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A human subject takes part 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?
A.
Adipocytes
B.
Endothelial cells
C.
Enterocytes
D.
Hepatocytes
E.
Myocytes
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The correct answer is B. 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
portion 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 (choice A). 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 (choice C) have the ability to pick up mixed
micelles from the gut lumen for repackaging in the
smooth endoplasmic reticulum as chylomicrons.
Hepatocytes (choice D) pick up the chylomicron
remnants after the lipoprotein lipase shrinks them.
Myocytes (choice E) are not involved in chylomicron
metabolism.
