Biochemistry: 32 - Digestion & Transport Of Dietary Lipids

Front 1

major fat in human diet; major storage lipid in plants/animals that we eat; consist of 3 fatty acids esterified to a glycerol backbone

Back 1

triacylglycerols

Front 2

amphipatic compounds (both hydrophilic/hydrophobic) synthesized in liver & secreted by gallbladder into intestinal lumen to emulsify fats in the intestine

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bile salts

Front 3

gut hormone secreted by intestinal cells when stomach content enters the intestine; stimulates contraction of gallbladder, release of bile acids & secretion of pancreatic enzymes

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cholecystokinin

Front 4

enzymes involved in limited digestion (due to low solubility of substrate) of triacylglycerols in mouth & stomach

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lingual & gastric lipase

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major enzyme produced in the pancreas that digests triacylglycerols

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pancreatic lipase

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peptide hormone released by small intestine in response to acidic materials entering the duodenum responsible for signalling the liver, pancreas & certain intestinal cells to secrete bicarbonate

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secretin

Front 7

enzyme produced only in salivary glands & acinar cells of pancreas that digests dietary starch

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amylase

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% of calories in typical U.S. diet that currently comes from fat

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38%

Front 9

According to current recommendations, fat should provide no more than ____ of the total calories in a diet.

Back 9

30%

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conditions related to increased serum amylase levels

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pancreatitis & mumps (salivary gland lesions)

Front 11

degradation products from breakdown of triacylglycerols

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free fatty acids & 2-monoacylglycerol

Front 12

major significant difference between fatty-acid composition of cow's milk & human milk

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cow's milk: mostly short- & medium-chain fatty acids

human milk: mostly long-chain, polyunsaturated fatty acids (palmitic, oleic & linoleic acids) → important in brain development

NOTE: lingual/gastric lipases most active in infants compensate for low concentrations of pancreatic lipase & bile salts; human mammary gland also produces lipases that enter the milk

Front 13

compound that raises the pH of the contents of the intestinal lumen into a range optimal for action of all digestive enzymes (pH ~6)

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bicarbonate

Front 14

substance derived from hemoglobin that is degraded to bilirubin & excreted by the liver in bile

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heme

Front 15

How do gallstones form specifically in patients with hemolytic anemia?

Back 15

increased red blood cell destruction → greater amount of heme degraded by liver & spleen into large quantities of bilirubin → liver is overwhelmed (cannot conjugate bilirubin & convert it into water-soluble bilirubin diglucoronide) → greater % of bilirubin entering hepatic biliary ducts is in less water-soluble form → precipitation as gallstones rich in calcium bilirubinate in gallbladder

- smaller stones may leave gallbladder thru cystic duct → common bile duct → small intestine → excretion in stool

- larger stones may become entrapped in lumen of common bile duct → obstruction of bile flow (cholectasis) → ductal spasm → pain

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Why are serum levels of pancreatic amylase & lipase elevated in patients with acute/chronic pancreatitis?

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Enzymes escape from inflamed exocrine cells of pancreas → surrounding pancreatic veins → bloodstream

NOTE: inflammatory pancreatic process can result from toxic effect of acute/chronic excessive alcohol ingestion

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fat-laden stools caused by malabsoprtion of dietary fats (due to lack of pancreatic secretions or insufficient production/secretion of bile salts)

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steatorrhea

Front 18

enzymes produced by pancreas that remove fatty acids from cholesterol esters (cholesterol esterified to fatty acids)

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esterases

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enzyme released by pancreas in its zymogen form & activated by trypsin that digests phospholipids to a free fatty acid & a lysophospholipid

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phospholipase A2

Front 20

compounds that act as detergents, binding to globules of dietary fat as they are broken up by peristaltic action of intestinal muscle & emulsifying them

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bile salts

Front 21

digestion of triacylglycerols in the intestinal lumen
(diagram)

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Front 22

enzyme that binds to dietary fat & lipase, relieving the bile salt inhibition & allowing triglyceride to enter the active site of lipase

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colipase

Front 23

tiny microdroplets emulsified by bile salts that package dietary lipids (fatty acids, 2-monoacylglycerols, cholesterol lysophospholipids, fat-soluble vitamins) produced by digestion for transport to intestinal epithelial cells & absorption

NOTE: bile salts are not absorbed, but left behind in lumen of gut

Back 23

micelles

Front 24

types of fatty acids that do not require bile salts for their absorption
(absorbed directly into intestinal epithelial cells → portal blood → transported to liver bound to albumin)

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short- & medium-chain fatty acids (C4-C12)

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concentration of bile salts necessary in lumen fir bile salt micelles to form

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5-15mm = critical micelle concentration (CMC)

[below CMC → bile salts are soluble / above CMC → micelles form

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% of bile salts recirculated when they reach the ileum

NOTE: bile salts are recycled & used multiple times in fat digestion
(diagram)

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greater than 95%

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fat-soluble vitamins

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A, D, E, K

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What could lead to a fat-soluble vitamin deficiency?

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prolonged obstruction of the common duct that carries exocrine secretions from the pancreas & gallbladder into the intestine

(fat-soluble vitamins are absorbed from micelles along with long-chain faty acids & 2-monoacylglycerols)

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soluble lipoprotein particles that do not coalesce in aqueous solutions

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chylomicrons

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protein constituents of lipoproteins

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apoproteins

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artificial fat substitute designed to allow individuals to obtain the taste & food consistency of fat without the calories from fat

- passes thru intestine intact & is eliminated by feces

- imparts a sweet taste (sucrose portion)

- passes thru digestive system unimpeded → can carry essential fat-soluble vitamins with it

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Olestra

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enzyme involved in the resynthesis of triacylglycerols

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fatty acyl coenzyme A (CoA)

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difference in triacylglycerol synthesis in intestinal cells vs. liver & adipose cells

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in intestinal cells: 2-monoacylglycerol serves as intermediate

in liver & adipose cells: phosphatidic acid serves as intermediate

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major apoprotein associated with chylomicrons as they leave the intestinal cells

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B-48

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resynthesis of triacylgycerols in intestinal epithelial cells
(diagram)

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Front 36

apoprotein synthesized in the liver that serves as major protein of very-low-density lipoprotein (VLDL)

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B-100

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Lipids (triacylglycerols) are synthesized in the ________.

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smooth endoplasmic reticulum (sER)

Front 38

Proteins are synthesized in the _______.

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rough endoplasmic reticulum (rER)

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organelles in which lipoproteins are assembled

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ER & Golgi complex

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process by which nascent chylomicrons are secreted by intestinal epithelial cells into chyle of lymphatic system

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exocytosis

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Nascent chylomicrons begin to enter the blood within _______ after the start of a meal & continue to enter as the meas is digested & absorbed.

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1-2 hours

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Chylomicrons transform from "nascent" to "mature" when ________.

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they accept proteins from HDL within the lymph & blood

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Why are chylomicrons the least dense of the blood lipoproteins?

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due to their high triacylglycerol content

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2 apoproteins acquired by chylomicrons from HDL

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apoprotein CII (apoCII) & apoprotein E (apoE)

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formation & secretion of nascent chylomicrons
(diagram)

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fate of chylomicrons
(diagram)

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the major component of chylomicrons

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triglycerides

Front 48

How are apoproteins B-48 & B-100 similar?

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they are synthesized/encoded from the same gene

[primary transcript of gene undergoes RNA editing in intestine → a stop codon causes the 48% difference in size between both apoproteins]

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apoprotein recognized by membrane receptors (on liver cells), allowing lipoproteins to enter cells by endocytosis for subsequent digestion by lysosomes

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apoE

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apoprotein that acts as an activator of lipoprotein lipase (LPL)

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apoCII

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enzyme on capillary endothelial cells (primarily within muscle & adipose tissue) that digests triacylglycerols of chylomicrons & VLDL in blood

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lipoprotein lipase (LPL)

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portion of chylomicron that remains in blood after LPL action

- binds o receptors on hepatocytes → apoE recognition → endocytosis → lysosome fusion with endocytotic vesicles → degradation by lysosomal enzymes

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chylomicron remnant

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products of lysosomal digestion of chylomicron remnants that can be reused by the cell

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fatty acids, amino acids, glycerol, cholesterol, phosphate

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complex polysaccharide (component of proteoglycans) used as an anticoagulant because it binds to antithrombin III (ATIII), which binds factors necessary for clotting & inhibits them from working

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heparin

[LPL is bound to capillary endothelium thru binding to proteoglycans → heparin can bind to LPL & dislodge it from capillary wall - LPL is bound to capillary endothelium thru binding to proteoglycans → loss of LPL activity & increase of triglyceride content in blood]

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fate of fatty acids & glycerol produced from triglyceride in chylomicron digested by LPL

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fatty acids → enter adjacent organs for energy production (muscle) or fat storage (adipocytes)

glycerol → metabolized in liver (for possible use in fed state)

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soluble protein that complexes with fatty acids to make them soluble in blood

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albumin

Front 57

hormone that stimulates the synthesis & secretion of adipose LPL, so that after a meal, when triglyceride levels increase in circulation, LPL is upregulated to facilitate hydrolysis of fatty acids from triglyceride

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insulin

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Km of LPL synthesized in adipose cells vs. muscle cells

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adipose cells → higher Km → adipose LPL is more active after a meal, when chylomicron levels are elevated in blood to facilitate hydrolysis of fatty acids from triglycerides

muscle cells → low Km → muscle cells can obtain fatty acids from blood lipoproteins when needed for energy (even if lipoprotein concentration is low)