section 12.3
Digestion and Absorption of Major Food Substances
217
1
II
O
K C — o — c —
II
|2
R— c — o — CH
O
Is
II
h2c — o — c —
Triacylglycerol
R
R
Pancreatic
lipase
+ 2Hj.O
O
H p— OH
R — C — O— (jîH
H P — OH
+ 2 R C O O " +
2H+
Fatty
acid
2-Monoacylglycerol
1
II
O H P — O — C — R,
II
\2
R — C — O — C H
O
Is
II
H P — O — p — o — C H — CH,-
C H 3
Phospholipase A 2
i,
+Tip
'
o
C H ,
Phosphatidycholine
O
H p — O — c — R,
\
H C — OH o
C H 3
I
II
l+
H P — O — P — O — C H — C H — N — C H 3
O-
C H 3
Lysophosphatidylcholine
RjCOO
+
H+
Fatty acid
FIGURE 12-13
H ydrolysis o f triacylglycerol and phosphatidylcholine by pancreatic lipase and phopholipase A 2, respectively.
The major functions of the stomach in fat digestion are
to store a fatty meal, to contribute to emulsification by the
shearing actions of the pylorus, and to gradually transfer
the partially digested emulsified fat to the duodenum by
controlling the rate of delivery. The hydrolysis of triacyl-
glycerol in the duodenum and jejunum requires bile and
pancreatic juice. Bile acids are powerful detergents that,
together with monoacylglycerol and phosphatidylcholine,
promote the emulsification of lipids. Emulsification is also
aided by the churning action of the GI tract which greatly
increases the area of the lipid-water interface that promotes
the action of pancreatic lipase. This triacylglycerol lipase
hydrolyzes ester linkages at the 1- and 3-positions to yield
2-monoacylglycerol and fatty acids (Figure 12-13). The
products of digestion are relatively insoluble in water but
are solubilized in micelles. Micelles also contain lipid-
soluble vitamins, cholesterol, and phosphatidylcholine.
Pancreatic lipase functions at the lipid-water interface, its
activity being facilitated by colipase (M.W. ~ 10,000),
also secreted by the pancreas as procolipase activated by
tryptic hydrolysis of an Arg-Gly bond in the N-terminal
region. Colipase anchors the lipase to the triacylglycerol
emulsion in the presence of bile salts by forming a
1 : 1
com-
plex with lipase and protects lipase against denaturation.
Colipase deficiency (with normal lipase) is accompanied
by significant lipid malabsorption, as is pancreatic lipase
deficiency. Pancreatic juice contains esterases that act on
short-chain triacylglycerols and do not require bile salts,
as well as cholesteryl esterase. Phosphatidylcholine in the
diet (4-8 g/day) and in bile secretions (17-22 g/day) is
hydrolyzed to lysophosphatidylcholine and fatty acid by
phospholipase A
2
, a pancreatic enzyme with an absolute
requirement for Ca2+ ions and for bile acids. The secreted
form, prophospholipase A
2
, is activated by tryptic hydrol-
ysis of an Arg-Ala bond in the N-terminal region. Phos-
pholipase A
2
also hydrolyzes fatty acids esterified at the
2
-position of phosphatidylethanolamine, phosphatidyl-
glycerol, phosphatidylserine, phosphatidylinositol, and
cardiolipin but has no effect on sphingolipids.
Lipid absorption in the duodenum and jejunum appears
to be a passive diffusion process. Lipid-laden micelles
migrate to the microvilli, and the fatty acids, monoa-
cylglycerols, and lysophosphoglycerols are transferred
across the membrane according to their solubility within
the lipid bilayer of the cell surfaces. Bile acids are not
absorbed into the enterocyte but migrate to the ileum,
where they are actively absorbed and transferred to the
liver via the portal venous system. The bile acid pool is
recycled several times daily (enterohepatic circulation)
to meet the demands of lipid digestion, and disorders
that interfere with this process lead to malabsorption of
lipids. A cytoplasmic fatty acid-binding protein with high
affinity for long-chain fatty acids transports fatty acids
to the smooth endoplasmic reticulum for resynthesis of
triacylglycerol.
Digestion
and
absorption of triacylglycerols
with
medium-chain fatty acids «
1 2
carbons) proceed by a dif-
ferent pathway. Medium-chain triacylglycerols are partly
water-soluble, are rapidly hydrolyzed by lingual and pan-
creatic lipases, and do not require the participation of bile
acids. Some are absorbed intact and hydrolyzed inside the
absorptive cell. Medium-chain fatty acids enter the portal
