BC Chap 32
Digestion and Transport of Dietary Lipids
 Triacyl glycerols – major fat in human diet, consisting of 3 fatty acids esterified to glycerol backbone
o Limited digestion occurs in mouth (lingual lipase) and stomach (gastric lipase) because of low solubility
of substrate
o In intestine, fats emulsified by bile salts released from gallbladder, increasing available surface area of
lipids for pancreatic lipase and colipase to bind and digest triglycerides
o Degradation products are free fatty acids and 2-monoacylglycerol
 When partially digested food enters intestine, hormone cholecystokinin (CCK) secreted by intestine, which
signals gallbladder to contract and release bile acids and pancreas to release digestive enzymes
 Phospholipids, cholesterol, and cholesterol esters (cholesterol esterified to fatty acids) present in foods
o Phospholipids hydrolyzed in intestinal lumen by phospholipase A2
o Cholesterol esters hydrolyzed by cholesterol esterase
o Both of above enzymes secreted by pancreas
 Products of enzymatic digestion (free fatty acids, glycerol, lysophospholipids, and cholesterol) form micells with
bile acids in intestinal lumen
o Micelles interact with enterocyte membrane and allow diffusion of lipid-soluble components across
enterocyte membrane into cell
o Bile acids don’t enter enterocyte at this time; remain in intestinal lumen, travel farther down, and are
reabsorbed and sent back to liver by enterohepatic circulation
o Allows bile salts to be used multiple times in fat digestion
 Intestinal epithelial cells resynthesize triacylglycerol from free fatty acids and 2-monoacylglycerol and package
them with protein (apolipoprotein B-48), phospholipids, and cholesterol esters into soluble lipoprotein particle
(chylomicron), which is secreted into lymph and eventually ends up in circulation where it can distribute dietary
lipids to all tissues of body
o Once in circulation, newly released (nascent) chylomicrons interact with lipoprotein particle (HDL) and
acquire 2 apoproteins from HDL (apoCII and apoE), converting it from nascent chylomicron to mature
o apoCII on mature chylomicron activates LPL located on inner surface of capillary endothelial cells of
muscle and adipose tissue
o LPL digests triglyceride in chylomicron, producing free fatty acids and glycerol
o Fatty acids enter adjacent organs either for energy production (muscle) or fat storage (adipocyte)
o Glycerol that is released is metabolized by liver
o As chylomicron loses triglyceride, density increases and becomes chylomicron remnant, which is taken
up by liver by receptors that recognize apoE
o In liver, chylomicron remnant degraded into component parts for further disposition by liver
 Lymph vessels designed so that under normal conditions, contents of blood can’t enter lymphatic system
o Lymph fluid similar to blood but without RBCs, WBCs, and platelets
 Amylase produced only in salivary glands and acinar cells of pancreas; elevated serum amylase can be sign of
pancreatitis or salivary gland lesions (such as mumps)
o Amylase – enzyme that digests starch
o Recent assay to detect amylase activity in serum or urine uses enzyme-coupled system linked to
increase in UV light absorption at 340 nm
 Maltopentose (5 glucose residues linked by α-(1,4)-bonds) is artificial substrate, and amylase
cleaves substrate to maltose (2 linked sugars) and maltotriose (3 linked sugars)
 α-glucosidase added to reaction mixture, and it cleaves products of amylase digestion to free
glucose; once free glucose formed, hexokinase (added to assay mixture) converts glucose 6-P to
6-phosphogluconate with production of NADPH from NADP+
 by measuring change in absorbance at 340 nm, one can determine how much NADPH produced,
which can be converted into units of amylase activity for production of glucose
o Chromogenic assays can use starch substrate to which chromogenic dye has been attached (renders
starch-dye complex insoluble
 As amylase cleaves starch substrate, smaller particles produced are soluble, which leads to
chromogen being solubilized, and a change of color in solution

Currently 38% of calories in typical U.S. diet come from fat; according to current recommendations, fat should
provide no more than 30% of total calories in healthy diet
Digestion of Triacylglycerols
 Triacylglycerols – major fat in human diet because they are major storage lipid in plants and animals that
constitute our food supply
o Contain glycerol backbone to which 3 fatty acids are esterified
o Main route of digestion involves hydrolysis to fatty acids and 2-monoacylglycerol in lumen of intestine
 Depends to some extent on chain length of fatty acids
o Lingual and gastric lipases preferentially hydrolyze short and medium-chain fatty acids from
triacylglycerols, so they are most active in infants and young children who drink relatively large
quantities of cow’s milk (contains high percentage of short and medium-chain fatty acids)
o Mammary gland produces milk (major source of nutrients for breastfed infant)
 Fatty acid composition varies depending on diet of mother, but long-chain fatty acids
predominate, particularly palmitic, oleic, and linoleic acids
 Although amount of total fat in human and cow milk is similar, cow’s milk has more short and
medium-chain fatty acids and doesn’t contain long-chain, polyunsaturated fatty acids in human
milk that are important in brain development
 Although concentrations of pancreatic lipase and bile salts low in intestinal lumen of newborn
infant, fat of human milk still readily absorbed because lingual and gastric lipases partially
compensate for lower levels of pancreatic lipase
 Human mammary gland also produces lipases that enter milk, one of which is not inactivated by
stomach acid and functions in intestine for several hours
 Lipases require lower levels of bile salts than pancreatic lipase
 Pancreatic amylase which digests dietary starch and pancreatic lipase elevated with acute pancreatitis
o Elevated levels of enzymes in blood are result of their escape from inflamed exocrine cells of pancreas
into surrounding pancreatic veins
o Can be caused by acute and chronic alcoholism
 Dietary fat leaves stomach and enters small intestine, where it is emulsified by bile salts
o Bile salts are amphipathic compounds (have both hydrophobic and hydrophilic components) synthesized
in liver and secreted via gallbladder into intestinal lumen
o Bile salts act as detergents, binding to globules of dietary fat as they are broken up by peristaltic action
of intestinal muscle
o Emulsified fat, which has increased SA compared with unemulsified fat, attacked by digestive enzymes
from pancreas
 Major enzyme that digests dietary triacylglycerols is lipase produced in pancreas
o Pancreatic lipase secreted along with colipase in response to release of CCK from intestine
 Peptide hormone secretin released by small intestine in response to acidic materials (such as partially digested
materials from stomach) entering duodenum
o Secretin signals liver, pancreas, and certain intestinal cells to secrete bicarbonate, which raises pH of
contents of intestinal lumen into range optimal for action of all digestive enzymes of intestine
 Bile salts inhibit pancreatic lipase activity by coating substrate and not allowing lipase access to substrate
o Colipase binds to dietary fat and lipase, relieving bile salt inhibition and allowing triglyceride to enter
active site of lipase
o Pancreatic lipase hydrolyzes fatty acids of all chain lengths from positions 1 and 3 of glycerol moiety of
triacylglycerol, producing free fatty acids and 2-monoacylglycerol (glycerol with fatty acid esterified at
position 2)
 Pancreas produces esterases that remove fatty acids from compounds (such as cholesterol esters) and
phospholipase A2 (released in zymogen form and activated by trypsin) that digests phospholipids to free fatty
acid and lysophospholipid
Absorption of Dietary Lipids
 Fatty acids and 2-monoacylglycerols produced by digestion packaged into micelles (tiny microdroplets
emulsified by bile salts)
o
For bile salt micelles to form, concentration of bile salts in intestinal lumen must be above critical
micelle concentration (CMC); below this concentration, bile salts soluble
o Other dietary lipids, such as cholesterol, lysophospholipids, and fat-soluble vitamins, also packaged in
micelles
o Micelles travel through layer of water (unstirred water layer) to microvilli on surface of intestinal
epithelial cells, where fatty acids, 2-monoacylglycerols, and other dietary lipids absorbed, but bile salts
left behind in lumen of gut
 Bile salts extensively resorbed when they reach ileum; greater than 95% of bile salts recirculated, traveling
through enterohepatic circulation to liver, which secretes them into bile for storage in gallbladder and ejection
into intestinal lumen during another digestive cycle
 Short and medium-chain fatty acids do not require bile salts for absorption; absorbed directly into intestinal
epithelial cells; because they don’t need to be packaged, they enter portal blood rather than lymph and are
transported to liver bound to serum albumin
 In patients with severe and recurrent episodes of hemolytic anemia, greater than normal amounts of heme must
be processed by liver and spleen; heme degraded to bilirubin, which is excreted by liver in bile
o If large quantities of bilirubin presented to liver as consequence of acute hemolysis, capacity of liver to
conjugate it (convert it to water-soluble bilirubin diglucuronide) can be overwhelmed, and greater
percentage of bilirubin entering hepatic biliary ducts is in less water-soluble forms
o In gallbladder, relatively insoluble forms of bilirubin tend to precipitate as gallstones rich in calcium
bilirubinate
o In some patients, stones may leave gallbladder through cystic duct and enter common bile duct; most of
these pass into small intestine and are excreted later in stools, but larger stones can become entrapped
in lumen of common bile duct, where they cause varying degrees of obstruction to bile flow (cholestasis)
with associated ductal spasm, producing pain
o If adequate amounts of bile salts do not enter intestinal lumen, dietary fats cannot readily by emulsified
and digested
 Steatorrhea (fat-laden stools caused by malabsorption of dietary fats) can be caused by lack of pancreatic
secretions, particularly pancreatic lipase, which normall digests dietary fat
o Steatorrhea may also be caused by insufficient production or secretion of bile salts
 Because fat-soluble vitamins (A, D, E, and K) absorbed from micelles along with long-chain fatty acids and 2monoacylglycerols, prolonged obstruction of duct that carries exocrine secretions from pancreas and gallbladder
into intestine (via common duct) could lead to deficiency of fats, which can lead to secondary vitamin deficiency
Synthesis of Chylomicrons
 Within intestinal epithelial cells, fatty acids and 2-monoacylglycerols condensed by enzymatic reactions in sER to
form triacylglycerols
o Fatty acids activated to fatty acyl-CoA by same process used for activation of fatty acids before βoxidation
o Fatty acyl-CoA then reacts with 2-monoacylglycerol to form diacylglycerol, which reacts with another
fatty acyl-CoA to form triacylglycerol
o Reactions for triacylclycerol synthesis in intestinal cells differ from those in liver and adipose cells in that
2-monoacylglycerol is intermediate in triacylglycerol synthesis in intestinal cells, whereas phospatidic
acid is necessary intermediate in other tissues
 Triacylglycerols transported in lipoprotein particles because they are insoluble in water (if they entered the
blood directly, they would coalesce, impeding blood flow)
o Intestinal cells package triacylglycerols together with proteins and phospholipids in chylomicrons
(lipoprotein particles that don’t readily coalesce in aqueous solutions)
o Chylomicrons contain cholesterol and fat-soluble vitamins as well, but major component is triglyceride
derived from diet
 Protein constituents of lipoproteins are apoproteins
o Major apoproteins associated with chylomicrons as they leave intestinal cells is B-48
o B-48 apoprotein structurally and genetically related to apoB100 synthesized in liver that serves as major
protein of VLDL (apoB48 and apoB100 encoded by same gene, but in intestine, primary transcript of
gene undergoes RNA editing, creating a stop codon that causes protein to be 48% of size of apoB100)

Protein component of lipoproteins synthesized on rER, and lipids synthesized in sER; lipids and proteins
complexed to form chylomicrons
 Olestra – artificial fat substitute designed to allow individuals to obtain taste and food consistency of fat without
the calories; structure of olestra consists of a sucrose molecule with fatty acids esterified to hydroxyl groups
o Fatty acids attached to sucrose resistant to hydrolysis by pancreatic lipase, so olestra passes through
intestine intact and is eliminated in feces; as result, no useful calories can be obtained from olestra
metabolism, although in mouth, sucrose proton of molecule imparts sweet taste
o Because olestra can pass through digestive system unimpeded, it can also carry with it essential fatsoluble vitamins, so foods prepared with olestra supplemented with vitamins
Transport of Dietary Lipids in Blood
 By process of exocytosis, nascent chylomicrons secreted by intestinal epithelial cells into chyle of lymphatic
system and enter blood through thoracic duct (nascent chylomicrons enter blood 1-2 hours after start of meal;
as meal digested and absorbed, they continue to enter blood for many hours)
o Initially, particles are nascent chylomicrons; as they accept proteins from HDL in lymph and blood, they
become mature chylomicrons
o HDL transfers proteins to nascent chylomicrons, particularly apoE and apoCII
 apoE recognized by membrane receptors, particularly those on surface of liver cells, allowing
apoE-bearing lipoproteins to enter cells by endocytosis for subsequent digestion by lysosomes
 apoCII acts as activator of LPL (enzyme on capillary endothelial cells primarily in muscle and
adipose tissue that digests triacylglycerols of chylomicrons and VLDL in blood)
 Because of high triacylglycerol content, chylomicrons are least dense of blood lipoproteins
o When blood collected from patients with certain types of hyperlipoproteinemias in which chylomicron
levels elevated, and blood allowed to stand in refrigerator overnight, chylomicrons float to top of liquid
and coalesce, forming creamy layer
 One manner in which individuals can lose weight is to inhibit activity of pancreatic lipase
o Results in reduced fat digestion and absorption and reduced caloric yield from diet
o Orlistat – drug that is a chemically synthesized derivative of lipstatin, a natural lipase inhibitor found in
certain bacteria
 Works in intestinal lumen and forms covalent bond with active-site serine residues of both
gastric and pancreatic lipase, thereby inhibiting their activities
 Nondigested triglycerides not absorbed by intestine and eliminated in feces
 Under normal use of drug, about 30% of dietary fat absorption inhibited
 Because excessive nondigested fat in intestines can lead to GI distress related to excessive
intestinal gas formation, individuals who take orlistat need to follow diet with reduced daily
intake of fat, which should be evenly distributed among meals of the day
Fate of Chylomicrons
 Triacylglycerols of chylomicrons digested by LPL attached to proteoglycans in basement membranes of
endothelial cells that line capillary walls
o LPL produced by adipose cells, muscle cells (particularly cardiac muscle), and cells of lactating mammary
gland – isozyme synthesis in adipose cells has higher Km than isozyme synthesized in muscle cells, so
adipose LPL is more active after a meal when chylomicron levels elevated in blood
o Insulin stimulates synthesis and secretion of adipose LPL so that after meal, when triglyceride levels
increase in circulation, LPL has been upregulated (through insulin release) to facilitate hydrolysis of fatty
acids from triglyceride
 Fatty acids released from triacylglycerols by LPL not very soluble in water; become soluble in blood by forming
complexes with albumin
o Major fate of fatty acids is storage as triacylglycerol in adipose tissue
o Fatty acids may also be oxidized for energy in muscle and other tissues
o LPL in capillaries of muscle cells has lower Km than adipose LPL, so muscle cells can obtain fatty acids
from blood lipoproteins whenever they are needed for energy, even if concentration of lipoproteins low
 Glycerol released from chylomicron triacylglycerols by LPL used for triacylglycerol synthesis in liver in fed state
 Portion of chylomicron that remains in blood after LPL action is chylomicron remnant
o
Remnant has lost many apoC molecules bound to mature chylomicron, which exposes apoE, which is
recognized by receptors on hepatocytes and binds to them
o Lysosomes fuse with endocytic vesicles, and chylomicron remnants degraded by lysosomal enzymes
o Products of lysosomal digestion (fatty acids, amino acids, glycerol, cholesterol, and phosphate) can be
reused by cell
 Heparin – complex polysaccharide that is a component of proteoglycans
o Isolated heparin frequently used as anticoagulant because it binds to antithrombin III (ATIII), and
activated ATIII then binds factors necessary for clotting and inhibits them from working
o As LPL bound to capillary endothelium through binding to proteoglycans, heparin also can bind to LPL
and dislodge it from capillary wall, leading to loss of LPL activity and increase of triglyceride content
Microsomal Triglyceride Transfer Protein
 Assembly of chylomicrons in ER of enterocyte requires activity of microsomal triglyceride transfer protein (MTP)
 MTP is dimer of 2 nonidentical subunits (smaller one is protein disulfide isomerase [PDI], and larger one contains
triglyceride transfer activity)
 MTP accelerates transport of triglycerides, cholesterol esters, and phospholipids across membranes of
subcellular organelles
 Lack of triglyceride transfer activity leads to disease called abetalipoproteinemia – affects both chylomicron
assembly in intestine and VLDL assembly in liver
o Both chylomicrons and VLDL require a B apoprotein for assembly (apoB48 for chylomicrons, and
apoB100 for VLDL), and MTP binds to B apoproteins
o For both chylomicron and VLDL assembly, small apoB-containing particle first produced in lumen of ER
 Appropriate apoB made on rER and inserted into ER lumen during synthesis
 As protein is translated, lipid (small amount of triglyceride) begins to associate with protein, and
lipid association catalyzed by MTP
 Leads to generation of small apoB-containing particles not formed by patients with
abetalipoproteinemia
o Symptoms include lipid malabsorption (and accompanying symptoms such as steatorrhea and vomiting),
which can result in caloric deficiencies and weight loss; because lipid-soluble vitamin distribution occurs
through chylomicron circulation, signs and symptoms of deficiencies in lipid-soluble vitamins may be
seen in patients
 MTP activity necessary to transfer triacylglycerol formed in ER to apoB protein
 Second stage of particle assembly is fusion of initial apoB particle with triacylglycerol within ER
 MTP inhibitors effective in lowering circulating lipid levels, but they also initiate severe hepatic steatosis (fatty
liver), which could lead to liver failure
o Steatosis comes about by accumulation of triglyceride in liver because of inability to form VLDL and
export triglyceride from liver
o Accumulation of triglyceride within hepatocytes eventually interferes with hepatic function and
structure
o Current research being done and is aiming toward reducing severity of fat accumulation in liver by
specifically targeting intestinal MTP without affecting hepatic MTP