OVERVIEW OF LIPID METABOLISM

The bile salts such as cholic acid contain a hydrophobic side and a
hydrophilic side
, thus allowing bile salts to dissolve
at an oil-water interface, with the
hydrophobic surface in contact with the nonpolar
phase and the hydrophilic surface in the
aqueous medium. This detergent action
emulsifies fats and yields mixed micelles,
which allow attack by wat
er-soluble digestive enzymes
and facilitate the absorption of
lipids through the intestinal mucosa.
Mixed Micelles also serve as transport vehicles
for those lipids that are less water-soluble than
fatty acids, such as
cholesterol or the
fat-soluble vitamins A, D, E, and K.
Thus, efficient absorption of lipids depends on the
presence of sufficient bile acids to
solubilize the ingested lipids.
2.
Digestion of Fats
.
The emulsification of fats rende
r them susceptible to hydrolysis by
enzymes secreted by the pancreas. Th
e most important enzyme involved is
pancreatic
lipase
.
Pancreatic lipase is virtually specific for
the hydrolysis of pr
imary ester linkages,
the 1 or the 3 ester bonds, but not the bond in
the central 2 position (see below). As a
result of this conversion,
2-monoglycerides (2-monoacylglycerols) are major end-
products of triglyceride digestion.
Less than 10% of trigly
cerides remain unhydrolyzed
in the intestine.
Absorption of Lipids
Short-chain fatty acids (up to 12
carbons) are absorbed directly
t
are
ithin the intestinal wa
ll, the triglycerides are
as shown
bed
s
H
2
C
CH
H
2
C
OC
O
R
1
OC
O
R
2
OC
O
R
3
Triacylglycerol
+ 2H
2
O
R
3
CO
2
H
R
2
CO
2
CH
CH
2
CH
2
OH
OH
R
1
CO
2
H
Fatty acids & monoacylglycerol
H
2
C
CH
H
2
C
OC
O
R
1
OC
O
R
2
OC
O
R
3
Triacylglycerol
+ 2H
2
O
R
3
CO
2
H
R
2
CO
2
CH
CH
2
CH
2
OH
OH
R
1
CO
2
H
Fatty acids & monoacylglycerol
duodenum of the 33 amino acid peptide hormone
pancreozymin-cholecystokinin
(P
CCK).
The
cholecystokinin
(CCK) activity induces
emptying of the gallbladder
, thu
leading to
increased concentration of bile salts
and other bile constituents
in the
intestine
, including cholesterol and phospholipids. The
pancreozymin
(PZ) activit
causes
release of pancreatic digestive enzymes
, including pancreatic lipase
.
ctivit
causes
release of pancreatic digestive enzymes
, including pancreatic lipase
.
through the villi of the intestinal mucosa
. They enter the blood via capillaries tha
eventually
empty into the portal vein and are transported
via lipid carrier proteins
directly to the liver
, where they are used for energy production.
2-Monoglycerides,
long-chain fatty acids (more than 12 carb
ons), cholesterol and lysophospholipids
absorbed from the lumen by intestinal
mucosal cells, where they are incorporated
into lipoproteins and directed
to the lymphatic system
.
through the villi of the intestinal mucosa
. They enter the blood via capillaries tha
eventually
empty into the portal vein and are transported
via lipid carrier proteins
directly to the liver
, where they are used for energy production.
2-Monoglycerides,
long-chain fatty acids (more than 12 carb
ons), cholesterol and lysophospholipids
absorbed from the lumen by intestinal
mucosal cells, where they are incorporated
into lipoproteins and directed
to the lymphatic system
.
WW
resynthesized
by the 2-monoacylglycerol pathway
on the right. The 2-monoacylglycerol pathway is unique for
the intestine. Triglycerides,
having been synthesized in the
intestinal mucosa, are not transported to any extent in the
portal venous blood. Instead,
the great majority of absor
lipids, including triglycerides, phospholipids, cholesterol
esters, and cholesterol, appear in the form of
chylomicron
resynthesized
by the 2-monoacylglycerol pathway
on the right. The 2-monoacylglycerol pathway is unique for
the intestine. Triglycerides,
having been synthesized in the
intestinal mucosa, are not transported to any extent in the
portal venous blood. Instead,
the great majority of absor
lipids, including triglycerides, phospholipids, cholesterol
esters, and cholesterol, appear in the form of
chylomicron
(2-monoacylglycerols
)
1,2-diglycerides
triglycerides
2-monoglycerides
(2-monoacylglycerols
)
1,2-diglycerides
triglycerides
2-monoglycerides
that pass to the lymphatic vessels of the abdominal region
and later to the