A young girl with a history of severe abdominal pain was taken to her local hospital at 5 a.m. in severe distress. Blood was drawn, and the plasma appeared milky, with the triacylglycerol level >2,000 mg/dl (normal = 4–150 mg/dl). The patient was placed on a diet extremely limited in fat but supplemented with medium-chain triglycerides.
8.3. Which of the following lipoprotein particles are most likely responsible for the appearance of the patient’s plasma?
A. Chylomicrons
B. High-density lipoproteins
C. Intermediate-density lipoproteins
D. Low-density lipoproteins
E. Very-low-density lipoproteins
Correct answer = A. The milky appearance of her plasma was a result of triacylglycerol-rich chylomicrons. Because 5 a.m. is presumably several hours after her evening meal, the patient must have difficulty degrading these lipoprotein particles. Intermediate-, low-, and high-density lipoproteins contain primarily cholesteryl esters, and, if one or more of these particles was elevated, it would cause hypercholesterolemia. Very-low-density lipoproteins do not cause the described milky appearance of plasma.
8.4. Which one of the following proteins is most likely to be deficient in this patient?
A. Apolipoprotein A-I B. Apolipoprotein B-48
C. Apolipoprotein C-II
D. Cholesteryl ester transfer protein
E. Microsomal triglyceride transfer protein
Correct answer = C. The triacylglycerol (TAG) in chylomicrons is degraded by endothelial lipoprotein lipase (LPL), which requires apolipoprotein (apo) C-II as a coenzyme. Deficiency of LPL or apo C-II results in decreased ability to degrade chylomicrons to their remnants, which get cleared (via apo E) by liver receptors. Apo A-I is the coenzyme for lecithin:cholesterol acyltransferase; apo B-48 is the characteristic structural protein of chylomicrons; cholesteryl ester transfer protein catalyzes the cholesteryl ester–TAG exchange between high-density and very-low-density lipoproteins (VLDL); and microsomal triglyceride transfer protein is involved in the formation, not degradation, of chylomicrons (and VLDL).
8.5. Complete the table below for an individual with classic 21-α-hydroxylase deficiency relative to a normal individual.
How might the results be changed if this individual were deficient in 17-αhydroxylase, rather than 21-α-hydroxylase?
Classic 21-α-hydroxylase deficiency causes mineralocorticoids (aldosterone) and glucocorticoids (cortisol) to be virtually absent. Because aldosterone increases blood pressure, and cortisol increases blood glucose, their deficiencies result in a decrease in blood pressure and blood glucose, respectively. Cortisol normally feeds back to inhibit adrenocorticotropic hormone (ACTH) release by the pituitary, and, so, its absence results in an elevation in ACTH. The loss of 21-α-hydroxylase pushes progesterone and pregnenolonetoandrogensynthesisand,therefore,causesandrostenedionelevelstorise.With17-α-hydroxylasedeficiency,sexhormonesynthesiswouldbedecreased.Mineralocorticoidproductionwouldbeincreased,leadingtohypertension.
UNIT IV Nitrogen Metabolism Amino Acids: Nitrogen Disposal 19
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I. OVERVIEW