Chapter Summary, Questions Answers - Pentose Phosphate Pathway and Nicotinamide Adenine Dinucleotide Phosphate

CHAPTER SUMMARY

The pentose phosphate pathway includes two irreversible oxidative reactions followed by a series of reversible sugar–phosphate interconversions (Figure 13.14). No ATP is directly consumed or produced in the cycle. The reduced nicotinamide adenine dinucleotide phosphate (NADPH)-producing oxidative portion of the pentose phosphate pathway is important in providing reducing equivalents for reductive biosynthesis and detoxification reactions. In this part of the pathway, glucose 6-phosphate is irreversibly converted to ribulose 5-phosphate, and two NADPH are produced. The regulated step is catalyzed by glucose 6-phosphate dehydrogenase (G6PD), which is strongly inhibited by NADPH. Reversible nonoxidative reactions interconvert sugars. This part of the pathway is the source of ribose 5-phosphate, required for nucleotide and nucleic acid synthesis. Because the reactions are reversible, they can be entered from fructose 6-phosphate and glyceraldehyde 3-phosphate (glycolytic intermediates) if ribose is needed and G6PD is inhibited. NADPH is a source of reducing equivalents in reductive biosynthesis, such as the production of fatty acids in liver, adipose tissue, and the mammary gland, and steroid hormones in the placenta, ovaries, testes, and adrenal cortex. It is also required by red blood cells (RBCs) for the reduction of hydrogen peroxide, providing the reducing equivalents required by glutathione (GSH). GSH is used by glutathione peroxidase to reduce peroxide to water. The oxidized glutathione (GSSH) produced is reduced by glutathione reductase, using NADPH as the source of electrons. NADPH provides reducing equivalents for the mitochondrial cytochrome P450 monooxygenase system, which is used in steroid hormone synthesis in steroidogenic tissue, bile acid synthesis in liver, and vitamin D activation in the liver and kidney. The microsomal system uses NADPH to detoxify foreign compounds (xenobiotics), such as drugs and a variety of pollutants. NADPH provides the reducing equivalents for phagocytes in the process of eliminating invading microorganisms. NADPH oxidase uses molecular oxygen and electrons from NADPH to produce superoxide radicals, which, in turn, can be converted to peroxide by superoxide dismutase. Myeloperoxidase catalyzes the formation of bactericidal hypochlorous acid from peroxide and chloride ions. Rare genetic defects in NADPH oxidase cause chronic granulomatous disease characterized by severe, persistent, infections and formation of granulomas. NADPH is required for the synthesis of nitric oxide (NO), an important free radical gas that causes vasodilation by relaxing vascular smooth muscle, acts as a neurotransmitter, prevents platelet aggregation, and helps mediate macrophage bactericidal activity. NO is made from arginine and O₂ by three different NADPH-dependent NO synthases (NOS). The endothelial (eNOS), and neuronal (nNOS) isozymes constantly produce very low levels of NO for vasodilation and neurotransmission, respectively. The inducible isozyme ( iNOS) produces large amounts of NO for defense against pathogens. G6PD deficiency impairs the ability of the cell to form the NADPH that is essential for the maintenance of the GSH pool.

The cells most affected are the RBCs because they do not have additional sources of NADPH. G6PD deficiency is an X-linked disease characterized by hemolytic anemia caused by the production of free radicals and peroxides following administration of oxidant drugs, ingestion of fava beans, or severe infections. The extent of the anemia depends on the amount of residual enzyme. Class I variants, the most severe (and least common), are associated with chronic nonspherocytic hemolytic anemia. Babies with G6PD deficiency may experience neonatal jaundice.

Figure 13.14 Key concept map for the pentose phosphate pathway and nicotinamide adenine dinucleotide phosphate (NADPH).

Study Questions

Choose the ONE best answer.

13.1 In preparation for a trip to an area of India where chloroquine-resistant malaria is endemic, a young man is given primaquine prophylactically. Soon thereafter, he develops a hemolytic condition due to a deficiency in glucose 6-phosphate dehydrogenase. A less-than-normal level of which of the following is a consequence of the enzyme deficiency and the underlying cause of the hemolysis?

A. Glucose 6-phosphate

B. Oxidized form of nicotinamide adenine dinucleotide

C. Reduced form of glutathione

D. Ribose 5-phosphate

Correct answer = C. Glutathione (GSH) is essential for red cell integrity and is maintained in its reduced (functional) form by nicotinamide adenine dinucleotide phosphate (NADPH)-dependent glutathione reductase. The NADPH is generated by the oxidative portion of the pentose phosphate pathway. Individuals with a deficiency of the initiating and regulated enzyme of this pathway, glucose 6-phosphate dehydrogenase (G6PD), have a decreased ability to generate NADPH and, therefore, a decreased ability to keep GSH functional. When treated with an oxidant drug such as primaquine, some patients with G6PD deficiency develop a hemolytic anemia. Primaquine does not affect glucose 6-phosphate levels. Nicotinamide adenine dinucleotide is neither produced by the pentose phosphate pathway nor used as a coenzyme by GSH reductase.

13.2 Septic shock, a state of acute circulatory failure characterized by persistent arterial hypotension (low blood pressure) and inadequate organ perfusion refractory to fluid resuscitation, results from a severe inflammatory response to bacterial infection. It has a high mortality rate and is associated with changes in the level of nitric oxide. Which statement concerning septic shock is most likely correct?

A. Activation of endothelial nitric oxide synthase causes an increase in nitric oxide.

B. High mortality is the result of the long half-life of nitric oxide.

C. Lysine, the nitrogen source for nitric oxide synthesis, is deaminated by bacteria.

D. Overproduction of nitric oxide by a calcium-independent enzyme is the cause of the hypotension.

Correct answer = D. Overproduction of short-lived (not long-lived) nitric oxide (NO) by calcium-independent, inducible nitric oxide synthase (iNOS) results in excessive vasodilation leading to hypotension. NOS uses arginine, not lysine, as the source of the nitrogen. The endothelial enzyme (eNOS) is constitutive and produces low levels of NO at a consistent rate.

13.3 An individual who has recently been prescribed a drug (atorvastatin) to lower cholesterol levels is advised to limit consumption of grapefruit juice, because high intake of the juice reportedly results in an increased level of the drug in the blood, increasing the risk of side effects. Atorvastatin is a substrate for the cytochrome P450 enzyme CYP3A4, and grapefruit juice inhibits the enzyme. Which statement concerning P450 enzymes is most likely correct?

A. They accept electrons from reduced nicotinamide adenine dinucleotide (NADH).

B. They catalyze the hydroxylation of hydrophobic molecules.

C. They differ from nitric oxide synthase in that they contain heme.

D. They function in association with an oxidase.

Correct answer = B. The P450 enzymes hydroxylate hydrophobic compounds, making them more water soluble. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) from the pentose phosphate pathway is the electron donor. The electrons are first transferred to the coenzymes of cytochrome P450 reductase and then to the P450 enzyme. Both the P450 enzymes and the nitric oxide synthase enzymes contain heme.

13.4 In male patients who are hemizygous for X-linked glucose 6-phosphate dehydrogenase deficiency, pathophysiologic consequences are more apparent in red blood cells (RBC) than in other cells such as in the liver. Which one of the following provides the most reasonable explanation for this different response?

A. Excess glucose 6-phosphate in the liver, but not in RBC, can be channeled to glycogen, thereby averting cellular damage.

B. Liver cells, in contrast to RBC, have alternative mechanisms for supplying the reduced nicotinamide adenine dinucleotide phosphate required for maintaining cell integrity.

C. Because RBC do not have mitochondria, production of ATP required to maintain cell integrity depends exclusively on the shunting of glucose 6-phosphate to the pentose phosphate pathway.

D. In RBC, in contrast to liver cells, glucose 6-phosphatase activity decreases the level of glucose 6-phosphate, resulting in cell damage.

Correct answer = B. Cellular damage is directly related to decreased ability of the cell to regenerate reduced glutathione, for which large amounts of reduced nicotinamide adenine dinucleotide phosphate (NADPH) are needed, and red blood cells (RBCs) have no means other than the pentose phosphate pathway of generating NADPH. It is decreased product (NADPH), not increased substrate (glucose 6-phosphate), that is the problem. RBCs do not have glucose 6-phosphatase. The pentose phosphate pathway does not generate ATP.

13.5 An essential prosthetic group for several enzymes of metabolism is derived from the vitamin thiamine. Measurement of the activity of what enzyme in red blood cells could be used to determine thiamine status in the body?

Red blood cells do not have mitochondria and, so, do not contain mitochondrial thiamine pyrophosphate (TPP)-requiring enzymes such as pyruvate dehydrogenase. However, they do contain the cytosolic TPP-requiring transketolase, whose activity can be used to assess thiamine status.