The small intestine is the major site for lipid digestion. There are specific enzymes for the digestion of triglycerides, phospholipids, and cleavage of esters from cholesterol. We will look at each in this section.
Triglycerides
The pancreas secretes pancreatic lipase into the duodenum as part of pancreatic juice. This major triglyceride digestion enzyme preferentially cleaves the sn-1 and sn-3 fatty acids from triglycerides. This cleavage results in the formation of a 2-monoglyceride and two free fatty acids as shown below.
Figure 3.441 Pancreatic lipase cleaves the sn-1 and sn-3 fatty acids of triglycerides
Figure 3.442 The products of pancreatic lipase are a 2-monoglyceride and two free fatty acids
To assist lipase, colipase serves as an anchor point to help lipase attach to the triglyceride droplet.
Figure 3.443 Colipase helps anchor lipase to the triglyceride droplet
Phospholipids
The enzyme phospholipase A2 cleaves the C-2 fatty acid of lecithin, producing lysolecithin and a free fatty acid.
Figure 3.444 Phospholipase A2 cleaves the C-2 fatty acid of lecithin
Figure 3.445 Products of phospholipase A2 cleavage
Cholesterol Esters
The fatty acid in cholesterol esters is cleaved by the enzyme, cholesterol esterase, producing cholesterol and a free fatty acid.
Figure 3.446 Cholesterol esterase cleaves fatty acids off of cholesterol
Figure 3.447 Products of cholesterol esterase
Formation of Mixed Micelles
If nothing else happened at this point, the 2-monoglycerides and fatty acids produced by pancreatic lipase would form micelles. The hydrophilic heads would be outward and the fatty acids would be buried on the interior. These micelles are not sufficiently water-soluble to cross the unstirred water layer to get to the brush border of enterocytes. Thus, mixed micelles are formed containing cholesterol, bile acids, and lysolecithin in addition to the 2-monoglycerides and fatty acids, as illustrated below1.
Figure 3.448 Normal (left) and mixed (right) micelles
Mixed micelles are more water-soluble, allowing them to cross the unstirred water layer to the brush border of enterocytes for absorption.
Figure 3.449 Mixed micelles can cross the unstirred water layer for absorption into the enterocytes
References & Links
1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
