Page 265 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Phospholipase A 2 (from pro-phospholipase A 2
Lipid Digestion in pancreatic juice—activated by trypsin)
The average intake of fats (butter, oil, mar- cleaves the 2nd ester bond of the phos-
garine, milk, meat, sausages, eggs, nuts etc.) is pholipids (mainly phosphatidylcholine =
roughly 60–100 g/day, but there is a wide lecithin) contained in micelles. The presence
range of individual variation (10–250 g/day). of bile salts and Ca 2+ is required for this reac-
Most fats in the diet (90%) are neutral fats or tion.
triacylglycerols (triglycerides). The rest are An unspecific carboxylesterase (= unspecific
phospholipids, cholesterol esters, and fat- lipase = cholesterol ester hydrolase) in pan-
soluble vitamins (vitamins A, D, E and K). Over creatic secretions also acts on cholesterol
95% of the lipids are normally absorbed in the esters on micelles as well as all three ester
small intestine. bonds of TG and esters of vitamins, A, D and E.
Nutrition and Digestion quired for their digestion in the watery en- digestive enzyme required to break down milk fat
Lipid digestion (! A). Lipids are poorly
This lipase is also present in human breast milk (but
not cow’s milk), so breast-fed infants receive the
soluble in water, so special mechanisms are re-
along with the milk. Since the enzyme is heat-sensi-
vironment of the gastrointestinal tract and for
tive, pasteurization of human milk significantly re-
their subsequent absorption and transport in
duces the infant’s ability to digest milk fat to a great
plasma (! p. 254). Although small quantities
extent.
of undegraded triacylglycerol can be absorbed,
and other lipids aggregate with bile salts
before they can be efficiently absorbed. Opti-
(! p. 248) to spontaneously form micelles in
mal enzymatic activity requires the prior me-
10 dietary fats must be hydrolyzed by enzymes 2-Monoacylglycerols, long-chain free fatty acids
chanical emulsification of fats (mainly in the the small intestine (! B3). (Since short-chain
distal stomach, ! p. 240) because emulsified fatty acids are relatively polar, they can be ab-
lipid droplets (1–2µm; ! B1) provide a much sorbed directly and do not require bile salts or
larger surface (relative to the mass of fat) for micelles). The micelles are only about
lipases. 20–50 nm in diameter, and their surface-to-
Lipases, the fat digesting enzymes, originate volume ratio is roughly 50 times larger than
from the lingual glands, gastric fundus (chief that of the lipid droplets in emulsions. They
and mucous neck cells) and pancreas (! A and facilitate close contact between the products
p. 246). About 10–30% of dietary fat intake is of fat digestion and the wall of the small in-
hydrolyzed in the stomach, while the remain- testine and are therefore essential for lipid ab-
ing 70–90% is broken down in the duodenum sorption. The polar side of the substances in-
and upper jejunum. Lingual and gastric lipases volved (mainly conjugated bile salts, 2-mono-
have an acid pH optimum, whereas pancreatic acylglycerol and phospholipids) faces the wa-
lipase has a pH optimum of 7–8. Lipases be- tery environment, and the non-polar side faces
come active at the fat/oil and water interface the interior of the micelle. Totally apolar lipids
(! B). Pancreatic lipase (triacylglycerol hy- (e.g., cholesterol esters, fat-soluble vitamins
drolase) develops its lipolytic activity (max. and lipophilic poisons) are located inside the
140 g fat/min) in the presence of colipase and micelles. Thus, the apolar lipids remain in the
2+
Ca . Pro-colipase in pancreatic juice yields lipophilic milieu (hydrocarbon continuum)
colipase after being activated by trypsin. In during all these processes until they reach the
most cases, the pancreatic lipases split triacyl- lipophilic brush border membrane of the
glycerol (TG) at the 1st and 3rd ester bond. This epithelium. They are then absorbed by the mu-
process requires the addition of water and cosa cells via dissolution in the membrane or
yields free fatty acids (FFA) and 2-monoacyl- by a passive transport mechanism (e.g., car-
glycerol. riers in the case of free fatty acids). Although
fat absorption is completed by the time the
A viscous-isotropic phase with aqueous and hydro- chyme reaches the end of the jejunum, the bile
phobic zones then forms around the enzyme (! B2). salts released from micelles are only absorbed
2+
252 Ca excesses or monoacylglycerol deficiencies result in in the terminal ileum and then recycled (en-
the conversion of the fatty acids into calcium soaps,
which are later excreted. terohepatic circulation; ! p. 249 B).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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