BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 15. THE PENTOSE PHOSPHATE PATHWAY AND GLUCONEOGENESIS

15.11. Glucose-6-phosphate dehydrogenase deficiency causes drug-induced hemolytic anemia

The Use of the antimalarial drug pamaquine began in 1926. While most patients tolerated the drug well, a small fraction developed acute pathological symptoms within a few days of starting Treatment. Their urine turned black, jaundice developed, and Blood Hemoglobin levels dropped sharply. In some cases, extensive destruction of red Blood Cells occurred, resulting in death.

The cause of this drug-induced hemolytic anemia was uncovered in 1956. The primary defect is a deficiency of glucose-6-phosphate dehydrogenase in red blood cells. The Pentose Phosphate Pathway is the sole source of NADPH in these cells; consequently, when glucose-6-phosphate dehydrogenase is deficient, The production of

NADPH is diminished. The principal role of NADPH in red blood cells is to reduce the disulfide form of Glutathione to its sulfhydryl form. This reaction is catalyzed by glutathione reductase.

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Reduced glutathione, a tripeptide with a free sulfhydryl group, serves as a sulfhydryl buffer that maintains the Cysteine residues of hemoglobin and other erythrocyte Proteins in the reduced state.

The ratio of reduced (G-SH) to oxidized (G-SS-G) glutathione normally stands at approximately 500. Reduced glutathione also plays a key role in detoxification processes by reacting with hydrogen peroxide and organic peroxides:

2Г-SH + R—О—ОН →Г- SS - Г - Н₂О + ROH.

Reduced glutathione is likewise essential for maintaining the normal Structure of red blood cells and preserving hemoglobin in the ferrous state. Cells with lowered levels of reduced glutathione exhibit an increased susceptibility to hemolysis, the exact cause of which remains unclear. It is possible that in the absence of reduced glutathione, drugs such as pamaquine alter the erythrocyte surface, rendering the cells more vulnerable to destruction and clearance by the Spleen. These drugs also increase The rate of toxic peroxide formation, which are normally eliminated via reaction with reduced glutathione.

Glucose-6-phosphate dehydrogenase deficiency is a relatively common condition. It is inherited as an X-linked trait. Heterozygous females possess two populations of red blood cells: one characterized by normal enzymatic activity, and the other by glucose-6-phosphate dehydrogenase deficiency. In most other Tissues, this enzyme is determined by a different Gene. The most common form of glucose-6-phosphate dehydrogenase deficiency (type A)—in which erythrocyte enzyme activity is reduced tenfold—is found in 11% of African Americans. Such a high frequency suggests that under certain environmental conditions, this deficiency may confer a selective advantage. Indeed, glucose-6-phosphate dehydrogenase deficiency in red blood cells apparently provides resistance to tropical malaria, because optimal growth of the parasite (Plasmodium falciparum) requires a normally functioning pentose phosphate pathway and reduced glutathione. Thus, glucose-6-phosphate deficiency and the sickle-Cell trait represent parallel MECHANISMS OF DEFENSE against malaria, which accounts for The high frequency of the corresponding gene in malaria-endemic Regions of the world.

The existence of glucose-6-phosphate dehydrogenase deficiency clearly demonstrates that atypical drug responses can have a genetic basis. Such inherited enzyme deficiencies may remain relatively harmless until specific drugs are administered. Here again we encounter the interplay between HEREDITY AND ENVIRONMENT in the Pathogenesis of disease. Galactosemia, hereditary fructose intolerance, phenylketonuria, and succinylcholine sensitivity serve as striking illustrations of this interaction.

15.12. Glutathione reductase transfers electrons from NADPH to oxidized glutathione with the participation of FAD

The regeneration of reduced glutathione is catalyzed by glutathione reductase, a dimer whose subunits have a molecular mass of 50 kDa. Electrons from NADPH are not transferred directly to the disulfide bond of oxidized glutathione. Instead, they pass from NADPH to a tightly bound flavin adenine dinucleotide (FAD), then to a disulfide bridge between two cysteine residues in the subunit, and finally to oxidized glutathione.

Each subunit consists of three Structural domains: an FAD-binding domain, an NADPH-binding domain, and an interface domain (Fig. 15.4). The domains binding FAD and NADP⁺ resemble each other and are similar to the nucleotide-binding domains found in Other dehydrogenases. FAD and NADP⁺ bind in an extended conformation, placing their isoalloxazine and nicotinamide rings in close proximity to one another (Fig. 15.4). Interestingly, the binding site for oxidized glutathione is formed by the FAD-binding domain of one subunit and the interface domain of the other subunit.

Fig. 15.4. Schematic representation of the domain structure of glutathione reductase. Each subunit of this dimeric enzyme consists of an NADP⁺ domain, an FAD domain, and an interface domain. Glutathione is bound to the FAD domain of one subunit and the interface domain of the other subunit

15.13. Glucose can be synthesized from noncarbohydrate precursors

We now turn to the Synthesis of glucose from noncarbohydrate precursors, a process known as Gluconeogenesis. This metabolic pathway is critically important because certain tissues, most notably the Brain, depend heavily on glucose as their primary fuel. The daily requirement of the adult brain

for glucose is approximately 120 g, which accounts for the lion's share of the body's total glucose demand (160 g). Body Fluids contain about 20 g of glucose, and roughly 190 g of glucose can be readily mobilized from Glycogen, its storage form (Ch. 16). Thus, "direct" glucose reserves are quite sufficient to meet physiological needs for a single day. During prolonged fasting, however, glucose must be synthesized from noncarbohydrate sources to maintain viability. Gluconeogenesis also plays a vital role during periods of intense physical exertion.

The primary non-carbohydrate precursors of glucose are lactate, Amino Acids, and glycerol. Lactate is produced in active Skeletal Muscle when the rate of Glycolysis exceeds that of The Tricarboxylic Acid Cycle and the Electron Transport Chain (Section 12.10). Amino acids originate from dietary proteins or, during starvation, from The breakdown of skeletal Muscle Proteins (Section 23.8). The Hydrolysis of triacylglycerols (Section 17.4) in adipocytes yields glycerol and Fatty acids. While glycerol serves as a glucose precursor, fatty acids cannot be converted into glucose in animal organisms for reasons that will be discussed later (Section 17.14). The gluconeogenic pathway drives The conversion of Pyruvate into glucose, with metabolites entering this pathway primarily at the levels of pyruvate, oxaloacetate, and dihydroxyacetone phosphate.

The Liver is the principal site of gluconeogenesis. This process also takes place in the renal cortex, though the total amount of glucose produced by the Kidneys is only about one-tenth of that produced by the liver, owing to the smaller mass of renal tissue. Very minor gluconeogenesis occurs in the brain, as well as in skeletal and cardiac Muscles. Most likely, hepatic and renal gluconeogenesis maintains a Blood Glucose Level sufficient to allow the brain and muscles to extract adequate amounts of glucose from the blood to meet their metabolic demands.