A human subject takes pa in a nutritional research study. After ingesting a very fatty meal, serum samples are taken for research studies at 1 hour and 3 hours. These studies measure the average diameter of the chylomicrons, showing an average chylomicron diameter of 500 nm at 1 hour, which drops to an average diameter of 150 nm at 3 hours. Where is the enzyme responsible for this change located?
Chylomicrons are produced by enterocytes (intestinal epithelial cells), using gut luminal triglycerides for the source of the lipid. The chylomicrons are secreted into the gut lymphatic system, and from there drain eventually into the systemic venous system from the thoracic duct, and hence into the serum poion of the blood. They are initially large and have a very high triglyceride content. With time, lipoprotein lipase releases triglycerides from the chylomicron core by hydrolyzing them to more easily absorbed fatty acids. The enzyme is located on the external surface of the vascular endothelium of tissues with triglyceride needs such as skeletal muscle, cardiac muscle tissue, and lactating breast. The result of lipoprotein lipase activity is that the chylomicrons shrink in size. While adipose tissue can utilize chylomicrons, lipoprotein lipase is located on the endothelial cells rather than adipocytes. Adipocytes have an adipose tissue lipase, which is an intracellular enzyme that can cleave triglycerides to glycerol and fatty acids, allowing them to be released into the circulation when chylomicrons are low. Enterocytes have the ability to pick up mixed micelles from the gut lumen for repackaging in the smooth endoplasmic reticulum as chylomicrons. Hepatocytes pick up the chylomicron remnants after the lipoprotein lipase shrinks them. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 26. Digestion, Absorption, & Nutritional Principles. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.
Adipocytes
Endothelial cells
Enterocytes
Hepatocytes