Class 12 of Spring 2018 – Lipids Part 2 – Chapter 6

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Adipose/Liver-Free Fatty Acid/VLDL cycle

Because the Dixon cycle (my favorite term for this system) is a

cycle, it can start anywhere and end anywhere. In the adipocyte

(fat cell–and we are mainly talking about the white adipose cell),

triglyceride (TAG) in the fat droplet is broken down (See #1) to

free fatty acids (FA), which then diffuse into blood and bind to

albumin, the predominant protein in blood. Albumin carries (via

blood) the free fatty acids (yellow arrows) throughout the

body–especially to heart, bone, and muscle (See #2). The left

over free fatty acids are taken up by liver (See #3), where they

are re-synthesized into TAGs. Apolipoprotein B (ApoB)–a very

large protein made in liver, wraps itself around (like a belt!) a

lipid droplet containing triglyceride (TAG) and cholesterol to form

the lipoprotein called Very Low Density Lipoprotein (VLDL). The

liver hepatocyte secretes VLDL particles into the blood (See #5)

and they permeate throughout the body and deliver fatty acids

(red arrows) to tissues like heart, muscle, and bone marrow. An

enzyme called Lipoprotein Lipase (LPL) is located on the surface

of the endothelium of blood vessels (facing the blood flow) of

many tissues and “catches” the VLDL particles. Lipoprotein

Lipase reaches into the droplet within the VLDL particle and

hydrolyzes the three fatty acids off the TAG molecule.

These fatty acids either diffuse or are transported into cells of the heart,

muscle, and other tissues. Eventually, the remaining fatty acids

in VLDL (in the form of TAG), are returned to adipose cells. In

this way, using two unique and overlapping systems–the albumin

system and the VLDL system, all of the tissues of the body have

access to energy, in the form of fatty acids (the body’s gasoline).

In the final step of the cycle (right bottom corner-See #6) about

50% of the VLDL particles, now largely depleted of TAG, are

converted to Low Density Lipoprotein (LDL) (also known as the

“Bad Cholesterol”), which, after a half-life of 3 days, is then taken

up by the liver using LDL receptors. This returns the remaining

extra cholesterol and other lipids back to the liver.

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