Lipid Metabolism Practice Questions

Q: Which of the following enzymes is not required for the formation of estradiol?

11β-hydroxylase. ***11β-hydroxylase*** - This enzyme is crucial for the **synthesis of cortisol** and **aldosterone** within the adrenal cortex, converting 11-deoxycortisol to cortisol and 11-deoxycorticosterone to corticosterone. - It plays no direct role in the synthesis pathway of **estrogen**, specifically estradiol, which is synthesized from androgens. *3β-hydroxysteroid dehydrogenase* - This enzyme is required for multiple steps in steroidogenesis, including the conversion of **pregnenolone to progesterone** and **DHEA to androstenedione**, both of which are precursors to estrogens like estradiol. - Its activity is essential for moving from **Δ5 steroids** to **Δ4 steroids**, an early and critical step in androgen and subsequent estrogen synthesis. *Aromatase* - **Aromatase (CYP19A1)** is the enzyme directly responsible for converting androgens (**androstenedione and testosterone**) into estrogens (**estrone and estradiol**, respectively). - It catalyzes the **aromatization of the A-ring** of the steroid structure, a defining step in estrogen synthesis. *17α-hydroxylase* - This enzyme (CYP17A1) is involved in crucial steps leading up to estrogen synthesis, including the conversion of **progesterone to 17α-hydroxyprogesterone** and **pregnenolone to 17α-hydroxypregnenolone**. - Its activity is necessary for the formation of **androgens** (like DHEA and androstenedione), which are direct precursors for estrogen synthesis.

Q: Which apolipoprotein is the primary structural component of LpA-I particles?

Apo A-I. ***Apo A-I*** - **Apolipoprotein A-I (Apo A-I)** is the main structural and functional protein of **high-density lipoprotein (HDL)**. - It plays a crucial role in **reverse cholesterol transport**, facilitating the removal of excess cholesterol from peripheral tissues back to the liver. *Apo B-48* - **Apo B-48** is found exclusively in **chylomicrons**, which are responsible for transporting dietary lipids from the intestines. - It is synthesized in the **intestine** and is critical for the assembly and secretion of chylomicrons. *Apo A-II* - **Apo A-II** is another apolipoprotein found in HDL particles, but it is not the primary structural component. - While present, it is less abundant than Apo A-I and its precise role is still being researched, though it may influence **HDL metabolism**. *Apo B-100* - **Apo B-100** is the primary structural protein of **low-density lipoprotein (LDL)** and very-low-density lipoprotein (VLDL). - It is essential for the binding of LDL to the **LDL receptor**, mediating the uptake of cholesterol into cells.

Q: Which of the following is not a ketone body produced by the liver?

Glycerol 3-phosphate. ***Glycerol 3-phosphate*** - **Glycerol 3-phosphate** is a molecule involved in **triglyceride synthesis** and glycolysis, not a ketone body produced by the liver. - It is formed from **dihydroxyacetone phosphate** (a glycolysis intermediate) or by phosphorylation of **glycerol**. *β-hydroxybutyrate* - **β-hydroxybutyrate** is one of the primary **ketone bodies** produced by the liver. - It is formed from **acetoacetate** and is a major energy source during prolonged fasting or ketogenic states. *Acetoacetate* - **Acetoacetate** is a principal **ketone body** synthesized by the liver. - It is an intermediate formed during the breakdown of **fatty acids** and supplies energy to peripheral tissues. *Acetone* - **Acetone** is a ketone body that arises from the **spontaneous decarboxylation of acetoacetate**. - While produced by the liver, it is primarily **excreted through respiration** and is not used as an energy source by peripheral tissues.

Q: What is the effect of progesterone on lipids?

Lowers HDL and increases LDL. ***Lowers HDL and increases LDL*** - This describes the effect of **synthetic progestins** (particularly older generation ones like levonorgestrel) on lipid profiles. - Synthetic progestins have been shown to **decrease HDL cholesterol** and **increase LDL cholesterol**, contributing to an unfavorable cardiovascular risk profile. - **Natural progesterone** has minimal or neutral effects on lipids, but this question refers to the progestin effects commonly discussed in contraceptive and hormone replacement therapy contexts. - This is the **classical teaching** for progesterone effects on lipids in most medical textbooks. *Lowers LDL, increases HDL* - This effect is characteristic of **estrogen**, not progesterone. - Estrogen improves lipid profiles by increasing HDL and lowering LDL cholesterol. - Progestins generally have opposite or antagonistic effects compared to estrogen on lipid metabolism. *Lowers HDL and lowers LDL* - While synthetic progestins do lower HDL, they typically **increase LDL**, not lower it. - A simultaneous decrease in both HDL and LDL is not a characteristic effect of progesterone or progestins. *Increases LDL and HDL* - Synthetic progestins tend to increase LDL, but they typically **lower HDL**, not increase it. - An increase in both LDL and HDL simultaneously is not a typical effect of progesterone on lipid metabolism.

Q: Lipoprotein lipase is activated by:

Apo C2. ***Apo C2*** - **Apo C2** (apolipoprotein C-II) acts as a **cofactor** for **lipoprotein lipase (LPL)**. - Its presence is essential for LPL to efficiently **hydrolyze triglycerides** within chylomicrons and VLDL, releasing fatty acids for tissue uptake. *Apo C3* - **Apo C3** is known to **inhibit** lipoprotein lipase activity, which is the opposite of activation. - It plays a role in slowing down the clearance of triglyceride-rich lipoproteins by interfering with LPL function. *Apo C1* - **Apo C1** (apolipoprotein C-I) plays a role in **activating lecithin-cholesterol acyltransferase (LCAT)** and may inhibit cholesteryl ester transfer protein (CETP), but it does not directly activate LPL. - Its primary functions are related to **cholesterol metabolism** and reverse cholesterol transport. *Apo A1* - **Apo A1** (apolipoprotein A-I) is the major protein component of **high-density lipoprotein (HDL)**. - It is a potent **activator of LCAT**, which is crucial for cholesterol esterification in HDL, but it does not activate LPL.

Q: Which of the following statements about trans fatty acids is false?

Refining reduces the level of TFA. ***Refining reduces the level of TFA*** - This statement is **false** because refining processes, particularly high-temperature deodorization during oil refining, can actually *increase* the formation of **trans fatty acids (TFAs)** through thermal isomerization of cis unsaturated fatty acids. - While refining removes impurities and improves oil stability, temperatures above 200°C during deodorization can convert some cis bonds to trans configuration, typically resulting in 1-3% TFA formation. *Hydrogenation increases the level of TFA* - **Partial hydrogenation** is the primary industrial source of **trans fatty acids (TFAs)**, converting liquid vegetable oils into semi-solid fats. - During this process, hydrogen is added to unsaturated fatty acids, and some double bonds shift from the natural *cis* configuration to the *trans* configuration, potentially creating 25-45% TFA content. - This statement is **true**. *Increases LDL cholesterol levels* - **Trans fatty acids (TFAs)** have a well-established dual negative effect: they raise **LDL cholesterol** ("bad" cholesterol) and lower **HDL cholesterol** ("good" cholesterol). - Even small amounts of TFA intake (1-3% of total energy) significantly increase **cardiovascular disease** risk. - This statement is **true**. *Fried rice has a high content of TFA* - This statement is **generally true** when referring to commercially prepared fried rice in settings where **partially hydrogenated oils** or repeatedly used/degraded cooking oils are employed. - However, home-cooked fried rice using fresh vegetable oils contains minimal TFAs, as stir-frying at typical cooking temperatures (150-200°C) produces negligible trans fat formation. - The high TFA content is primarily associated with commercial/restaurant preparation using poor quality or hydrogenated fats, not the dish itself.

Q: The major carrier of cholesterol in plasma is:

Low-Density Lipoprotein (LDL). ***Low-Density Lipoprotein (LDL)*** - **LDL** is the **major carrier of cholesterol in plasma**, transporting approximately **60-70% of total plasma cholesterol**. - It is primarily responsible for delivering **cholesterol** from the liver to peripheral tissues for **membrane synthesis**, **steroid hormone production**, and other cellular functions. - LDL cholesterol levels are the primary target for cardiovascular risk assessment and management. *Very-Low-Density Lipoprotein (VLDL)* - **VLDL** primarily transports **triglycerides** (55-65% of its content) synthesized in the liver to peripheral tissues. - While it contains some cholesterol (~10-15%), its main function is **triglyceride delivery**, and it serves as a precursor to LDL in the circulation. *Chylomicrons* - **Chylomicrons** are responsible for transporting **dietary triglycerides** and **cholesterol** from the intestines to tissues. - They are the largest lipoproteins and primarily transport **exogenous (dietary) lipids**. - Cholesterol represents only 3-5% of chylomicron content. *High-Density Lipoprotein (HDL)* - **HDL** carries approximately **20-30% of plasma cholesterol** and plays a crucial role in **reverse cholesterol transport**. - It collects excess cholesterol from peripheral tissues and returns it to the liver for excretion. - While functionally important for cholesterol homeostasis (protective against atherosclerosis), it carries significantly less cholesterol than LDL.

Q: More than 50% of polyunsaturated fatty acids are seen in all, except:

Canola oil. ***Canola oil*** - While canola oil is a source of **polyunsaturated fatty acids (PUFA)**, its total PUFA content typically ranges from 25-35%, which is not more than 50%. - It is particularly rich in **monounsaturated fatty acids (MUFA)**, primarily **oleic acid**, which can make up a higher percentage of its fat content. *Soybean oil* - Soybean oil is known for its high content of **polyunsaturated fatty acids (PUFA)**, particularly **linoleic acid (omega-6)** and **alpha-linolenic acid (omega-3)**. - Its total PUFA content commonly exceeds **50%**, making it a significant source of these essential fatty acids. *Safflower oil* - Safflower oil is exceptionally rich in **polyunsaturated fatty acids (PUFA)**, especially **linoleic acid (omega-6)**. - Depending on the variety, its linoleic acid content alone can be over **70%**, ensuring its total PUFA content is well over 50%. *Corn oil* - Corn oil is another common vegetable oil with a high proportion of **polyunsaturated fatty acids (PUFA)**, predominantly **linoleic acid**. - Its PUFA content typically ranges from **50-60%**, meeting the criterion of having more than 50% PUFA.

Q: From which of the following cellular locations are ketone bodies synthesized?

Mitochondria. ***Mitochondria*** - **Ketone bodies**, namely **acetoacetate** and **beta-hydroxybutyrate**, are primarily synthesized in the mitochondrial matrix of liver cells. - This process, known as **ketogenesis**, occurs when **acetyl-CoA** levels are high, typically due to increased fatty acid oxidation during fasting or uncontrolled diabetes. *Cytosol* - The **cytosol** is the site of many metabolic pathways, including **glycolysis** and the pentose phosphate pathway, but not ketone body synthesis. - While some steps of fatty acid synthesis occur in the cytosol, a distinct process from ketogenesis, ketone body formation happens in a separate compartment. *ER* - The **endoplasmic reticulum (ER)** is involved in protein synthesis and folding, lipid synthesis, and detoxification. - It does not play a direct role in the synthesis of ketone bodies. *Peroxisomes* - **Peroxisomes** are involved in processes like **beta-oxidation of very long-chain fatty acids** and the synthesis of plasmalogens. - While involved in lipid metabolism, they are not the primary site for the synthesis of ketone bodies.

Question 1

Which of the following enzymes is not required for the formation of estradiol?

Accepted Answer

11β-hydroxylase

Answer

Aromatase

Answer

3β-hydroxysteroid dehydrogenase

Answer

17α-hydroxylase

Question 2

Which apolipoprotein is the primary structural component of LpA-I particles?

Accepted Answer

Apo A-I

Answer

Apo B-48

Answer

Apo A-II

Answer

Apo B-100

Question 3

What is the primary role of cholesterol in low-density lipoprotein (LDL) metabolism?

Accepted Answer

Excess cholesterol in cells reduces the number of LDL receptors.

Answer

Cholesterol binds to receptors on cell membranes.

Answer

Cholesterol regulates the activity of enzymes involved in cholesterol metabolism.

Answer

Cholesterol in LDL is primarily involved in transporting cholesterol to tissues.

Question 4

Which of the following is not a ketone body produced by the liver?

Accepted Answer

Glycerol 3-phosphate

Answer

Acetone

Answer

β-hydroxybutyrate

Answer

Acetoacetate

Question 5

What is the effect of progesterone on lipids?

Accepted Answer

Lowers HDL and increases LDL

Answer

Lowers HDL and lowers LDL

Answer

Lowers LDL, increases HDL

Answer

Increases LDL and HDL

Question 6

Lipoprotein lipase is activated by:

Accepted Answer

Apo C2

Answer

Apo C3

Answer

Apo C1

Answer

Apo A1

Question 7

Which of the following statements about trans fatty acids is false?

Accepted Answer

Refining reduces the level of TFA

Answer

Hydrogenation increases the level of TFA

Answer

Increases LDL cholesterol levels

Answer

Fried rice has a high content of TFA

Question 8

The major carrier of cholesterol in plasma is:

Accepted Answer

Low-Density Lipoprotein (LDL)

Answer

Very-Low-Density Lipoprotein (VLDL)

Answer

Chylomicrons

Answer

High-Density Lipoprotein (HDL)

Question 9

More than 50% of polyunsaturated fatty acids are seen in all, except:

Accepted Answer

Canola oil

Answer

Soybean oil

Answer

Safflower oil

Answer

Corn oil

Question 10

From which of the following cellular locations are ketone bodies synthesized?

Accepted Answer

Mitochondria

Answer

Cytosol

Answer

ER

Answer

Peroxisomes

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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