Chapter Summary, Questions Answers - Amino Acids: Disposal of Nitrogen

CHAPTER SUMMARY

Nitrogen enters the body in a variety of compounds present in food, the most important being amino acids contained in dietary protein. Nitrogen leaves the body as urea, ammonia, and other products derived from amino acid metabolism (Figure 19.21). Free amino acids in the body are produced by hydrolysis of dietary protein by proteases activated from their zymogen form in the stomach and intestine, degradation of tissue proteins, and de novo synthesis. This amino acid pool is consumed in the synthesis of body protein, metabolized for energy, or its members used as precursors for other nitrogen-containing compounds. Free amino acids from digestion are taken up by intestinal cells via sodium-linked secondary active transport. Note that body protein is simultaneously degraded and resynthesized, a process known as protein turnover. The concentration of a cellular protein may be determined by regulation of its synthesis or degradation. The adenosine triphosphate (ATP)-dependent, cytosolic, selective ubiquitin– proteasome and ATP-independent, nonselective lysosomal acid hydrolases are the two major enzyme systems that are responsible for degrading proteins. Nitrogen cannot be stored, and amino acids in excess of the biosynthetic needs of the cell are quickly degraded. The first phase of catabolism involves the transfer of the α-amino groups through transamination by pyridoxalphosphate–dependent aminotransferases (transaminases), followed by oxidative deamination of glutamate by glutamate dehydrogenase, forming ammonia and the corresponding α-keto acids. A portion of the free ammonia is excreted in the urine, some of which is used in converting glutamate to glutamine for safe transport, but most is used in the hepatic synthesis of urea, which is quantitatively the most important route for disposing of nitrogen from the body. The two major causes of hyperammonemia (with its neurologic effects) are liver disease and inherited deficiencies of urea cycle enzymes such as X-linked ornithine transcarbamolyase.

Figure 19.21 Key concept map for nitrogen metabolism. GI = gastrointestinal; PEST = proline, glutamate, serine, threonine; NH₃ = ammonia.

Study Questions:
Choose the ONE best answer.

19.1 In the transamination reaction shown to the right, which of the following are the products X and Y?

A. Alanine, α-ketoglutarate

B. Asparate, α-ketoglutarate

C. Glutamate, alanine

D. Pyruvate, aspartate

Correct answer = B. Transamination reactions always have an amino acid and an α-keto acid as substrates. The products of the reaction are also an amino acid (corresponding to the α-keto substrate) and an α-keto acid (corresponding to the amino acid substrate). Three amino acid α-keto acid pairs commonly encountered in metabolism are: alanine/pyruvate, aspartate/oxaloacetate, and glutamate/α-ketoglutarate. In this question, glutamate is deaminated to form α-ketoglutarate, and oxaloacetate is aminated to form aspartate.

19.2 Which one of the following statements about amino acids and their metabolism is correct?

A. Free amino acids are taken into the enterocytes by a proton-linked transport system.

B. In healthy, fed individuals, the input to the amino acid pool exceeds the output.

C. Liver uses ammonia to buffer protons.

D. Muscle-derived glutamine is metabolized in liver and kidney tissue to ammonia plus a gluconeogenic precursor.

E. The first step in the catabolism of most amino acids is their oxidative deamination.

F. The toxic ammonia generated from the amide nitrogen of amino acids is transported through blood as arginine.

Correct answer = D. Glutamine, produced by the catabolism of branched-chain amino acids in muscle, is deamidated to ammonia plus glutamate. The glutamate is deaminated to ammonia plus α-ketoglutarate, which can be used for gluconeogenesis. Free amino acids are taken into enterocytes by a sodium-linked transport system. Healthy, fed individuals are in nitrogen balance, in which nitrogen input equals output. Liver converts ammonia to urea, and kidney uses ammonia to buffer protons. Amino acid catabolism begins with transamination that generates glutamate. The glutamate undergoes oxidative deamination. Toxic ammonia is transported as glutamine and alanine. Arginine is synthesized and hydrolyzed in the hepatic urea cycle.

For Questions 19.3– 19.5:

A female neonate did well until approximately age 24 hours, when she became lethargic. A sepsis workup proved negative. At 56 hours, she started showing focal seizure activity. The plasma ammonia level was found to be 887 µmol/l (normal 5–35 µmol/l). Quantitative plasma amino acid levels revealed a marked elevation of citrulline but not argininosuccinate.

19.3 Which one of the following enzymic activities is most likely to be deficient in this patient?

A.  Arginase

B. Argininosuccinate lyase

C. Argininosuccinate synthetase

D. Carbamoyl phosphate synthetase I

E. Ornithine transcarbamoylase

Correct answer = C. Genetic deficiencies of each of the five enzymes of the urea cycle, as well as deficiencies in N-acetyglutamate synthase, have been described. The accumulation of citrulline (but not argininosuccinate) in the plasma of this patient means that the enzyme required for the conversion of citrulline to argininosuccinate (argininosucinate synthetase) is defective, whereas the enzyme that cleaves argininosuccinate (argininosuccinate lyase) is functional.

19.4 Which one of the following would also be elevated in the blood of this patient?

A. Asparagine

B. Glutamine

C. Lysine

D. Urea

Correct answer = B. Deficiencies of the enzymes of the urea cycle result in the failure to synthesize urea and lead to hyperammonemia in the first few weeks after birth. Glutamine will also be elevated because it acts as a nontoxic storage and transport form of ammonia. Therefore, elevated glutamine always accompanies hyperammonemia. Asparagine and lysine do not serve this sequestering role. Urea would be decreased due to impaired activity of the urea cycle. [Note: Alanine would also be elevated in this patient.]

19.5 Why might supplementation with arginine be of benefit to this patient?

The arginine will be cleaved by arginase to urea and ornithine. Ornithine will be combined with carbamoyl phosphate by ornithine transcarbamoylase to form citrulline. Citrulline, containing one waste nitrogen, will be excreted.