Disorders of lipid metabolism/Questions and case studies

Questions

1. Which of the following statements regarding LDL is true?
   • A – participates in the supply of triacylglycerols through peripheral tissue
   • B – the main component of the surface part is cholesterol esters
   • C – apolipoprotein C is needed for its function
   • D – this particle is removed from the circulation by receptor-mediated endocytosis
2. Which of the following is not involved in esterification in centripetal cholesterol transport?
   • A – lecithin-cholesterol acyltransferase (LCAT)
   • B – phosphatidylcholine
   • C – apolipoprotein A
   • D – apolipoprotein B
3. Squalene is an intermediate metabolite in the biosynthesis of:
   • A – cholesterol
   • B – β-hydroxybutyrate
   • C – unsaturated fatty acids
   • D – sphingomyelin
   • E – acylcarnitine
4. The following does not apply to triacylglycerol hormone-sensitive lipase:
   • A – catalyzes the breakdown of triacylglycerols, which are transported in the core of VLDL particles
   • B – is induced by insulin
   • C – is regulated by phosphorylation and dephosphorylation
   • D – is an integral part of HDL
5. What results from a complete lack of apolipoprotein B-48?
   • A – VLDL cannot be formed
   • B – chylomicrons cannot be formed
   • C – LDL particles cannot be formed
   • D – cholesterol synthesis in the liver is impaired
6. Fatty acids released from adipose tissue are transported by circulation:
   • A – bound to albumin
   • B – as triacylglycerols carried by chylomicrons
   • C – as lecithin on the surface of HDL particles
   • D – by no specified mechanism
7. Fatty acid catabolism stimulates gluconeogenesis in all of the following ways except:
   • A – by contributing to the production of NADH
   • B – by providing carbons for the glucose backbone
   • C – by activating pyruvate carboxylase
   • D – by contributing to the production of ATP
8. Fatty acids serve as an energy source for (list all):
   • A – liver
   • B – brain
   • C – musculature
   • D – red blood cells
9. Which statement about HDL is correct?
   • A – has an antiatherogenic effect
   • B – arise in adipose tissue
   • C – carry triacylglycerols in their nucleus
   • D – enable centripetal transport of cholesterol
   • E – in the circulation they transmit apolipoprotein E and C to chylomicrons
10. The proliferation of LDL particles is more dangerous from the point of view of atherogenicity than the proliferation of chylomicrons or VLDL because:
    • A – LDL particles are smaller than VLDL or chylomicrons, so they penetrate the pores of the blood capillary wall and reach the target cells, to which they deliver cholesterol.
    • B – contain lecithin-cholesterol acyltransferase, which esterifies cholesterol into non-polar cholesteryl esters.
    • C – are taken up by target cells by endocytosis controlled by specific receptors
    • D – their increased amount in circulation stimulates the endogenous formation of cholesterol in the endothelium of vascular capillaries
11. Why are VLDL particles increased in alcoholics?
12. Which lipoprotein particles are increased in the absence of lipoprotein lipase?

Answers

Question 1. A – - wrong - supply of triacylglycerol to peripheral tissues is via VLDL or chylomicrons; LDL is the main transport particle for cholesterol B – wrong – cholesterol esters are hydrophobic and are therefore located not on the surface but in the core of lipoprotein particles C – wrong - apolipoprotein C is a cofactor of lipoprotein lipase D – correct - LDL passes through the pores of the endothelial wall of the capillary, is taken up by the target cells with the help of specific receptors that bind apolipoprotein B; the resulting complex is enveloped by absorption of part of the cell membrane (endocytosis); this creates a vesicle that combines with lysosomes; Their hydrolytic enzymes degrade the CDL particle; the released cholesterol is used to build cell membranes or (if there is excess) is transported back to the liver (centripetal transport) by HDL particles. Question 2. A – – wrong – LCAT participates; catalyzes cholesterol esterification B - wrong - phosphatidylcholine participates as a fatty acid donor after esterification C – wrong – apolipoprotein A is a cofactor of LCAT D – - correct - apolipoprotein B is not important for the centripetal cholesterol transport system; its role is in the metabolism of VLDL particles (mediates their secretion from hepatocytes) and LDL particles (is a binding protein for receptor-mediated endocytosis) E – wrong – esterification with LCAT and phosphatidylcholine takes place on HDL particles after cholesterol has been taken up from the cell membrane Question 3. A – correct - squalene is formed from 6 isoprene units (isopentenyl pyrophosphate); this is then cyclized to lanosterol B – wrong – β-hydroxybutyrate is a keto substance that is formed from acetoacetate during ketogenesis; this takes place in the mitochondria and not in the cytoplasm (unlike cholesterol biosynthesis) C – wrong – unsaturated fatty acids do not have squalene precursor D - wrong - sphingomyelin is formed from serine and palmityl-CoA, which give sphingosine, which is converted to ceramide by adding another fatty acid; reaction with cytidine diphosphate choline produces sphingomyelin E – wrong – acylcarnitine is involved in the transport of fatty acids across the mitochondrial membrane Question 4. A – wrong - it catalyzes the breakdown of triacylglycerols, which are transported in the core of VLDL particles B – wrong – is induced by insulin C - wrong - is regulated by phosphorylation and dephosphorylation D – correct - is an integral part of HDL Question 5. A – wrong - VLDL particles contain apolipoprotein B-100, which allows them to be secreted from the hepatocyte B – correct - chylomicrons that are formed in the intestinal mucosa need the presence of apolipoprotein B-48 for secretion into the lymphatic circulation C – wrong - LDL particles contain apolipoprotein B-100, which functions as a binding protein for receptor-mediated endocytosis D - wrong - cholesterol synthesis in the liver is not affected by the presence of apolipoprotein B-48 Question 6. A – correct - fatty acids after being released by lipolysis in adipose tissue are transported in the circulation bound to albumin B – wrong – as triacylglycerols inside chylomicrons, fatty acids are transported from the intestine to adipose tissue or muscle C – wrong – lecithin on the surface of HDL particles provides a fatty acid for cholesterol esterification D – wrong Question 7. A – wrong - the breakdown of fatty acids leads to the production of NADH, which is needed for gluconeogenesis; however, it is necessary for NADH to be transferred from the mitochondria (the mitochondrial membrane is not permeable for NADH) to the cytoplasm using the so-called malate or glycerol phosphate "shuttle" B – correct - Fatty acid catabolism by Đ-oxidation does not provide carbon atoms for the glucose backbone. Fatty acids are broken down into acetyl-CoA and acetoacetate, which is used to form ketone bodies. Most of the carbon atoms for gluconeogenesis are provided from the carbon skeleton of amino acids. Some amino acids are catabolized to pyruvate and thus enter the gluconeogenic pathway via the pyruvate carboxylase reaction. Others are converted to 4- to 5-carbon intermediates of the citric acid cycle and thus increase the content of oxaloacetate and malate in the mitochondria. Only leucine is completely degraded to acetyl CoA, so it is only ketogenic. Other amino acids are glucogenic or both. C – wrong – fatty acid catabolism increases acetyl CoA, which promotes (allosteric activation) pyruvate carboxylase D - wrong - most of the energy for gluconeogenesis is provided by fatty acid catabolism Question 8. A – yes - fatty acid catabolism takes place in the liver, which provides 129 ATP molecules, i.e. 8.1 ATP/ 1 C atom, during the complete oxidation of palmitate to CO 2, i.e. β-oxidation + breakdown of acetyl-CoA in the citric acid cycle B – no - the brain is dependent on glucose for energy consumption C – yes - skeletal muscle and myocardium use fatty acids as an energy source D – no - red blood cells cannot use fatty acids as an energy source because they do not have mitochondria Question 9. A – yes - HDL has an anti-atherogenic effect, because they rid the cells of excess cholesterol, convert it into cholesterol esters and, in this form, transport it to the liver, where the cholesterol is metabolized B – no – HDL is created in the liver, some is created during the conversion of chylomicrons to chylomicron remnants C - no - HDL carry cholesterol esters in their core D – yes - HDLs mediate the centripetal (reverse) transport of cholesterol from peripheral tissues to the liver E - yes - HDLs in the circulation transfer apolipoproteins E and C to the newly formed chylomicrons Question 10. A – correct - the smaller size of LDL particles allows them to penetrate the pores of the vessel walls of the capillaries and thus get out of circulation; it is not possible for VLDL and chylomicrons B – wrong – LDLs do not contain the enzyme LCAT; HDL particles have this C – correct - endocytosis controlled by specific receptors or a defect in the synthesis of these receptors results in hyper-LDL-lipoproteinemia, which is the cause of so-called familial hypercholesterolemia. D - correct - an increase in LDL in the circulation reduces the synthesis of a specific LDL-receptor and thus reduces the possibility of endocytosis of LDL particles; this leads to the stimulation of endogenous cholesterol formation in the cells Question 11. Alcohol oxidation in the liver shifts the NAD+/NADH ratio in favor of NADH. An increased amount of NADH inhibits the oxidation of fatty acids, which are reesterified in the liver to triacylglycerols and are incorporated into VLDL particles and secreted into the circulation. Question 12. Chylomicrons and VLDL. Lipoprotein lipase anchored on the membrane of the vascular endothelium hydrolyzes triacylglycerols from chylomicrons and VLDL, which then turn into so-called "remnants". other particles.. In case of deficiency of LPL or its cofactor (Apo CII), the cleavage of triacylglycerols is deficient and chylomicrons and VLDL accumulate in the circulation.

Case reports

Patient with hypertension and ischemic heart disease

A woman, 52 years old, with hypertension (controlled by medication), ischemic heart disease was examined for a lipid profile. The findings showed an increase in LDL-cholesterol (4.62 mmol/l), a decrease in HDL-cholesterol (0.90 mmol/l) and an increase in triacylglycerols (2.85 mmol/l). Her brother had mild hypercholesterolemia, but a significant decrease in HDL-cholesterol and normotriacylglycelemia, her sister had marked triacylglycerolemia.

Questions:

1. What are the main risk factors for coronary heart disease?
2. What adverse effect can poorly treated diabetes have on lipoprotein metabolism?
3. What does oxidized LDL-particles cause?

Answers

Age over 45 years, family history of CHD, cigarette smoking (over 15 per day), hypertension, obesity, lack of exercise, diabetes mellitus, hyperlipoproteinemia. Poorly treated diabetes with more persistent attacks of hyperglycemia leads to glycation of proteins and also the LDL-receptor protein and Apo B100. Proteins altered in this way are not metabolized normally, LDL particles accumulate in the circulation. The endothelium of the arterial wall can be damaged in various ways (mechanically, cytotoxicly, also by oxidized LDL particles). The damaged area attracts monocytes, which turn into macrophages. These trap oxidized LDL and turn it into foam cells. Damaged endothelium begins to produce thromboxane A2, which induces platelet aggregation.

Patient after acute myocardial infarction

A 53-year-old man who suffered an acute myocardial infarction was followed by the attending physician. Triacylglycerol level: 1.6 mmol/l, HDL-cholesterol: 0.89 mmol/l, cholesterol: 9.5 mmol/l, calculated LDL-cholesterol also significantly increased. His younger brother had a family history of "high cholesterol".

Questions:

1. What is the most likely form of hyperlipoproteinemia in this patient?
2. What is the mechanism of the therapeutic effect of cholestyramine (bile acid sequestrant) and statins?

Answers

Familial hypercholesterolemia (type Iia according to Fredrickson). Preparations of the cholestyramine type (e.g. Colestipol) bind bile acids in the intestine and thus prevent their reabsorption (they block entero-hepatic circulation). This results in an increased metabolism of cholesterol in the liver to bile acids. Statins are inhibitors of the key enzyme of cholesterol biosynthesis in cells – hydroxymethyl-glutarylCoA reductase.

A patient with hyperlipidemia

In a 65-year-old man, hyperlipidemia was discovered during a routine examination: cholesterol: 8.8 mmol/l, triacylglycerols: 2.4 mmol/l. The patient had periorbital edema , dry skin and hair, there was no family history of cardiovascular disease.

Questions:

1. What can be the causes of combined hyperlipoproteinemia?
2. What other examinations do you recommend for differential diagnosis?

Answers

Primary: Familial combined hyperlipoproteinemia (type IIb according to Fredrickson), familial dysbetalipoproteinemia (type II), unclassified, Secondary: type 2 diabetes mellitus, hypothyroidism, nephropathy, immunoglobulinopathy, cholestasis, hepatopathy, systemic lupus erythematosus, Tests for examination of the thyroid gland, liver, kidneys, protein electrophoresis. Results: aminotransferases normal, GMT normal, bilirubin or ALP not increased, chemical examination of urine and urine sediment within normal limits, TSH – 43 U/l (norm below 5.5), thyroxine – 12 nmol/l (norm: 60–160).

Answers

Hypothyroidism, secondary hyperlipoproteinemia.

Note: the thyroid disorder must be treated primarily, hyperlipoproteinemia usually resolves.

Patient with xanthomas and hypercholesterolemia

A 55-year-old, obese woman came to the general practitioner with nodules about 2 cm in diameter on her forearm. She had yellowish streaks on her palms (palmar striae).

Laboratory examination:

• cholesterol: 11,9 mmol/l
• triacylglycerides: 8.7 mmol/l
• ELFO of lipoproteins: broad β fraction

Questions:

1. What type of hyperlipoproteinemia is it?

Answers

It is familial dysbetalipoproteinemia (type III according to Fredrickson). It is accompanied by the apoprotein 2/2 phenotype. It is caused by insufficient degradation of VLDL that accumulates in the circulation (defect in the ligand of VLDL-residues for the apoE/apoB-receptor in the liver).

A 46-year-old manager on a preventive check-up

His father had a history of acute myocardial infarction at the age of 57, but he lived to be 79 years old. The patient is obese (104 kg, 175 cm), blood pressure 170/100 mmHg.

Laboratory examination:

• cholesterol: 6,5 mmol/l
• triacylglycerols: 2.9 mmol/l
• HDL cholesterol: 0.84 mmol/l

Questions:

1. What are the health risks for this patient?

Answers

Illness with ischemic heart disease: hypertension, mild hypercholesterolemia, triacylglycerolemia, reduced HDL-cholesterol, obesity, lack of exercise, stressful job.

Links

related articles

• Lipids
• Cholesterol
• Atherosclerosis
• CHD

Source

• MASOPUST, Jaroslav – PRŮŠA, Richard. Patobiochemie metabolických drah. 1. edition. 1999. 182 pp. pp. 80-85. ISBN 80-238-4589-6.