Phospholipases, lipoprotein degradation

Lipoprotein lipase and hepatic lipase are two lipases that degrade triacylglycerols in lipoproteins (Chapter 19) and also demonstrate significant PL A, activity. The enzymes have 50% sequence identity and are members of a superfamily of lipases and phospholipases that share the G-X-S-X-G motif at the active site and an Asp-His-Ser triad that is required for catalysis. Hepatic lipase is about 2-3-fold more efficient at hydrolyzing phospholipids than is lipoprotein lipase and has lysophospholipase activity. Another lipase, intestinal lipase, has lysophospholipase activity and is also referred to as a PLB. 

It was generally assumed that oxidation products from oils and fats are poorly absorbed. Indeed, animals can be resistant to the effects of abused oils in their diet (29,30). Nevertheless, orally administered oxidized C-linoleic acid was incorporated into chylomicrons and very low density lipoprotein (VLDL) particles in rats. The absorbed oxidation products were subsequently identified as hydroxy fatty acids (31). Interestingly, dietary glutathione (GSH) reduced the absorption of peroxidized linoleate (32), presumably due to the activity of selenium-dependrait GSH peroxidase present in the intestinal epithelial cell (33). However, these absorption studies used free fatty adds, not triglycerides thus, they cannot address the role of pancreatic lipase and phospholipase. The fate of polymerized fatty adds has not been studied specifically. Whether the intestinal flora could degrade fliese polymers and hence allow their absorption remains to be seen. What is clear is that very little is known about the absorption of oxidized fat in animals and even less in humans. 

Whether oxidants affect the lipase action or promote the synthesis and release of TG-rich lipoproteins has not been determined. It is also possible that TG containing peroxidized fatty acid components are resistant to lipolysis. Earlier studies have demonstrated that antioxidants enhanced the (hepatic) lipase actions [66] however, these studies have not been corroborated or confirmed by more robust studies. In contrast, phospholipases seem to act more robustly on peroxidized or oxidatively tailored phospholipids [67-70] although the enzyme itself could be inactivated by oxidants [70], Recently it was reported that the protein disulfide isomerase, a redox-sensitive enzyme, could contribute to the endoplasmic reticulum-associated degradation of apoB through its chaperone activity [71],... 

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