not precipitate is the one from the stomach, while the others precipitate under these conditions.