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pob5e_solutions_ch23 - 2608T_ch23sm_S266-S273 12:03 am...

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Hormonal Regulation and Integration of Mammalian Metabolism S-266 1. Peptide Hormone Activity Explain how two peptide hormones as structurally similar as oxytocin and vasopressin can have such different effects (see Fig. 23–10). Answer Oxytocin and vasopressin are recognized by different receptors, typically found in different cell types. These receptors are coupled to different downstream effects in their target cells. 2. ATP and Phosphocreatine as Sources of Energy for Muscle During muscle contraction, the con- centration of phosphocreatine in skeletal muscle drops while the concentration of ATP remains fairly constant. However, in a classic experiment, Robert Davies found that if he first treated muscle with 1-fluoro-2,4-dinitrobenzene (p. 94), the concentration of ATP declined rapidly while the concentration of phosphocreatine remained unchanged during a series of contractions. Suggest an explanation. Answer Muscle contraction results in a net hydrolysis of ATP. Although the amount of ATP in muscle is very small, the supply can be rapidly replenished by phosphoryl group transfer from the phosphocreatine reservoir, catalyzed by creatine kinase: Phosphocreatine ADP 34 creatine ADP Because this reaction is rapid relative to the use of ATP by muscle, [ATP] remains in a steady state. The effect of pretreatment with fluoro-2,4-dinitrobenzene suggests that this is an effec- tive inhibitor of creatine kinase. Under working conditions, the small amount of muscle ATP is quickly depleted and cannot be replenished. 3. Metabolism of Glutamate in the Brain Brain tissue takes up glutamate from the blood, transforms it into glutamine, then releases it into the blood. What is accomplished by this metabolic conversion? How does it take place? The amount of glutamine produced in the brain can actually 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 3 is very toxic to the brain.) Answer Ammonia is very toxic to nervous tissue, especially the brain. Excess NH 3 in brain cells is removed by the transformation of glutamate to glutamine, catalyzed by glutamine syn- thetase. Glutamine is then exported to the blood and travels to the liver, where it is trans- formed to urea. The additional glutamine in the brain arises from the action of aminotransferases that transfer amino groups from amino acids to a -ketoglutarate (a citric acid cycle intermediate), forming glutamate, which is then converted to glutamine. chapter 23 2608T_ch23sm_S266-S273 02/26/2008 12:03 am Page S-266 pinnacle OS9:Desktop Folder:WHQY028:
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4. Proteins as Fuel during Fasting When muscle proteins are catabolized in skeletal muscle during a fast, what are the fates of the amino acids?
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