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Amino Acid Oxidation and the Production of Urea chapter 18 S-211 1. Products of Amino Acid Transamination Name and draw the structure of the a -keto acid resulting when each of the following amino acids undergoes transamination with a -ketoglutarate: (a) aspartate, (b) glutamate, (c) alanine, (d) phenylalanine. Answer 2. Measurement of Alanine Aminotransferase Activity The activity (reaction rate) of alanine aminotransferase is usually measured by including an excess of pure lactate dehydrogenase and NADH in the reaction system. The rate of alanine disappearance is equal to the rate of NADH disappearance measured spectrophotometrically. Explain how this assay works. Answer The measurement of the activity of alanine aminotransferase by measurement of the reaction of its product with lactate dehydrogenase is an example of a “coupled” assay. The prod- uct of the transamination (pyruvate) is rapidly consumed in the subsequent “indicator reaction,” catalyzed by an excess of lactate dehydrogenase. The dehydrogenase uses the cofactor NADH, the disappearance of which is conveniently measured by observing the rate of decrease in NADH absorption at 340 nm. Thus, the rate of disappearance of NADH is a measure of the rate of the aminotransferase reaction, if NADH and lactate dehydrogenase are added in excess. 3. Alanine and Glutamine in the Blood Normal human blood plasma contains all the amino acids re- quired for the synthesis of body proteins, but not in equal concentrations. Alanine and glutamine are present in much higher concentrations than any other amino acids. Suggest why. Answer Muscle tissue can convert amino acids to their keto acids plus ammonia, then oxidize the keto acids to produce ATP for muscle contraction. However, urea cannot be formed in -Ketoglutarate O CH 3 C Phenylpyruvate CH 2 O COO B C OOC COO CH 2 C O COO Oxaloacetate Pyruvate (a) (b) (c) (d) OOC CH 2 CH 2 COO C O 2608T_ch18sm_S211-S222 02/22/2008 2:46 pm Page S-211 pinnacle 111:WHQY028:Solutions Manual:Ch-18:
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S-212 Chapter 18 Amino Acid Oxidation and the Production of Urea muscle. Alanine and glutamine transport amino groups in the bloodstream to the liver (see Fig. 18–2) from muscle and other nonhepatic tissues. In muscle, amino groups from all amino acids are transferred to pyruvate or glutamate to form alanine or glutamine, and these latter amino acids are transported to the liver. 4. Distribution of Amino Nitrogen If your diet is rich in alanine but deficient in aspartate, will you show signs of aspartate deficiency? Explain. Answer No; aspartate is readily formed by the transfer of the amino group of alanine to ox- aloacetate. Cellular levels of aminotransferases are sufficient to provide all of the amino acids in this fashion, if the a -keto acids are available. 5. Lactate versus Alanine as Metabolic Fuel: The Cost of Nitrogen Removal The three carbons in lactate and alanine have identical oxidation states, and animals can use either carbon source as a meta- bolic fuel. Compare the net ATP yield (moles of ATP per mole of substrate) for the complete oxidation (to CO 2
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