14.4 Pancreatic and Biliary Digestion & Intestinal Nutrient Absorption

14.4 Pancreatic and Biliary Digestion & Intestinal Nutrient Absorption

Pancreatic and Biliary Digestion in the Duodenum

The duodenum functions as the master biochemical hub of the digestive tract. Acidic chyme entering the duodenum stimulates enteroendocrine signaling to orchestrate neutralizing and digestive secretions from the exocrine pancreas and hepatobiliary system.

1. Duodenal Enteroendocrine Signaling

• Secretin: Synthesized by enteroendocrine S cells in duodenal crypts. Released in response to luminal free fatty acids and protons ($pH < 4.5$). Secretin travels via the bloodstream to pancreatic ductal epithelial cells, binding $G_s$-coupled receptors to stimulate cAMP production and open apical CFTR chloride channels. This drives the apical $\text{Cl}^-/\text{HCO}_3^-$ exchanger, pumping large volumes of isosmotic, $\text{HCO}_3^-$-rich alkaline fluid into the duodenum. This neutralizes gastric acid, raising the luminal pH to 7.0–8.0, which prevents duodenal ulceration and provides the optimal alkaline pH for pancreatic enzymes.
• Cholecystokinin (CCK): Synthesized by enteroendocrine I cells in the duodenum and jejunum in response to luminal peptides, amino acids, and long-chain fatty acids. CCK exerts three coordinated actions:
  1. Gallbladder Contraction: Stimulates smooth muscle contraction of the gallbladder wall.
  2. Sphincter of Oddi Relaxation: Relaxes the hepatopancreatic ampullary sphincter, allowing bile and pancreatic juice to flow into the duodenum.
  3. Pancreatic Acinar Secretion: Binds $CCK_A$ receptors on pancreatic acinar cells, activating a $G_q$-protein pathway that triggers exocytosis of digestive zymogens.

2. The Pancreatic Zymogen Activation Cascade

Premature activation of digestive proteases within the pancreas causes acute necrotizing pancreatitis. To prevent autodigestion, the pancreas synthesizes proteases as inactive zymogens packaged with a trypsin inhibitor protein (SPINK1). Activation occurs strictly within the duodenal lumen:

1. Enteropeptidase (Enterokinase): A brush-border serine protease anchored to duodenal enterocyte microvilli. Enteropeptidase recognizes the specific Asp-Asp-Asp-Asp-Lys sequence of trypsinogen, cleaving off a hexapeptide to yield active trypsin.
2. Trypsin as Master Activator: Once generated, trypsin autocatalytically activates additional trypsinogen molecules and cleaves all other pancreatic zymogens:
   • Chymotrypsinogen $\to$ Chymotrypsin (endopeptidase cleaving aromatic bonds).
   • Proelastase $\to$ Elastase (endopeptidase cleaving bonds adjacent to small aliphatic amino acids).
   • Procarboxypeptidases A & B $\to$ Carboxypeptidases A & B (zinc exopeptidases cleaving C-terminal neutral/aromatic or basic amino acids).
   • Prophospholipase A2 $\to$ Phospholipase A2 (hydrolyzes fatty acids from the sn-2 position of phospholipids).
   • Procolipase $\to$ Colipase (anchors pancreatic lipase to bile salt-coated lipid droplets).

Non-proteolytic pancreatic enzymes are secreted in active forms:

• Pancreatic Amylase: Cleaves $\alpha$-1,4-glucosidic bonds of starch to yield maltose, maltotriose, and $\alpha$-limit dextrins.
• Pancreatic Lipase: Hydrolyzes dietary triacylglycerols at the sn-1 and sn-3 positions, yielding 2-monoacylglycerol and two free fatty acids.
• Ribonuclease and Deoxyribonuclease: Hydrolyze dietary RNA and DNA into mononucleotides.

3. Bile Synthesis, Storage, and Micellar Emulsification

Bile is an aqueous solution of bile salts, cholesterol, phospholipids (lecithin), bilirubin, and electrolytes:

• Synthesis: Hepatocytes synthesize primary bile acids (cholic acid and chenodeoxycholic acid) from cholesterol via the rate-limiting microsomal enzyme cholesterol $7\alpha$-hydroxylase (inhibited by bile acids via FXR nuclear receptors). Hepatocytes conjugate these bile acids to glycine or taurine (forming glycocholic and taurocholic acids), dropping their $pK_a$ from ~6.0 to ~2.0–4.0. At physiological intestinal pH, these molecules exist as ionized, amphipathic bile salts.
• Storage and Concentration: Bile is continuously secreted into bile canaliculi, flows through hepatic ducts, and is diverted into the gallbladder via the cystic duct during interdigestive periods. The gallbladder absorbs water and inorganic salts, concentrating bile up to 5- to 10-fold.
• Physiological Role: Emulsification and Micelle Formation: Dietary lipids are insoluble in water and form large oil droplets. Bile salts act as biological detergents; their hydrophobic steroid face binds lipid droplets while their hydrophilic hydroxyl and amino faces orient outward toward aqueous chyme. Mechanical shearing by intestinal peristalsis fractures these droplets into stable sub-micron emulsion droplets, expanding the surface area for pancreatic lipase and colipase. Once lipase digests triacylglycerols, bile salts, 2-monoacylglycerols, free fatty acids, cholesterol, and fat-soluble vitamins aggregate into mixed micelles (~4–7 nm). Micelles diffuse across the unstirred water layer to the enterocyte brush border, where lipids dissociate and enter the epithelial cell.
• Enterohepatic Circulation: Bile salts are not absorbed in the duodenum or jejunum. Instead, they travel to the terminal ileum, where ~95% are actively reabsorbed via the Apical Sodium-Dependent Bile Acid Transporter (ASBT). Reabsorbed bile salts enter the portal vein, are extracted with high efficiency by hepatocytes, and are re-secreted into bile (cycling 4 to 12 times per day). Resection of the terminal ileum disrupts this recycling, causing bile acid-induced secretory diarrhea in the colon and fat malabsorption (steatorrhea) with deficiencies of fat-soluble vitamins (A, D, E, K).

Nutrient Absorption Across the Intestinal Epithelium

Carbohydrate Absorption Pathways:
- Glucose & Galactose: Secondary active transport across apical membrane via SGLT1 (coupled with 2 Na+).
- Fructose: Facilitated diffusion across apical membrane via GLUT5 (sodium-independent).
- Basolateral Exit: All three monosaccharides exit via GLUT2 facilitated diffusion into portal capillaries.

Lipid Processing and Chylomicron Assembly:
1. Luminal Uptake: Fatty acids & 2-monoacylglycerols diffuse across the apical membrane.
2. Re-esterification: Inside enterocyte Smooth Endoplasmic Reticulum (SER), triacylglycerols are re-synthesized.
3. Assembly: Triacylglycerols, cholesterol esters, and fat-soluble vitamins are packaged with Apolipoprotein B-48
   into giant lipoprotein particles called Chylomicrons (75-1200 nm).
4. Exocytosis & Lacteal Uptake: Chylomicrons exit basolaterally and enter lymphatic lacteals, bypassing the portal vein.

Protein Absorption Pathways:
- Free Amino Acids: Apical uptake via specialized Na+-dependent amino acid symporters.
- Di- and Tri-peptides: Apical uptake via PepT1 (H+-dependent secondary active transport).
- Intracellular Cleavage: Cytoplasmic peptidases hydrolyze oligopeptides into free amino acids.
- Basolateral Exit: Facilitated diffusion and Na+-independent exchangers export amino acids into portal blood.