Digestion of dietary phospholipids
In the previous section we discussed the digestion of the class of lipids known as triacylglycerols. In this section, we shall discuss phospholipids, the major form of lipid in all cell membranes. Phospholipids, like triacylglycerols, contain fatty acids, as discussed in triacylglycerol metabolism. Phospolipids are different because, as the name implies, they also contain phosphorus.
The phospholipid, lecithin (also called phosphatidylcholine) . This phosphorus containing lipid is composed of 2 fatty acids bonded to a glycerol backbone, phosphorus and choline
Digestion of phospolipids occurs in the small intestine. The pancreas liberates the proenzyme phospholipase A2 into the small intestine. Phospholipase A2 is then converted into the active enzyme from trypsin (another pancreatic enzyme). Phospholipase A2 removes the fatty acid at position 2 of the phospholipid.
Once the fatty acid is removed, the remaining molecule is called a lysophospholipid. If we remove the the fatty acid from position 2 in the phospholipid lecithin, above, it becomes lysolecithin. The prefix "lyso" simply gives us an indication of the type of structural change.
Since the fatty acid at position 2 on the glycerol backbone has been removed, there is only one remaining fatty acid. The remaining fatty acid resides at position 1. This fatty acid is removed by lysophospholipase, another pancreatic enzyme. Once both fatty acids are removed, all that remains is a glyceroylphosphoryl base. This may pass through in the faeces or further metabolized and absorbed and used.
Cholesterol and steroids