Page 267 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Lipid Distribution and Storage High-density lipoproteins (HDL) exchange
certain apoproteins with chylomicrons and
Lipids in the blood are transported in lipo- VLDL and absorb superfluous CHO from the
proteins, LPs (! A), which are molecular extrahepatic cells and blood (! B). With their
aggregates (microemulsions) with a core of ApoAI, they activate the plasma enzyme LCAT
very hydrophobic lipids such as triacylglycerols (lecithin–cholesterol acyltransferase), which
(TG) and cholesterol esters (CHO-esters) sur- is responsible for the partial esterification of
rounded by a layer of amphipathic lipids CHO. HDL also deliver cholesterol and CHO-
(phospholipids, cholesterol). LPs also contain esters to the liver and steroid hormone-pro-
several types of proteins, called apolipo- ducing glands with HDL receptors (ovaries,
proteins. LPs are differentiated according to testes, adrenal cortex).
their size, density, lipid composition, site of Triacylglycerol (TG)
Nutrition and Digestion elements of LPs (e.g. ApoAII and ApoB48), lig- acids (FFA) and 2-monoacylglycerol (MG) in the
synthesis, and their apolipoprotein content.
Apolipoproteins (Apo) function as structural
Dietary TGs are broken down into free fatty
gastrointestinal tract (! C and p. 252). Since
ands (ApoB100, ApoE, etc.) for LP receptors on
the membranes of LP target cells, and as
short-chain FFAs are water-soluble, they can
enzyme activators (e.g. ApoAI and ApoCII).
be absorbed and transported to the liver via
Chylomicrons transport lipids (mainly tri-
the portal vein. Long-chain FFAs and 2-mono-
acylglycerol, TG) from the gut to the periphery
re-synthesized to TG in the mucosa cells (! C).
! D), where their ApoCII activates endothelial
(The FFAs needed for TG synthesis are carried
10 (via intestinal lymph and systemic circulation; acylglycerols are not soluble in water. They are
lipoprotein lipase (LPL), which cleaves FFA
by FFA-binding proteins from the cell mem-
from TG. The FFA are mainly absorbed by myo- brane to their site of synthesis, i.e., the smooth
cytes and fat cells (! D). With the aid of ApoE, endoplasmic reticulum.) Since TGs are not
the chylomicron remnants deliver the rest of soluble in water, they are subsequently loaded
their TG, cholesterol and cholesterol ester load onto chylomicrons, which are exocytosed into
to the hepatocytes by receptor-mediated en- the extracellular fluid, then passed on to the
docytosis (! B, D). intestinal lymph (thereby by-passing the
Cholesterol (CHO) and the TG imported liver), from which they finally reach the
from the gut and newly synthesized in the liver greater circulation (! C, D). (Plasma becomes
are exported inside VLDL (very low density cloudy for about 20–30 minutes after a fatty
lipoproteins) from the liver to the periphery, meal due to its chylomicron content). The liver
where they by means of their ApoCII also acti- also synthesizes TGs, thereby taking the re-
vate LPL, resulting in the release of FFA (! D). quired FFAs from the plasma or synthesizing
This results in the loss of ApoCII and exposure them from glucose. Hepatic TGs are loaded
of ApoE. VLDL remnants or IDL (intermediate- onto VLDL (see above) and subsequently
density lipoproteins) remain. Ca. 50% of the IDL secreted into the plasma (! D). Since the ex-
returns to the liver (mainly bound by its ApoE port capacity of this mechanism is limited, an
on LDL receptors; see below) and is re- excess of FFA or glucose (! D) can result in the
processed and exported from the liver as VLDL accumulation of TGs in the liver (fatty liver).
(! B). Free fatty acids (FFAs) are high-energy sub-
The other 50% of the IDL is converted to LDL strates used for energy metabolism (! p. 228).
(low density lipoprotein) after coming in con- Fatty acids circulating in the blood are mainly
tact with hepatic lipase (resulting in loss of transported in the form of TG (in lipoproteins)
ApoE and exposure of ApoB100). Two-thirds of whereas plasma FFA are complexed with al-
the LDLs deliver their CHO and CHO-esters to bumin. Fatty acids are removed from TGs of
the liver, the other third transfers its CHO to chylomicrons and VLDL by lipoprotein lipase
extrahepatic tissue (! B). Binding of ApoB100 (LPL) localized on the luminal surface of the
to LDL receptors is essential for both processes capillary endothelium of many organs (mainly
254 (see below). in fat tissue and muscles) (! D). ApoCII on the
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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