L15: Lipid Digestion

Question 1

LO1: Name the major classes of lipids, the primary function of each, and describe the relationship of each to fatty acids.

Fatty Acids
Triglycerides
Ketones
Cholesterol
Phospholipids
Sphingolipids

Answer 1

Fatty acids: metabolic fuel, precursors for eicosanoids, building blocks for phospholipids and sphingolipids

Triglycerides: storage form, transport form of fatty acids (three hydroxyl groups+fatty acid)

Ketones: soluble metabolic fuels for skeletal muscle, heart, kidney, brain (acids formed when excessive fatty acid degradation occurs; incl. acetoacetate and B-hydroxybutyrate)

Cholesterol: structural component of plasma membrane, precursor of steroid hormones, Vit. D, and bile acids (cholesterol ring+fatty acid)

Phospholipids: major building block of membranes, storage site for poly-unsaturated fatty acids, component of signal transduction pathways (polar head+fatty acid tails)

Sphingolipids: structural component of membranes, surface antigens (sphingosine+long chain saturated fatty acid)

Question 2

LO2: Describe how changes in the insulin/glucagon ratio affect the interconversion between acetyl CoA, fatty acids, and triglycerides.

Answer 2

INSULIN

• promotes synthesis of fatty acids
• promotes storage of lipids

GLUCAGON

• promotes mobilization and oxidation of lipids
• promotes ketone synthesis

Question 3

LO3: Name three pancreatic enzymes that are important in intestinal lipid digestion and describe the reactions they catalyze.

Answer 3

PANCREATIC LIPASE

• degrades triglycerides/diglycerides in the intestine to form 2-monoglyceride+FAs
• contains colipase for optimal activity

CHOLESTEROL ESTERASE
-degrades cholesterol esters to form cholesterol+FAs

PHOSPHOLIPASE A2
-degrades phospholipids to form lysophospholipid+FAs

Question 4

LO4: Name the primary emulsification agents and describe the structural features that contribute to their ability to emulsify fat

Answer 4

BILE SALTS: disperse fat globules into smaller particles to that digestive enzymes have large surface area to act upon
-amphipathic nature breaks lipid droplets up into micelles

Question 5

LO5: Describe the process by which dietary lipids are repackaged for secretion in the intestinal cell, naming key enzymes and proteins involved.

CoASH
Fatty Acyl CoA
Fatty Acyl CoA Synthetase
ACAT
Fatty Acyl CoA transferase
Chylomicrons
Enterocytes
Medium/short chain FAs

Answer 5

1. FAs broken down to 2-monoglycerides, FAs, and cholesterol
2. Coenzyme A (CoASH) +FAs produce fatty acyl CoA (“activated,” done by Fatty Acyl CoA Synthetase), which can be repackaged into triglycerides
3. Cholesterol esters reformed by ACAT (acyl CoA: cholesterole acyltransferase)
   Triglycerides reformed by Fatty Acyl CoA transferase
4. Neutral lipids, triglycerides, cholesterol esters assembled into chylomicrons in the enterocytes (absorptive cells) that line the intestine
5. Chylomicrons released by exocytosis into lymphatic system—>thoracic duct—>right atrium of heart—>extrahepatic tissues
6. Medium and short chain FAs can be released directly into portal blood (bound by albumin) and transported to the liver for processing

Question 6

LO6: Describe the effect that a deficiency in each of the following compounds or proteins would have on lipid digestion/absorption: bile salts, pancreatic lipase, and colipase

Answer 6

BILE SALTS
-help break up fat droplets, so deficiency would reduce digestion/absorption of fatty acids and they’d be excreted in feces (chalky/clay colored stool)

PANCREATIC LIPASE

• deficiency would result in lack of 2-monoglycerides/FAs to the intestinal cells (tri/diglycerides wouldn’t get properly digested)
• seen in pancreatitis/CF
• if activity <10% of normal, severe weight loss can occur due to loss of fuel through feces

COLIPASE
-contained in pancreatic lipase for optimal activity, so a deficiency would slightly reduce pancreatic lipase activity

Question 7

LO5: Describe the process by which dietary lipids are repackaged for secretion in the intestinal cell, naming key enzymes and proteins involved.

Apo B-48
ApoCII
APO-E
LPL/Chylomicron TG

Answer 7

Apo B-48: makes up the hydrophillic shell of the nascent chylomicron, along with phospholipids and unesterified cholesterol

ApoCII/Apo-E: chylomicrons acquire these after entering the general circulation (from HDL)

LPL: hydrolyzes chylomicron triglycerides, located extracellularly in the capillary bed of most tissues, but particularly enriched in muscle/adipose
-requires ApoCII

Question 8

LO7: Explain why a person with lipid malabsorption might have any of the following conditions:
Nightblindness
Defective blood clotting
Anemia
Osteoporosis
Inability to maintain weight

Answer 8

All result from malabsorption of fat-soluble vitamins or lipids
Nightblindness: Vitamin A
Defective blood clotting: Vitamin K
Anemia: Vitamin E
Osteoporosis: Vitamin D

Inability to maintain weight: if lipids can’t be absorbed, they are excreted through feces and result in severe weight loss

Question 9

LO8: Provide an explanation of why a diet enriched in triglycerides containing medium chain fatty acids would be beneficial to a patient suffering from a lipid malabsorption syndrome

Answer 9

Medium chain fatty acids can be taken up directly by the intestinal mucosal cells and released to blood circulation via the portal vein; absorption not affected by most lipid malabsorption defects