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.