Chapter 18

Name and draw the structure of the \(a\)-keto acid resulting when each of the four amino acids listed undergoes transamination with \(a\) ketoglutarate: (a) aspartate, (b) glutamate, (c) alanine, (d) phenylalanine.

The measurement of alanine aminotransferase activity (reaction rate) usually includes 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.

Normal human blood plasma contains all the amino acids required 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.

If your diet is rich in alanine but deficient in aspartate, will you show signs of aspartate deficiency? Explain.

In a study, cats were fasted overnight then given a single meal complete in all amino acids except arginine. Within 2 hours, blood ammonia levels increased from a normal level of \(18 \mu \mathrm{g} / \mathrm{L}\) to \(140 \mu \mathrm{g} / \mathrm{L}\), and the cats showed the clinical symptoms of ammonia toxicity. A control group fed a complete amino acid diet or an amino acid diet in which arginine was replaced by ornithine showed no unusual clinical symptoms. a. What was the role of fasting in the experiment? b. What caused the ammonia levels to rise in the experimental group? Why did the absence of arginine lead to ammonia toxicity? Is arginine an essential amino acid in cats? Why or why not? c. Why can ornithine be substituted for arginine?

A weightreducing diet heavily promoted some years ago required the daily intake of a "liquid protein" soup made of hydrolyzed gelatin (derived from collagen), water, and an assortment of vitamins. All other food and drink were to be avoided. People on this diet typically lost 10 to \(14 \mathrm{lb}\) in the first week. a. Opponents argued that the weight loss was almost entirely due to water loss and would be regained very soon after a normal diet was resumed. What is the biochemical basis for this argument? b. A few people on this diet died. What are some of the dangers inherent in the diet, and how can they lead to death?

Which amino acids are exclusively ketogenic?

A two-year-old child was taken to the hospital. His mother said that he vomited frequently, especially after feedings. The child's weight and physical development were below normal. His hair, although dark, contained patches of white. A urine sample treated with ferric chloride \(\left(\mathrm{FeCl}_{3}\right)\) gave a green color characteristic of the presence of phenylpyruvate. Quantitative analysis of urine samples gave the results shown in the table. a. Suggest which enzyme might be deficient in this child. Propose a treatment. b. Why does phenylalanine appear in the urine in large amounts? c. What is the source of phenylpyruvate and phenyllactate? Why does this pathway (normally not functional) come into play when the concentration of phenylalanine rises? d. Why does the boy's hair contain patches of white?

Vegetarian Diets Vegetarian diets can provide high levels of antioxidants and a lipid profile that can help prevent coronary disease. However, there can be some associated problems. Blood samples were taken from a large group of volunteer subjects who were vegans (strict vegetarians: no animal products), lactovegetarians (vegetarians who eat dairy products), or omnivores (individuals with a varied diet, including meat). In each case, the volunteers had followed the diet for several years. The blood levels of both homocysteine and methylmalonate were elevated in the vegan group, somewhat lower in the lactovegetarian group, and much lower in the omnivore group. Explain.

Vitamin \(\mathrm{B}_{12}\) deficiency can arise from a few rare genetic diseases that lead to low \(\mathrm{B}_{12}\) levels despite a normal diet that includes \(\mathrm{B}_{12}\)-rich meat and dairy sources. These conditions cannot be treated with dietary \(\mathrm{B}_{12}\) supplements. Explain.