LIPID AND LIPOPROTEIN METABOLISM OBJECTIVES: After completion of this lesson, the participant will be able to: 1. Define the following ‘terms’:
a. Lipid
b. Lipoprotein
c. Apolipoprotein
d. Endogenous
e. Exogenous
2. Discuss the importance of specimen collection requirements for lipid studies 3. Identify at least 2 preanalytical sources of variation in lipid test results
4. Discuss lipid/lipoprotein biosynthesis and metabolism, including dietary sources, liver synthesis, digestion, absorption, transport and storage
5. Discuss the role of the liver in lipoprotein metabolism
6. Describe the basic composition of the following lipoprotein fractions, indicating which lipid is primarily found in each lipoprotein:
a. Chylomicron
b. Very low density lipoprotein (VLDL) c. Low density lipoprotein (LDL) d. High density lipoprotein (HDL)
7. Identify positive risk factors associated with coronary heart disease as determined by the National Cholesterol Education Program (NCEP) Adult Treatment Panel (ATP)
8. Identify the reference ranges for cholesterol, triglyceride, HDLc and LDLc as recommended by the American Heart Association/National Cholesterol Education Program
9. Assess the relative risk for developing coronary heart disease (CHD) when given lipid, and/or lipoprotein values and patient history
10. Discuss causes and related features of the following dyslipoproteinemia and lysosomal storage disorders:
a. Tangiers disease
b. Tay-Sachs disease
c. Gaucher’s disease
LIPID AND LIPOPROTEIN METABOLISM I. Lipids: What are they?
A. Characteristics
1. Class of compounds we know as ‘fats’ that are nearly insoluble in water (polar solutions), but soluble in organic solvents (non-polar solutions).
2. Most lipids are dependent on protein for transport through plasma and combine with protein to form water-soluble ‘lipoproteins’
2. Clinically important lipids include cholesterol and triglyceride, which are directly measured by the laboratory; also phospholipids and glycolipids
B. Lipids play important role in virtually all aspects of life. Major functions include: 1. Precursor of steroid hormones and bile salts (cholesterol)
2. Acts as functional and structural components in cell membranes of tissues, brain, central nervous system (cholesterol, phospholipid, glycolipid)
3. Provides energy storage and metabolic fuels (triglyceride)
4. Forms insulation (as adipose tissue) to prevent heat loss and protect vital organs from damage (triglyceride)
5. Provides maturity to fetal lung as pulmonary surfactants (phospholipid) C. Source of lipids
1. Exogenous lipids (from the diet)
a. Approx 40% of the caloric intake of a typical American diet consists of lipids: 35% from saturated animal lipids, 5% from polyunsaturated vegetable lipids
b. Diet contributes approx 300-500 mg of cholesterol daily c. Exogenous cholesterol and triglyceride are introduced to the
circulation through intestinal villi as chylomicrons, a triglyceride-rich lipoprotein
2. Endogenous lipids (hepatic origin)
b. Almost 90% of cholesterol synthesis occurs in the liver and gut, with the liver synthesizing approx 1.5 grams (1500 mg) daily
c. Endogenous cholesterol and triglyceride are introduced to the circulation as VLDL, a triglyceride-rich lipoprotein
II. Lipoproteins
A. Definition and function
1. In physiological fluids and tissues, most lipids are relatively water insoluble and must be ‘packaged’ as lipoproteins (a macromolecular complex) for
transport through plasma.
2. Lipoproteins are spherical particles with nonpolar lipids (triglyceride, unesterified cholesterol) located in the core and polar lipids (phospholipids, cholesterol esters)
located near the surface
3. Lipoproteins contain one or more specific proteins called apolipoproteins, which are primarily located on the lipoprotein surface.
4. Apolipoproteins are the protein components of lipoprotein and function to a. Activate important enzymes in lipoprotein metabolic pathways b. Maintain structural integrity of the lipoprotein complex
c. Facilitate uptake of lipoprotein into cells
5. Research indicates apolipoprotein abnormalities (synthesis and/or function) may lead to abnormal lipid metabolism and metabolic disease such as atherosclerosis, cardiovascular disease, stroke and Alzheimer’s disease. Major apolipoproteins (Apo) and the major lipoprotein it is associated with are:
a. Apo A-I: HDL
b. Apo B-100: LDL and VLDL
c. Apo B-48: chylomicrons
d. Apo(a): Lipoprotein(a)
B. Four lipoprotein fractions may be separated by electrophoresis and
ultracentrifugation techniques. Current lipoprotein classification based on relative densities
1. Chylomicrons: least dense of the lipoproteins and will float on top of plasma (aqueous solution)
2. VLDL: very low density lipoproteins 3. LDL: low density lipoproteins 4. HDL: high density lipoproteins
C. Approximate composition (% weight) of lipoproteins
Lipoprotein Cholesterol Triglyceride Protein Phospholipid
Chylomicrons VLDL LDL HDL 5-9 18-20 50-55 15-20 80-85 45-60 6-9 3-8 2-5 7-10 20 45-55 7 20-25 20 25-30
1. Chylomicrons (least dense of all the lipoproteins) a. Assembled by the mucosal cells in the intestine
b. Composed primarily of exogenous (dietary) triglycerides
c. Persist in plasma for several hours after a meal; thus requires 12-14 hours for total clearance from plasma
d. In the nonfasting state, chylomicrons contribute significantly to the total plasma triglyceride level
e. Will cause well-mixed plasma to be turbid; if only chylomicrons are present, will float on top of clear plasma as a creamy layer, after
overnight standing test
2. VLDL (very low density lipoproteins) a. Synthesized by the liver
b. Composed primarily of endogenous triglycerides
c. In the fasting state, most plasma triglyceride is present as VLDL d. Increased levels cause plasma to appear turbid
IDL (intermediate density lipoprotein): transition of VLDL to LDL in peripheral circulation
3. LDL (low density lipoprotein)
a. Synthesized from peripheral conversion of VLDL to LDL b. Composed primarily of cholesterol
c. LDL deposits cholesterol in fibroblasts and tissue cells resulting in
plaque formation
d. Increased LDL levels associated with increased risk of coronary heart
disease (CHD)
e. Referred to as ‘bad cholesterol’
Lp(a) is called “lipoprotein little a”; is structurally related to LDL ; associated with increased risk of CHD and stroke
4. HDL (high density lipoproteins)
a. Synthesized by the liver; ‘takes up’ lipids during catabolism of
chylomicrons, VLDL, IDL and LDL
b. Composed primarily of phospholipid and protein
d. Increased HDL levels associated with decreased risk of CHD e. Referred to as ‘good cholesterol’
III Lipoprotein Biosynthesis and Metabolism A. Overview
1. Ingests, absorbs, and transports 60-130 g of fat per day (most of the fat ingested is in the form of triglycerides; approx 500 mg of cholesterol/day)
2. >90% of dietary triglycerides are absorbed by the intestine
3. ~50% of dietary cholesterol is absorbed by the intestine (compared to the liver synthesizing 1.5 grams (1500 mg) of cholesterol per day)
B. Pathways of lipoprotein metabolism are complex; simplified overview follows:
C. Exogenous pathway (digestion and absorption) 1. Most digestion of fats occurs in the intestine
a. Bile salts and pancreatic/intestinal enzymes (lipoprotein lipase) break up (emulsify) the large dietary fat molecules into smaller particles that can be absorbed into the mucosal cells of the intestine
b. In the mucosal cells of the intestine the small particles of fat are reassembled into chylomicrons composed of exogenous lipids, primarily triglyceride
2. Chylomicrons enter the lymphatics and then released into the bloodstream a. Transported to tissue sites such as capillaries of skeletal muscle, heart
and adipose tissue for metabolic fuel or storage as fat (adipose tissue) b. Majority is transported to the liver
1) Broken down into free fatty acids, free cholesterol, and amino acids: used for metabolic processes, energy storage
2) Majority is ‘repackaged’ into VLDL
D. Endogenous pathway (liver repackaging, peripheral conversion and reverse transport)
1. Liver repackages chylomicrons into VLDL (composed of endogenous lipids, primarily triglyceride) which are released into the bloodstream. Liver also releases a “disc-shaped nascent HDL particle” which contains primarily phospholipid and apo A-I
2. In the bloodstream (peripheral circulation) VLDL undergoes lipolysis (catalyzed by lipoprotein lipase) resulting in formation of LDL (major cholesterol carrier)
3. LDL is taken up by cells (via receptors) and hydrolyzed releasing a. Free fatty acids and glycerol (used as energy, stored as fat)
b. Free cholesterol (used for membrane biosynthesis, steroid hormone synthesis in specific cells, bile synthesis by hepatocytes, and stored as
fat)
4. Reverse Cholesterol Transport Pathway
a. Cells derive cholesterol from LDL, but can also synthesize their own.
Cellular cholesterol ‘equilibrium’ is maintained by HDL
b. HDL functions to
1) Transport cholesterol away from peripheral cells to the liver
2) Scavenger lipid material during catabolism of chylomicrons,
VLDL and LDL
IV. Laboratory measurement
A. Sample collection requirements are essential, else false results are obtained: 1. Fasting specimen is required
a. Prolonged fasting period required (12-14 hr) to ensure clearance of exogenous triglycerides (chylomicrons) from plasma
b. Non-fasting specimen will falsely increase triglyceride levels
c. Fasting preferred for cholesterol (but not mandatory if only cholesterol
ordered by clinician)
2. Serum is specimen of choice (heparinized plasma also used)
3. Must be collected in glycerol free tube (glycerol falsely elevates triglyceride)
B. Lipid Profile (or Lipoprotein Profile)
1. Cholesterol and/or triglyceride levels can be measured separately, if ordered as such by the clinician, or as part of a lipid profile
2. To assess a patient’s risk of developing coronary heart disease, a Lipid Profile is recommended by NCEP (National Cholesterol Education Program)
3. Lipid Profile (Lipid Panel) as defined by CPT code 80061 includes:
a. Total cholesterol
b. Total triglyceride
c. HDL
d. LDL: calculated using the Friedwald calculation
e. Cholesterol:HDL ratio: calculated as total cholesterol/HDL 4. LDL is often calculated using Friedwald equation
a. LDLc = Total cholesterol - HDL - VLDL (VLDL = triglyceride/5) b. Assumes total cholesterol composed primarily of VLDL cholesterol,
LDL cholesterol, and HDL cholesterol
c. Assumes fasting plasma does not contain chylomicrons
d. Invalid when triglyceride > 400 mg/dl or if non-fasting specimen V. Clinical Application: Diagnosis of Lipid and Lipoprotein Disorders
A. National Cholesterol Education Program (NCEP) Recommendations:
1. Hypercholesterolemia in adults has been ‘redefined’ by the NCEP-ATP in terms of risk of developing coronary heart disease (CHD)
2. Cholesterol screening to diagnose hypercholesterolemia is now
recommended for all adults over the age of 20 years. The NCEP recommends measurement of total cholesterol and HDLc at the initial screening stage.
B.
C. Secondary hyperlipidemia and dyslipoproteinemia: generally the diagnosis of a primary disorder is made after secondary causes have been ruled out
D. Dyslipoproteinemia: Tangiers disease
1. Caused by an increased catabolism of HDL resulting in severely decreased or absent HDL with accumulation of cholesterol in many tissues throughout the body
2. Total cholesterol ~70-160 mg/dl; triglyceride varies with diet
3. Three major clinical signs: hyperplastic orange tonsils, splenomegaly and peripheral neuropathy; also heptomegaly and corneal opacities; increased incidence of CHD
E. Lipid Storage Disorders (lipidoses) (Lysosomal Storage Disease)
Inherited metabolic defect
1. Genetic defect where a critical lysosomal enzyme needed for lipid catabolism is decreased, absent or defective
2. Results in accumulation of substrate (fatty material) in lysosomes of vital organs especially the liver, spleen, bone marrow, brain, peripheral nervous system causing permanent damage
3. Symptoms/findings are dependent upon specific enzyme that is affected 4. Often fatal at early age
Tay-Sachs disease
1. Most thoroughly studied lysosomal storage disease; high prevalence in people who are of Ashkenazi descent (Jewish people of Eastern European ancestry) 2. The lysosomal enzyme hexosaminidase A is defective resulting in a build
up of partially hydrolyzed gangliosides in the lysosomes of the CNS leading to gradual collapse of the central nervous system
3. Results in blindness, deafness, paralysis and death by age 3 or 4
Gaucher's disease, type 1 (type 1 is the milder adult form; infant form type 2 is more severe with death by age 1)
1. Most frequently occurring lysosomal storage disease; highest incidence in Ashkenazi Jews
2. Caused by deficiency of beta-glucosidase enzyme resulting in accumulation of phospholipid (glucosylceramide) in lysosomes of liver, spleen, bone, CNS 3. Symptoms/findings: hepatosplenomegaly, Gaucher’s cells in bone marrow
aspirate, anemia, elevated phosphate, bone pain, bone fractures, lytic lesions, bleeding tendency (low platelets), yellow spots in eyes
Niemann-Pick disease, type IA (most severe form)
1. Three mutations account for 92% of Ashkenazi Jewish patients
2. Due to a defect in sphingomyelinase, the enzyme that cleaves phosphocholine from sphingomyelin; sphingomyelin (fat and cholesterol) accumulates in lysosomes of liver, spleen, lungs, bone marrow, and brain, resulting in mental retardation and early death
3. Symptoms/findings: foamy histiocytes in bone marrow, acute neuropathic, progressive loss of motor and intellectual capacity early in life,
Lipid and Lipoprotein Post Lecture Problem Case Three patients are seen in the clinic:
1. The first patient is a 40-year-old male with hypertension who smokes cigarettes. He has not been previously diagnosed with coronary heart disease (CHD). His father developed CHD at age 53. This patient’s fasting laboratory results follow:
Glucose 98 mg/dl triglyceride 150 mg/dl
Cholesterol 210 mg/dl HDL 45 mg/dl
1. Identify the abnormal test results
2. Calculate the LDL cholesterol level using the Friedwald calculation Friedwald calculation: LDL = total cholesterol - HDL - triglyceride
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3. Identify the CHD risk factors for this patient
2. The second patient is a 60-year-old woman with no family history of CHD. She is normotensive and has never smoked. This patient’s fasting laboratory results follow:
glucose 85 mg/dl triglyceride 85 mg/dl
cholesterol 220 mg/dl HDL 80 mg/dl
1. Identify the abnormal test results
2. Calculate the LDL cholesterol level using the Friedwald calculation
3. Identify the CHD risk factors for this patient
3. The third patient is a 49-year-old man with no personal or family history of CHD, does not smoke cigarettes and is normotensive. This patient’s fasting laboratory results follow:
glucose 134 mg/dl triglyceride 505 mg/dl
cholesterol 260 mg/dl HDL 25 mg/dl
1. Identify the abnormal test results
2. Calculate the LDL cholesterol level using the Friedwald calculation. Is this calculation valid? Why or why not
