Preparation for Hydrolysis: Emulsification
Up to 90% of dietary lipids are triacylglycerols (TAGs). Because fats are hydrophobic while enzymes function in an aqueous environment, lipids must undergo preparation. The initial stage of assimilation is emulsification.
Driven by peristalsis and bile acids, large fat droplets are broken down into a finely dispersed emulsion. This vastly increases the fat-water interfacial surface area, creating optimal conditions for the efficient action of digestive enzymes.
Action of Pancreatic Lipase
The main phase of digestion occurs in the intestinal lumen via pancreatic lipase. Lipase alone cannot firmly anchor to the surface of an emulsified fat droplet. It requires a cofactor protein—colipase.
Colipase is synthesized in the pancreas in an inactive form and activated in the intestine via limited proteolysis. It acts as a bridge between the enzyme and the micelle, significantly accelerating hydrolysis.
Pancreatic lipase cleaves ester bonds specifically at the 1st and 3rd positions of TAGs. The resulting products are 2-monoacylglycerol (2-MAG) and two free fatty acids. Operating in parallel is phospholipase A2 (activated by trypsin), which hydrolyzes dietary phospholipids, converting them into amphiphilic compounds capable of absorption.
Micelle Formation and Absorption
Lipid hydrolysis products cannot simply dissolve and absorb on their own. To cross the aqueous unstirred water layer of the intestinal mucosa, they assemble into mixed micelles.
These structures contain:
- 2-monoacylglycerols and fatty acids;
- Bile acids;
- Cholesterol;
- Fat-soluble vitamins.
Mixed micelles deliver lipids to the surface of intestinal mucosal cells (enterocytes). At the cell membrane, the micelles disintegrate: hydrophobic components cross into the enterocyte via passive diffusion, while bile acids remain in the intestinal lumen.
Resynthesis of Fats in Enterocytes
Once inside the cell, hydrolysis products do not enter the bloodstream directly. Enterocytes use them as building blocks to assemble their own, organism-specific TAG molecules—a process known as resynthesis.
- Fatty acid activation: The enzyme acyl-CoA synthetase consumes a molecule of ATP to attach coenzyme A to a fatty acid, yielding acyl-CoA.
- TAG assembly: Acyltransferases sequentially attach acyl-CoA molecules to 2-MAG.
A key feature of resynthesis is that the cell can utilize both dietary fatty acids and endogenous fatty acids (synthesized de novo). Consequently, the fatty acid profile of the newly synthesized TAGs differs from that of the ingested fats.
Disorders of Fat Digestion
Impaired lipid digestion typically arises from two main causes:
- Reduced pancreatic lipase activity (e.g., in chronic pancreatitis).
- Defective emulsification due to bile deficiency (e.g., in cholelithiasis).
The hallmark symptom of lipid malabsorption is steatorrhea (undigested fats in feces). Prolonged malabsorption inevitably leads to fat-soluble vitamin deficiencies. For instance, vitamin K deficiency impairs blood clotting factor synthesis and causes bleeding, while vitamin A deficiency leads to night blindness (nyctalopia).
Tissue Lipid Hydrolysis
Resynthesized fats are packaged into chylomicrons and released into the bloodstream. The degradation of TAGs within chylomicrons and VLDLs occurs in the capillaries of adipose, skeletal muscle, and cardiac tissues.
Here, another enzyme is at work—lipoprotein lipase (LPL), anchored to the capillary endothelium. It is activated by apolipoprotein apoC-II and hydrolyzes TAGs into glycerol and three fatty acids. The released fatty acids are promptly taken up by cells for storage as lipid droplets or oxidation for energy.