Glucose-6-phosphate dehydrogenase (G6PDH) is expressed in all cells and is responsible for the first reaction of the pentose phosphate pathway in which glucose-6-phosphate is oxidized to 6-phosphogluconolactone with concomitant production of NADPH.
Oxidative reactions of the pentose phosphate pathwy. G6PDH: glucose-6-phopsphate dehydrogenase. PGLS: 6-phosphogluconolactonase. PGD: 6-phosphogluconate dehydrogenase.
The NADPH produced via the pentose phosphate pathway is required for a variety of reductive biosynthetic reactions as well as for the regeneration of the reduced form of glutathione (GSH). GSH is essential for the detoxification of hydrogen peroxide (H2O2) and therefore, cellular defense against the oxidizing effects of H2O2 is absolutely dependent upon the generation of NADPH. GSH converts H2O2 to H2O via the action of glutathione peroxidase and requires one mole of NADPH per mole of H2O2. The critical need for production of NADPH via the pentose phosphate pathway is especially true in red blood cells because there are no other NADPH-producing reactions in these cells and they are extremely sensitive to oxidative damage.
Pathway for reactive oxygen species (ROS) removal in erythrocytes. Formation of the reactive oxygen species (ROS), H2O2, can occur spontaneously from O2 or is generated via superoxide dismutase (SOD) action on superoxide anion produced as a result of O2 oxidation of the ferrous iron (Fe2+) to ferric iron (Fe3+) in hemoglobin. The consequence of the latter reaction is the formation of methemoglobin Since hemoglobin is a heterotetramer, and each subunit contains Fe2+ iron in its heme there is the potential for multiple Fe3+ irons to be present in methemoglobin. The Fe3+ form of iron does not bind O2, however, the presence of at least one Fe3+ in the hemoglobin tetramer results in enhanced binding of the O2 to the remaining Fe2+ irons causing reduced delivery of the O2 to the tissues with potential for cyanosis. Normal daily levels of methemoglobin range from 0.5%–3%. The ferric iron in methemoglobin is reduced to ferrous via the action of the NADH-requiring enzyme, methemoglobin reductase (cytochrome b5 reductase 3: CYB5R3). The NADPH produced in the PPP, and the antioxidant glutathione (GSH), are both necessary for the continual removal of ROS from within the erythrocyte (red blood cell, RBC). The enzyme, glutathione peroxidase (GPx) utilizes reduced glutathione (GSH) as the electron donor in the process of reducing H2O2 to H2O while simultaneously generating oxidized glutathione (GS-SG). GPx is one of several selenocysteine-containing red-ox enzymes. In order for continued use of GSH, the oxidized molecule needs to be reduced via the action of the NADPH-requiring enzyme, glutathione reductase.
The glucose-6-phosphate dehydrogenase gene (symbol G6PD) is located on the X chromosome (Xq28) about 1 Mb (million base pairs) from the telomeric end. The gene spans 18 kb and is composed of 14 exons that generate two alternatively spliced mRNAs. These two mRNAs encode G6PDH isoform a (545 amino acids) and isoform b (515 amino acids). The first exon of the gene is a non-coding exon with the translational initiation codon present in exon 2. The 545 amino acid isoform a protein is inactive but post-translational processing results in a 515 amino acid functional protein containing an acetylated alanine residue at the N-terminus. Biologically active G6PDH is functional as either a homodimer or a homotetramer and both forms co-exist in equal proportions at neutral pH.
Deficiencies in G6PDH are the most commonly inherited enzyme deficiencies (enzymopathies) world wide with estimates of greater than 400 million affected individuals. The incidence of deficiency approaches 25% of the population of persons of Mediterranean, tropical African, and tropical and subtropical Asian descent. In fact, the incidence of G6PDH deficiency is so high in some populations that the occurrence of homozygous females is not at all rare as would normally be expected for an X-linked disorder. The inheritance of G6PDH deficiency is clinically classified as an X-linked recessive disorder, however, because heterozygous females can develop hemolytic episodes the disorder is not truly recessive in the Mendelian sense. The phenomenon of symptom manifesting heterozygous females is explained by the co-existence of populations of G6PDH positive and negative cells in the same female as a result of X-chromosome inactivation.
Deficiency in G6PDH represents one of the most genetically heterogeneous disorders. There are over 400 different variants of G6PDH defined by their diverse biochemical characteristics. To date a total of 130 different point mutations have been identified in the G6PD gene. Surprisingly, only five in-frame deletions have been identified and no large deletions or insertions have been found. This fact is likely explained by the observation that in mouse models of G6PDH deficiency, complete loss of enzyme activity is associated with embryonic lethality.
G6PDH deficiency cannot be classified by a single mutation but is manifest as a consequence of numerous structural allelic mutants. In addition, G6PDH deficiencies are also classifiable by a variety of physicochemical parameters including chromatographic properties, thermostability, pH dependence, KM for glucose-6-phosphate, and KM for NADP. G6PD gene variants are currently divided into five classifications dependent upon enzyme activity and clinical manifestations. In addition, the deficiencies are classified as to whether or not they are sporadic or polymorphic.
The five classifications are:
Class 1: enzyme deficiency with chronic nonspherocytic hemolytic anemia
Class 2: severe enzyme deficiency, less than 10% of normal activity
Class 3: moderate to mild enzyme deficiency, 10-60% of normal activity
Class 4: very mild or no enzyme deficiency, at least 60% of normal activity
Class 5: increased enzyme activity.
Mutations in the G6PD gene that result in nonspherocytic hemolytic anemia have all been found to cluster near the carboxy terminal end of the protein, whereas, the mutations that result in clinically mild symptoms are clustered near the amino terminal end of the protein. Nearly all G6PDH mutations are single nucleotide missense mutations resulting in the substitution of a single amino acid. In most instances of G6PDH mutation resulting in disease it is due to protein instability as a result of the amino acid substitution.
The vast majority of individuals that harbor a mutant form of the G6PD gene go their entire lives without knowing they carry a mutation. The acute hemolysis that is the only common clinical manifestation in G6PD mutations can rapidly be compensated for and thus may remain undetectable. The most common symptoms of G6PDH deficiency are neonatal jaundice and acute hemolytic anemia. The acute hemolytic attacks are the result of reduced antioxidant capacity of red blood cells with the result being membrane lipid peroxidation and consequent cell lysis. The accumulation of reactive oxygen species in G6PDH deficiency also leads to aberrant oxidation of the cysteine sulhydryl groups in the globin proteins in hemoglobin. This results in significant cross-linking of hemoglobin tetramers which can be visualized microscopically. The cross-linked hemoglobin forms what is referred to histologically as Heinz bodies. The presence of Heinz bodies is diagnostic for G6PDH deficiency versus erythrocyte pyruvate kinase deficiency which is not associated with the formation of these inclusion bodies.
In addition, red cell deficiency in G6PDH is the basis of favism, primaquine sensitivity (a drug used in the treatment of malaria similar to quinine) and some other drug-sensitive hemolytic anemias (e.g. due to sulfonamides used to treat bacterial infections). Favism is a term relating to a hemolytic anemia that results from the ingestion of the fava bean (Vicia faba) also known as broad beans. In the case of primaquine-induced hemolytic anemia, it was this clinical manifestation, particularly in Blacks, that actually led to the discovery of G6PDH deficiencies. Aside from favism and drug-induction of hemolytic anemia, infection is the most common cause of hemolysis in G6PDH deficient individuals. With respect to favism, the occurrence of acute hemolysis after ingestion of the fava bean was recognized as far back as the time of the Greek mathematician, Pythagoras. Analysis of extracts from fava beans demonstrated that the toxic compounds are the pyrimidine aglycones, divicine and isouramil, in combination with ascorbic acid.