Proton pump inhibitors (omeprazole, esomeprazole, lansoprazole…) are prodrugs: weak bases with a pyridine nitrogen of pKa ≈ 4. The Henderson–Hasselbalch equation predicts how strongly they concentrate in an acidic compartment relative to neutral tissue:
[protonated] / [neutral] = 10^(pKa − pH)
pH 7.4 (blood): ratio ≈ 10^(4 − 7.4) ≈ 0.0004 (stays neutral, crosses membranes freely)
pH 1.0 (canaliculus): ratio ≈ 10^(4 − 1.0) = 1000 (protonates → trapped, can't diffuse back out)
This "ion trapping" concentrates the drug up to 1000-fold specifically inside the acid-secreting canaliculus of the gastric parietal cell. There, the acid catalyzes an intramolecular rearrangement into a reactive sulfenamide, which forms a covalent disulfide bond with cysteine residues (Cys813/Cys822) on the luminal face of the H⁺/K⁺-ATPase — the pump that exchanges cytoplasmic H⁺ for luminal K⁺ to acidify the stomach. The bond is essentially irreversible.
Because inhibition is covalent, acid secretion only recovers as the parietal cell synthesizes brand-new pump protein — modeled here as first-order recovery with a half-life τ (clinically ≈ 24–48 h, longer than the drug's own ~1 h plasma half-life, which is why once-daily dosing works):
dActive/dt = −k·[trapped drug]·Active + ln(2)/τ · (Total − Active)
- PPI dose — how fast new drug molecules enter the tissue and diffuse toward the canaliculus.
- Stop Dosing — halts new drug supply so you can watch the pump population recover on its own synthesis clock.
- Canalicular pH — how strongly the acid gradient traps and activates the drug; raise it toward neutral to see trapping collapse (this is also why PPIs only inhibit actively secreting pumps — take them before a meal).
- Pump synthesis half-life — how quickly the cell replaces covalently blocked pumps with new ones.