Chapter Summary, Questions Answers - Nucleotide Metabolism

CHAPTER SUMMARY

Nucleotides are composed of a nitrogenous base (adenine = A, guanine = G, cytosine = C, uracil = U, and thymine = T); a pentose; and one, two, or three phosphate groups (Figure 22.24). A and G are purines, and C, U, and T are pyrimidines. If the sugar is ribose, the nucleotide is a ribonucleoside phosphate (for example, adenosine monophosphate [AMP]), and it can have several functions in the cell, including being a component of RNA. If the sugar is deoxyribose, the nucleotide is a deoxyribonucleoside phosphate (for example, deoxyAMP), and will be found almost exclusively as a component of DNA. The committed step in purine synthesis uses 5-phosphoribosyl-1- pyrophosphate ([PRPP], an “activated pentose” that provides the ribose-phosphate group for de novo purine and pyrimidine synthesis and salvage) and nitrogen from glutamine to produce phosphoribosyl amine. The enzyme is glutamine:PRPP amidotransferase and is inhibited by AMP and guanosine monophosphate ([GMP] the end products of the pathway) and activated by PRPP. Purine nucleotides can also be produced from preformed purine bases by using salvage reactions catalyzed by adenine phosphoribosyltransferase (APRT) and hypoxanthine-guanine phosphoribosyltransferase (HGPRT). A near-total deficiency of HGPRT causes Lesch-Nyhan syndrome, a severe, heritable form of hyperuricemia accompanied by compulsive self-mutilation. All deoxyribonucleotides are synthesized from ribonucleotides by the enzyme ribonucleotide reductase. This enzyme is highly regulated (for example, it is strongly inhibited by deoxyadenosine triphosphate [dATP], a compound that is overproduced in bone marrow cells in individuals with adenosine deaminase deficiency). This syndrome causes severe combined immunodeficiency disease. The end product of purine degradation is uric acid, a compound whose overproduction or undersecretion causes hyperuricemia that, if accompanied by the deposition of monosodium urate crystals in joints and soft tissues and an inflammatory response to those crystals, results in gout. The first step in pyrimidine synthesis, the production of carbamoyl phosphate by carbamoyl phosphate synthetase II, is t he regulated step in this pathway (it is inhibited by uridine triphosphate [UTP] and activated by PRPP). The UTP produced by this pathway can be converted to cytidine triphosphate. Deoxyuridine monophosphate can be converted t o deoxythymidine monophosphate using thymidylate synthase, an enzyme targeted by anticancer drugs such as 5-fluorouracil. The regeneration of tetrahydrofolate from dihydrofolate produced in the thymidylate synthase reaction re q ui re s dihydrofolate reductase, an enzyme targeted by the drug, methotrexate. Pyrimidine degradation results in soluble products.

Figure 22.24 Key concept map for nucleotide metabolism. THF = tetrahydrofolate; ADA= adenosine deaminase; XO = xanthine oxidase; TS = thymidylate synthase; RNR = ribonucleotide reductase; CPS II = carbamyl phosphate synthetase II; AMP = adenosine monophosphate; GMP = guanosine monophosphate; CMP = cytidine monophosphate; TMP = thymidine monophosphate; IMP = inosine monophosphate; d = deoxy; PPi = pyrophosphate.

Study Questions
Choose the ONE correct answer.

22.1 Azaserine, a drug with research applications, inhibits glutamine-dependent enzymes. Incorporation of which of the ring nitrogens (N) in the generic purine structure shown would most likely be affected by azaserine?

A. 1

B. 3

C. 7

D. 9

Correct answer = D. The nitrogen (N) at position 9 is supplied by glutamine in the first step of purine de novo synthesis, and its incorporation would be affected by azaserine. The N at position 1 is supplied by aspartate and at position 7 by glycine. The N at position 3 is also supplied by glutamine, but azaserine would have inhibited purine synthesis prior to this step.

22.2 A 42-year-old male patient undergoing radiation therapy for prostate cancer develops severe pain in the metatarsal phalangeal joint of his right big toe. Monosodium urate crystals are detected by polarized light microscopy in fluid obtained from this joint by arthrocentesis. This patient’s pain is directly caused by the overproduction of the end product of which of the following metabolic pathways?

A. De novo pyrimidine biosynthesis

B. Pyrimidine degradation

C. De novo purine biosynthesis

D. Purine salvage

E. Purine degradation

Correct answer = E. The patient’s pain is caused by gout, resulting from an inflammatory response to the crystallization of excess urate in his joints. Radiation therapy caused cell death, with degradation of nucleic acids and their constituent purines. Uric acid, the end product of purine degradation, is a relatively insoluble compound that can cause gout (and kidney stones). Pyrimidine metabolism is not associated with uric acid production. Overproduction of purines can indirectly result in hyperuricemia. Purine salvage decreases uric acid production.

22.3 Which one of the following enzymes of nucleotide metabolism is correctly paired with its pharmacologic inhibitor?

A. Dihydrofolate reductase—methotrexate

B. Inosine monophosphate dehydrogenase—hydroxyurea

C. Ribonucleotide reductase—5-fluorouracil

D.Thymidylate synthase—allopurinol

E. Xanthine oxidase—probenecid

Correct answer = A. Methotrexate interferes with folate metabolism by acting as a competitive inhibitor of the enzyme dihydrofolate reductase. This starves cells for tetrahydrofolate and makes them unable to synthesize purines and thymidine monophosphate. Inosine monophosphate dehydrogenase is inhibited by mycophenolic acid. Ribonucleotide reductase is inhibited by hydroxyurea. Thymidylate synthase is inhibited by 5-fluorouracil. Xanthine oxidase is inhibited by allopurinol. Probenecid increases renal excretion of urate but does not inhibit its production.

22.4 A 1-year-old female patient is lethargic, weak, and anemic. Her height and weight are low for her age. Her urine contains an elevated level of orotic acid. Activity of uridine monophosphate synthase is low. Administration of which of the following is most likely to alleviate her symptoms?

A. Adenine

B. Guanine

C. Hypoxanthine

D. Thymidine

E. Uridine

Correct answer = E. The elevated excretion of orotic acid and low activity of uridine monophosphate (UMP) synthase indicate that the patient has orotic aciduria, a rare genetic disorder affecting de novo pyrimidine synthesis. Deficiencies in one or both catalytic domains of UMP synthase leave the patient unable to synthesize pyrimidines. Uridine, a pyrimidine nucleoside, is a useful treatment because it bypasses the missing activities and can be salvaged to UMP, which can be converted to all the other pyrimidines. Although thymidine is a pyrimidine nucleoside, it cannot be converted to other pyrimidines. Hypoxanthine, guanine, and adenine are all purine bases and cannot be converted to pyrimidines.

22.5 What laboratory test would help in distinguishing an orotic aciduria caused by ornithine transcarbamylase deficiency from that caused by uridine monophosphate synthase deficiency?

Blood ammonia level would be expected to be elevated in ornithine transcarbamylase deficiency but not in uridine monophosphate synthase deficiency.