3. Lipids & Lipoproteins

What are the 2 lipid products that are released when lipids are broken down?

fatty acids or alcohols that can join with them to form esters (aka cholesterol ester)

What is the hydrophilic portion of a fatty acid?

a carboxylic acid group (COOH)

What are the 4 primary functions of lipids?

critical structural component of bio membranes

store excess calories

provide readily available energy reserves

can be essential vitamins/hormones (Vit A,D,E,K)

What is the major storage form of fat in the body?

triglycerides

Describe the structure of cholesterol

27 C atoms

Slightly polar b/c of a free OH at C3

flat rigid structure that polarizes light

Describe the structure of bile acids. What is their primary role in lipid metabolism?

24 C atoms, C24 carboxyl group, OH groups (4)

promote fat absorption in the intestine as an emulsifier (detergent)

What must happen in order for lipids to be transported in blood?

Lipids must combine w/ apolipoprotein (apoproteins)

What are the 3 primary functions of an apoprotein?

1. maintain structural integrity of lipoprotein

2. activate lipid metabolism enzymes

3. be recognition signals for cell surface receptors

Lipoproteins are divided into 7 classes based on what 3 features?

1. size

2. apoprotein composition

3. lipid composition

What are the 7 classes of lipoproteins?

Chylomicrons, Chylomicron remnants, VLDL, IDL, LDL, HDL, Lp(a)

How are Low-Density Lipoproteins (LDL) formed? What 2 things can take up the LDL?

formed by lipolysis of VLDL

liver & peripheral cells via LDL receptors

Which apoproteins are released from LDL? What are the 2 ways those apoproteins can be utilized in lipid metabolism?

Apo-C & Apo-E

Can be 1) Taken up by circulating HDL 2) Degraded into their A.A. constituents

What 2 organs can synthesize High-Density Lipoproteins (HDL)? What is it capable of removing?

liver & small intestines

capable of removing excess cholesterol from peripheral cells back to the liver for removal (good cholesterol)

What apoproteins does HDL contain?

A-1 (E and CII buried inside)

What is Apo A-1 a cofactor for? And what does the enzyme it cofactors with do?

LCAT (Lecithin cholesterol acyltransferase)

Esterifies a FA to a cholesterol molecule (opposite of cholesterol esterase)

Where are apoproteins synthesized and which lipoproteins are they packaged with?

liver

HDL & VLDL

Out of the chylomicrons, VLDL, IDL, LDL, and HDL, which lipids are more predominant?

Triglycerides: Chylomicrons & VLDL

Cholesterol: IDL, LDL, HDL

For lipoprotein electrophoresis, what is the order from the cathode end to anode end?

Chylomicrons, LDL (β), IDL/VLDL, Lp(a) (Pre-β), HDL (α)

What charge does the lipoprotein carry if 1) pH > pI 2) pI > pH? What is the pH of the electrophoresis?

pH > pI: protein carries net negative charge and migrates toward anode

pI > pH: protein carries net positive charge and migrates towards cathode

8.6

Describe the Lipid Absorption Pathway (6 steps)

1. Bile acids (which are amphipathic) form micelles around either triglyceride or cholesterol ester

2. Formation of micelles allow either lipase or cholesterol esterase to hydrolyze the lipid

3. Triglycerides broken down into glycerol + 3 fatty acids. Cholesterol ester broken down into cholesterol + fatty acid.

4. Either are absorbed into the enterocyte (epithelial lining cells of intestines) via passive transfer or w/ transporters

5. Once in the enterocyte, they are re-synthesized with apoproteins A1,B48,CII, and small amounts of E to form chylomicrons.

6. They are secreted into intestinal lymphatics, ultimately → blood

Describe the Exogenous Metabolism Pathway (4 steps)

1. Chylomicrons w/ Apo-B48 & Apo-A enter the bloodstream

2. HDL provides the chylomicrons w/ Apo-C and Apo-E for further metabolism

3. Lipase breaks down triglycerides → Chylomicron remnants with ApoB-48 and Apo-E

4. Remnant taken by the liver (which has B&E receptors)

Describe the Endogenous Metabolism Pathway

1. Cholesterol & Triglycerides produced in the liver are packaged into VLDL with Apo-B100, Apo-C, and Apo-E

2. VLDL released into the bloodstream

3. VLDL’s undergo lipolysis by lipoprotein lipase → becomes IDL

4. Further lipolysis transforms IDL → LDL which only has Apo-B100 apoprotein left

5. LDL taken by the liver or tissues (which has B receptor for Apo-B100), completing cycle

What is coronary artery disease (CAD) a result of?

Life-long process of atherosclerosis (deposition of lipids in various tissues within the body)

What are the 7 risk factors for atherosclerosis?

1. >45 years old in men, >55 years old in women

2. family history of CHD

3. Hypertension (aka high blood pressure)

4. Diabetes

5. Low HDL, high LDL & total cholesterol

6. severe obesity & physical inactivity

7. stress

What are the lab guidelines for desirable lipid levels for TC, LDL, HDL, and triglyceride?

Total cholesterol: ≤ 200 mg/dL

LDL: ≤ 100 mg/dL

HDL: ≥ 50 mg/dL

Triglyceride: < 150 mg/dL

How is Lp(a) acquired and what is it “known as”?

genetically inherited

silent killer

What is Lp(a) structured similarly to? What is the mechanism that makes it so dangerous?

similarly structured to plasminogen

inhibits plasmin formation (which degrades incidental clots) by blocking plasminogen activators → increased risk of circulatory blocks

What are the 3 populations in which high HDL levels are associated with?

pre-menopausal women, people who exercise regularly, and people who maintain a healthy weight

What is the relationship between 1) estrogen and total cholesterol. 2) HDL in menstruating women and LDL

both have inverse relationships. If estrogen ↑, total cholesterol ↓. If HDL is ↑, LDL is ↓

What are the 3 forms of treatment for lipid disorders?

statin drugs: ↑ HDL and ↓ LDL

fibrate drugs: ↓ secretion of VLDL

cholestyramine: ↓ plasma cholesterol

What is the sample of choice for testing lipids & how long must a fast be for triglycerides?

serum

12 hours minimum

What does it mean when your sample is visibly lipemic? What does it mean if the lipids float on top of the serum?

means triglycerides MUST be elevated (>300 mg/dL)

Chylomicrons present if they float

What are the 4 reactions for triglyceride analysis?

1. triglycerides + H₂O + lipase → fatty acids + glycerol

2. glycerol + ATP + glycerol kinase → glycerol-1-phosphate + ADP

3. glycerol-1-phosphate + O₂ + glycerol phosphate oxidase → DHAP + H₂O₂

4. H₂O₂ + dye precursor + peroxidase → H₂O + quinonimine dye

What are the 2 potential issues that can happen with triglyceride analysis? Why is glycerol used instead of triglyceride for the standard?

1. free glycerol can accidentally be read/calculated as triglyceride

2. contaminants like Vit. C or bleach can react w/ H₂O₂ or the dye

large error can occur if detergent or lipase step doesn’t work

What are the 3 reactions for total cholesterol analysis?

1. cholesterol ester + H₂O + cholesterol esterase → cholesterol + fatty acid

2. cholesterol + O₂+ cholesterol oxidase → cholestenon + H₂O₂

3. 2H₂O₂ + reduced dye + peroxidase → 4H₂O + oxidized dye

What are the 3 steps to HDL analysis?

1. Precipitate LDL & VLDL w/ phospho-tungstate, heparin + Mn⁺², or dextran + MG⁺²

2. Centrifuge to separate supernatant HDL from precipitate

3. Measure cholesterol from HDL using same rxn as total cholesterol analysis

What must be considered when doing HDL analysis?

Because there is much less HDL compared to total cholesterol, a larger aliquot must be used

What are the steps to the alternative reaction for HDL analysis?

1. Mix serum with a detergent that dissolves only HDL particles

2. LDL & VLDL coated but not dissolved

3. Apply same color reagent to free cholesterol from HDL & follow same steps as total cholesterol analysis

What is the advantage & disadvantage of the alternative reaction for HDL analysis?

advantage: only 1 combination reagent → easily automated

disadvantage: lack of specificity for HDL for unusual patients

What is the assumption when doing the LDL calculation? What is the equation for calculating LDL?

That all triglyceride is present as VLDL IF TRIGLYCERIDE IS <400 mg/dL

Total Cholesterol = HDL + LDL + VLDL (triglyceride/5)

How is LDL measured asides from the calculation? What are the 2 details for the assay?

Direct Immunoassay for LDL

1. Latex beads coated w/ Ab’s against VLDL & HDL apoproteins (most like Apo-C & Apo-E) → only LDL left in solution

2. LDL in supernatant is measured