Chapter Review in Chapter 23 Hormonal Regulation and Integration of Mammalian Metabolism

Chapter Review

Key Terms

Terms in bold are defined in the glossary.

hormone
neuroendocrine system
metabotropic
ionotropic
endocrine
paracrine
autocrine
hypothalamus
hepatocyte
white adipose tissue (WAT)
adipocyte
brown adipose tissue (BAT)
uncoupling protein 1 (UCP1)
thermogenesis
beige adipose tissue
myocyte
plasma proteins
ATP-gated $K^{+}$ $upper K Superscript plus$ channels
sulfonylurea drugs
glucagon
cortisol
body mass index (BMI)
adipokines
leptin
arcuate nucleus
orexigenic
anorexigenic
α-melanocyte–stimulating hormone (α-MSH)
adiponectin
AMP-activated protein kinase (AMPK)
mTORC1 (mechanistic target of rapamycin complex 1)
PPAR (peroxisome proliferator-activated receptor)
ghrelin
endocannabinoids
probiotics
prebiotics
diabetes mellitus
type 1 diabetes
type 2 diabetes
glucose-tolerance test
ketosis
acidosis
ketoacidosis
metabolic syndrome

Problems

1. Peptide Hormone Activity Explain how two peptide hormones as structurally similar as oxytocin and vasopressin can have such different effects (see Fig. 23-8).

The sequence of amino acids in oxytocin is as follows: H 3 N bonded to C y s – T y r – red highlighted I l e – G l n – A s n – C y s – P r o – red highlighted L e u – G l y – C double bonded to O above and bonded to N H 2 to the right. The two C y s amino acids are bonded to yellow highlighted S above and these two S atoms are bonded together, forming a loop above the main chain. Vasopressin is similar except that red highlighted P h e is substituted for I l e and red highlighted A r g is substituted for L e u.

2. Metabolism of Glutamate in the Brain Brain tissue takes up glutamate from the blood, transforms it into glutamine, and releases the glutamine into the blood. What does this metabolic conversion accomplish? How does this conversion take place? The amount of glutamine produced in the brain can exceed the amount of glutamate entering from the blood. How does this extra glutamine arise? (Hint: You may want to review amino acid catabolism in Chapter 18; recall that ${NH}_{4}^{+}$ $NH Subscript 4 Superscript plus$ is very toxic to the brain.)
3. Proteins as Fuel during Fasting When muscle proteins undergo catabolism in skeletal muscle during a fast, what are the fates of the amino acids?
4. Absence of Glycerol Kinase in Adipose Tissue The biosynthesis of triacylglycerols requires glycerol 3-phosphate. Adipocytes, specialized for the synthesis and degradation of TAGs, cannot use glycerol directly because they lack glycerol kinase, which catalyzes the reaction

$Glycerol + ATP \to glycerol 3-phosphate + ADP$ $Glycerol plus ATP right-arrow glycerol 3 hyphen phosphate plus ADP$

How does adipose tissue obtain the glycerol 3-phosphate necessary for TAG synthesis?
5. Oxygen Consumption during Exercise A sedentary adult consumes about 0.05 L of $O_{2}$ $upper O Subscript 2$ in 10 seconds. A sprinter running a 100 m race consumes about 1 L of $O_{2}$ $upper O Subscript 2$ in 10 seconds. After finishing the race, the sprinter continues to breathe at an elevated (but declining) rate for some minutes, consuming an extra 4 L of $O_{2}$ $upper O Subscript 2$ above the amount consumed by the sedentary individual.
1. Why does the need for $O_{2}$ $upper O Subscript 2$ increase dramatically during the sprint?
2. Why does the demand for $O_{2}$ $upper O Subscript 2$ remain high after the sprinter finishes the race?
6. Thiamine Deficiency and Brain Function Individuals with thiamine deficiency show some characteristic neurological signs and symptoms, including loss of reflexes, anxiety, and mental confusion. Why might thiamine deficiency manifest as changes in brain function?
7. Potency of Hormones Under normal conditions, the human adrenal medulla secretes epinephrine $(C_{9} H_{13} {NO}_{3})$ $left-parenthesis upper C Subscript 9 Baseline upper H Subscript 13 Baseline NO Subscript 3 Baseline right-parenthesis$ at a rate sufficient to maintain a concentration of $10^{- 10} M$ $10 Superscript negative 10 Baseline upper M$ in circulating blood. To appreciate what that concentration means, calculate the volume of water that you would need to dissolve 1.0 g (about 1 teaspoon) of epinephrine to a concentration equal to that in blood.
8. Regulation of Hormone Levels in the Blood The half-life of most hormones in the blood is relatively short. For example, when researchers inject radioactively labeled insulin into an animal, half of the labeled hormone disappears from the blood within 30 min.
1. What is the importance of the relatively rapid inactivation of circulating hormones?
2. In what ways can the organism make rapid changes in the level of a circulating peptide hormone?
9. Water-Soluble versus Lipid-Soluble Hormones On the basis of their physical properties, hormones fall into one of two categories: those that are very soluble in water but relatively insoluble in lipids (e.g., epinephrine) and those that are relatively insoluble in water but highly soluble in lipids (e.g., steroid hormones). In their role as regulators of cellular activity, most water-soluble hormones do not enter their target cells. The lipid-soluble hormones, by contrast, do enter their target cells and ultimately act in the nucleus. What is the relationship between solubility, the location of receptors, and the mode of action of these two classes of hormones?
10. Metabolic Differences between Muscle and Liver in a “Fight-or-Flight” Situation When an animal confronts a “fight-or-flight” situation, the release of epinephrine promotes glycogen breakdown in the liver and skeletal muscle. The end product of glycogen breakdown in the liver is glucose; the end product in skeletal muscle is pyruvate.
1. What is the reason for the different products of glycogen breakdown in the two tissues?
2. What is the advantage of these specific glycogen breakdown routes to an animal that must fight or flee?
11. Excessive Amounts of Insulin Secretion: Hyperinsulinism Certain malignant tumors of the pancreas cause excessive production of insulin by β cells. Affected individuals exhibit shaking and trembling, weakness and fatigue, sweating, and hunger.
1. What is the effect of hyperinsulinism on the metabolism of carbohydrates, amino acids, and lipids by the liver?
2. What are the causes of the observed symptoms? Suggest why this condition, if prolonged, leads to brain damage.
12. Thermogenesis Caused by Thyroid Hormones Thyroid hormones are intimately involved in regulating the basal metabolic rate. Liver tissue of animals given excess thyroxine shows an increased rate of $O_{2}$ $upper O Subscript 2$ consumption and increased heat output (thermogenesis), but the ATP concentration in the tissue is normal. Different explanations have been offered for the thermogenic effect of thyroxine. One is that excess thyroxine causes uncoupling of oxidative phosphorylation in mitochondria. How could such an effect account for the observations? Another explanation suggests that thermogenesis is due to an increased rate of ATP utilization by the thyroxine-stimulated tissue. Is this a reasonable explanation? Why or why not?
13. Function of Prohormones What are the possible advantages of synthesizing hormones as prohormones?
14. Sources of Glucose during Starvation The typical human adult uses about 160 g of glucose per day. Of this, the brain alone uses 120 g. The body’s available reserve of glucose (∼20 g of circulating glucose and ∼190 g of glycogen) is adequate for about one day. After the glucose reserve has been depleted during starvation, how does the body obtain more glucose?
15. Parabiotic ob/ob Mice By careful surgery, researchers can connect the circulatory systems of two mice so that the same blood circulates through both animals. In these parabiotic mice, products released into the blood by one animal reach the other animal via the shared circulation. Both animals are free to eat independently. Suppose a researcher parabiotically joins a mutant ob/ob mouse (both copies of the OB gene are defective) and a normal OB/OB mouse (both copies of the OB gene are functional). What would happen to the weight of each mouse in the parabiotic pair?

16. Calculation of Body Mass Index A biochemistry professor weighs 260 lb (118 kg) and is 5 feet 8 inches (173 cm) tall. What is his body mass index (BMI)? How much weight would he have to lose to bring his BMI down to 25 (normal)?

17. Insulin Secretion Predict the effects on insulin secretion by pancreatic β cells of exposure to the potassium ionophore valinomycin. Explain your prediction.

18. Effects of a Deleted Insulin Receptor A strain of mice specifically lacking the insulin receptor of liver is found to have mild fasting hyperglycemia ( $blood glucose equals 132 mg slash dL$ $blood glucose equals 132 mg slash dL$ , vs. 101 mg/dL in controls) and a more striking hyperglycemia in the fed state ( $glucose equals 363 mg slash dL$ $glucose equals 363 mg slash dL$ , vs. 135 mg/dL in controls). The mice have higher than normal levels of glucose 6-phosphatase in the liver and elevated levels of insulin in the blood. Explain these observations.

19. Decisions on Drug Safety The drug rosiglitazone (Avandia) is effective in lowering blood glucose levels in patients with type 2 diabetes, but a few years after rosiglitazone came into widespread use, it seemed that using the drug came with an increased risk of heart attack. In response, the U.S. Food and Drug Administration (FDA) severely restricted the conditions under which it could be prescribed. Two years later, after additional studies had been completed, the FDA lifted the restrictions, and today rosiglitazone is available by prescription in the United States, with no special limitations. Many other countries ban it completely. If it were your responsibility to decide whether this drug should remain on the market (labeled with suitable warnings about its side effects) or should be withdrawn from the market altogether, what factors would you weigh in making your decision?

20. Type 2 Diabetes Medication The drugs acarbose (Precose) and miglitol (Glyset), used in the treatment of type 2 diabetes mellitus, inhibit α-glucosidases in the brush border of the small intestine. These enzymes degrade oligosaccharides derived from glycogen or starch to monosaccharides. Suggest a possible mechanism for the salutary effect of these drugs for individuals with diabetes. What side effects, if any, would you expect from these drugs? Why? (Hint: Review lactose intolerance, p. 523.)

Data Analysis Problem

21. Cloning the Sulfonylurea Receptor of the Pancreatic β Cell Glyburide, a member of the sulfonylurea family of drugs, is used to treat type 2 diabetes. It binds to and closes the ATP-gated $K^{+}$ $upper K Superscript plus$ channel shown in Figures 23-26 and 23-27.

Given the mechanism shown in Figure 23-27, would treatment with glyburide result in increased or decreased insulin secretion by pancreatic β cells? Explain your reasoning.

How does treatment with glyburide help reduce the symptoms of type 2 diabetes?

Would you expect glyburide to be useful for treating type 1 diabetes? Explain your answer.
Aguilar-Bryan and coauthors (1995) cloned the gene for the sulfonylurea receptor (SUR) portion of the ATP-gated $K^{+}$ $upper K Superscript plus$ channel from hamsters. The research team went to great lengths to ensure that the gene they cloned was, in fact, the SUR-encoding gene. Here we explore how it is possible for researchers to demonstrate that they have cloned the gene of interest rather than another gene.

The first step was to obtain pure SUR protein. As was already known, drugs such as glyburide bind SUR with very high affinity $left-parenthesis upper K Subscript d Baseline less-than 10 nM right-parenthesis$ $left-parenthesis upper K Subscript d Baseline less-than 10 nM right-parenthesis$ , and SUR has a molecular weight of 140 to 170 kDa. Aguilar-Bryan and coworkers made use of the high-affinity glyburide binding to tag the SUR protein with a radioactive label that would serve as a marker to purify the protein from a cell extract. First, they made a radiolabeled derivative of glyburide, using radioactive iodine $left-parenthesis Superscript 125 Baseline upper I right-parenthesis$ $left-parenthesis Superscript 125 Baseline upper I right-parenthesis$ :

In preliminary studies, the $^{125} I$ $Superscript 125 Baseline upper I$ -labeled glyburide derivative (hereafter, $[^{125} I]$ $left-bracket Superscript 125 Baseline upper I right-bracket$ glyburide) was shown to have the same $K_{d}$ $upper K Subscript d$ and binding characteristics as unaltered glyburide. Why was it necessary to demonstrate this? (What alternative possibilities did it rule out?)
Even though $[^{125} I]$ $left-bracket Superscript 125 Baseline upper I right-bracket$ glyburide bound to SUR with high affinity, a significant amount of the labeled drug would probably dissociate from the SUR protein during purification. To prevent this, $[^{125} I]$ $left-bracket Superscript 125 Baseline upper I right-bracket$ glyburide had to be covalently cross-linked to SUR. There are many methods for covalent cross-linking; Aguilar-Bryan and coworkers used UV light. When aromatic molecules are exposed to short-wave UV, they enter an excited state and readily form covalent bonds with nearby molecules. By cross-linking the radiolabeled glyburide to the SUR protein, the researchers could simply track the $^{125} I$ $Superscript 125 Baseline upper I$ radioactivity to follow SUR through the purification procedure.

The research team treated hamster HIT cells (which express SUR) with $[^{125} I]$ $left-bracket Superscript 125 Baseline upper I right-bracket$ glyburide and UV light, purified the $^{125} I$ $Superscript 125 Baseline upper I$ -labeled 140 kDa protein, and sequenced its 25 residue amino-terminal segment; they found the sequence PLAFCGTENHSAAYRVDQGVLNNGC. The investigators then generated antibodies that bound to two short peptides in this sequence, one binding to PLAFCGTE and the other to HSAAYRVDQGV, and showed that these antibodies bound the purified $^{125} I$ $Superscript 125 Baseline upper I$ -labeled 140 kDa protein.

Why was it necessary to include this antibody-binding step?
Next, the researchers designed PCR primers based on the sequences above, and then cloned a gene from a hamster cDNA library that encoded a protein with these sequences (see Chapter 9 on biotechnology methods). The cloned putative SUR cDNA hybridized to an mRNA of the appropriate length that was present in cells known to contain SUR. The putative SUR cDNA did not hybridize to any mRNA fraction of the mRNAs isolated from hepatocytes, which do not express SUR.

Why was it necessary to include this putative SUR cDNA–mRNA hybridization step?
Finally, the cloned gene was inserted into and expressed in COS cells, which do not normally express the SUR gene. The investigators mixed these cells with $[^{125} I]$ $left-bracket Superscript 125 Baseline upper I right-bracket$ glyburide, with or without a large excess of unlabeled glyburide, exposed the cells to UV light, and measured the radioactivity of the 140 kDa protein produced. Their results are shown in the table.

Experiment

Cell type

Added putative SUR cDNA?

Added excess unlabeled glyburide?

$^{125} I$ $Superscript bold 125 Baseline bold upper I$ label in 140 kDa protein

1

HIT

No

No

+ + +

2

HIT

No

Yes

−

3

COS

No

No

−

4

COS

Yes

No

+ + +

5

COS

Yes

Yes

−

Why was no $^{125} I$ $Superscript 125 Baseline upper I$ -labeled 140 kDa protein found in experiment 2?

How would you use the information in the table to argue that the cDNA encoded SUR?

What other information would you want to collect to be more confident that you had cloned the SUR gene?

Reference

Aguilar-Bryan, L., C.G. Nichols, S.W. Wechsler, J.P. Clement, IV, A.E. Boyd, III, G. González, H. Herrera-Sosa, K. Nguy, J. Bryan, and D.A. Nelson. 1995. Cloning of the β cell high-affinity sulfonylurea receptor: a regulator of insulin secretion. Science 268:423–426.

Experiment	Cell type	Added putative SUR cDNA?	Added excess unlabeled glyburide?	$^{125} I$ $Superscript bold 125 Baseline bold upper I$ label in 140 kDa protein
1	HIT	No	No	+ + +
2	HIT	No	Yes	−
3	COS	No	No	−
4	COS	Yes	No	+ + +
5	COS	Yes	Yes	−