Dietary B12 Release in the Stomach
Placeholder sentence: acid and pepsin liberate B12 from food proteins.
- ~1.5–3: Gastric pH (Placeholder short caption)
- Pepsin: Enzyme (Placeholder short caption)
- Animal protein: B12 source (Placeholder short caption)
- Stomach lumen: Site (Placeholder short caption)
Gastric liberation of protein-bound B12
Placeholder short paragraph describing acid/pepsin release of B12.
Placeholder highlight: achlorhydria impairs this first release step.
R-protein Binding and Pancreatic Cleavage
Placeholder sentence: haptocorrin shields B12 until pancreatic enzymes act.
- Haptocorrin: Carrier (Placeholder short caption)
- Trypsin/chymotrypsin: Cleaving enzymes (Placeholder short caption)
- Duodenum: Location (Placeholder short caption)
- Acidic→alkaline: pH shift (Placeholder short caption)
R-protein transfer and proteolytic release
Placeholder short paragraph describing R-protein handoff and cleavage.
Placeholder highlight: pancreatic insufficiency can trap B12 on R-protein.
Intrinsic Factor Binding
Placeholder sentence: parietal-cell intrinsic factor captures freed B12.
- Parietal cells: Secreted by (Placeholder short caption)
- Very high: Affinity (Placeholder short caption)
- B12–IF: Complex (Placeholder short caption)
- Protease-resistant: Stability (Placeholder short caption)
Formation of the stable B12–IF complex
Placeholder short paragraph describing intrinsic factor binding.
Placeholder highlight: pernicious anemia arises from IF autoantibodies.
Terminal Ileum Cubilin Receptor Uptake
Placeholder sentence: cubilin-amnionless receptors internalize the complex.
- Cubilin/amnionless: Receptor (Placeholder short caption)
- Terminal ileum: Site (Placeholder short caption)
- Receptor-mediated endocytosis: Process (Placeholder short caption)
- ~3–4 hours: Time lag (Placeholder short caption)
Cubilin-mediated endocytosis in enterocytes
Placeholder short paragraph describing cubilin receptor uptake.
Placeholder highlight: ileal resection removes this absorption site.
Transport to Circulation via Transcobalamin
Placeholder sentence: transcobalamin carries B12 from enterocyte to blood.
- Transcobalamin II: Transporter (Placeholder short caption)
- Basolateral membrane: Exit route (Placeholder short caption)
- Portal circulation: Destination (Placeholder short caption)
- Liver B12 pool: Storage (Placeholder short caption)
Basolateral export and systemic delivery
Placeholder short paragraph describing transcobalamin-bound transport.
Placeholder highlight: liver stores buffer years of B12 supply.