Substrate Preparation: Optimal pH and Emulsification
Chyme entering the duodenum from the stomach is highly acidic, whereas small intestinal enzymes—most notably pancreatic lipase—operate exclusively in a neutral environment. To establish this, the pancreas secretes bicarbonate ions. Protons are neutralized by bicarbonate, producing carbonic acid, which rapidly dissociates into water and carbon dioxide, thereby shifting the pH toward neutrality.
Simultaneously, emulsification is initiated. Dietary fats are insoluble in water, and enzymes function solely in the aqueous phase. To maximize the contact surface area between lipase and its substrate, large fat globules must be broken down into a fine emulsion. Bile acid salts act as detergents to accomplish this task.
Bile micelles consist of three key components:
- Bile acids.
- Phospholipids.
- Cholesterol.
Clinical significance: Bile acids and phospholipids keep hydrophobic cholesterol in solution. If the ratio of these substances is altered and emulsifier levels drop, cholesterol precipitates out of solution. This exact mechanism underlies gallstone formation in cholelithiasis.
Intestinal Lipid Hydrolysis
Once fats are emulsified, enzymatic breakdown begins. The primary drivers of this phase are pancreatic secretions: pancreatic lipase and its accessory protein, colipase.
Lipase attacks emulsified triacylglycerols. Through a complex hydrolytic cascade, the following products are formed:
- β-Monoacylglycerol (2-monoacylglycerol) — the primary end product, accounting for up to 80% of digested fats.
- Free fatty acids.
- Diacylglycerol (acting purely as an intermediate in hydrolysis).
To transport these hydrophobic products to the intestinal mucosa, they associate with bile acid salts to form mixed micelles. Only within these water-soluble complexes can digestive products cross the unstirred water layer to reach enterocytes for absorption.
Absorption and Intracellular Resynthesis
Mixed micelles deliver their cargo to the mucosal cells of the small intestine (enterocytes). Once inside, lipid resynthesis begins in the endoplasmic reticulum, assembling the body's own triacylglycerols from the absorbed building blocks.
The core biochemical reaction of resynthesis is as follows: A molecule of β-monoacylglycerol reacts with two activated fatty acid molecules (acyl-CoA). This yields a triacylglycerol (TAG) molecule and releases two molecules of coenzyme A (HS-CoA).
Newly synthesized TAGs form a hydrophobic core that must be safely transported via blood and lymph. To achieve this, the core is surrounded by a specialized hydrophilic shell composed of phospholipids and apolipoproteins. A critical integral protein of this shell is apoB-48. This dense packing results in the formation of nascent chylomicrons, which are then secreted from the enterocyte.