G6PD is an enzyme widely distributed in cells of all organs. In erythrocytes, it catalyzes the first reaction of the pentose phosphate pathway, which performs an essential function in oxidoreductive balance, as it is the only source of hydride ions (one proton and two electrons) for the reduction of NADP+ to NADPH and thus for the reduction of glutathione and the elimination of hydrogen peroxide (H2O2) (Fig. 1). The gene encoding the enzyme is in the telomeric region of the long arm of the X chromosome. More than 300 variants of the enzyme are known, of which four are the most common, with different properties (Table 1). Since the hereditary transmission is linked to the X chromosome, males are hemizygous for the relevant gene, so in them, there are two possibilities of expression, normal or deficient G6PD. In females, since there are two X chromosomes, there are three possibilities of expression: normal, heterozygous, and homozygous (in populations with a high frequency of deficiency). Heterozygous females have mosaicism due to random inactivation of the X chromosome. Therefore, there can also be conditions of complete inactivation and deficit similar to that of the male, but these conditions are rarer as usually, nonheterozygous females do not manifest severe deficits as the male. G6PD variants are classified according to their activity values compared to those of the normal con textual population. Specifically, class I variants cause con genital nonspherocytic hemolytic anemia (activity <10% of normal); class II variants exhibit severe deficiency but not hemolytic anemia (activity <10% of normal); class III variants exhibit mild to moderate deficiency (activity 10–60% of normal); class IV variants exhibit very mild deficiency (activity 60–100% of normal); and class V variants are very rare and exhibit increased enzyme activity compared to normal, over 150%. Although many individuals with deficiency are asymptomatic throughout life, acute hemolytic crises can be observed unpredictably because of oxidative stress, which can be caused by drugs, infections, ingestion, or simple con tact with broad beans, especially in Mediterranean forms. The precise mechanism by which the crises are triggered is not known, and they appear to be due to increased sensitivity to oxidative damage.
Fig1. Metabolism of the red blood cell with the cycle based on G6PD, 6PGD and glutathione oxidation-reduction by NADP+/NADPH and glutathione-peroxidase/glutathione-reductase highlighted, with a final reduction of H2O2 to H2O. (Copyright EDISES 2021. Reproduced with permission)
Table1. Common variants of glucose-6-phosphate dehydrogenase
During a crisis, fatigue, back pain, anemia, jaundice with increased unconjugated bilirubin, reticulocytosis, increased LDH, the appearance of Heinz bodies, hemoglobinemia, hemoglobinuria, and other symptoms common to acute hemolytic manifestations are generally observed. A G6PD deficiency may be revealed at birth in the form of neonatal hemolytic jaundice, which should be considered in the differential diagnosis. To prevent such occurrences, in many countries, screening for G6PD deficiency is part of neonatal prevention programs. Seizures are more characteristic of GdMed enzyme variants, while chronic hemolysis with chronic jaundice and cholelithiasis are observed more in GdA variants. In Africa and the Mediterranean, the selection of the deficient gene has been operated by malaria, particularly the one due to Plasmodium falciparum, as the parasite entering the erythrocytes of a subject with deficiency is in an unfavorable metabolic environment, with limited ability to replicate.
The laboratory diagnosis of G6PD deficiency is based on the evaluation of enzymatic activity by quantitative spectrophotometric analysis of NADPH produced by NADP+ in a physiological pH system having as substrate glucose-6- phosphate to which is added hemolysate of the sample to be examined. There are standardized commercial methods based on a procedure initially recommended by the World Health Organization (WHO). Screening tests are also available, which can make use of samples collected on bibulous paper. Since the activity of the enzyme in reticulocytes is much higher than the average of red blood cells (the half-life of the enzyme is 62 days), it is necessary to interpret very carefully the results obtained from subjects with reticulocytosis, as can happen at the end of a hemolytic crisis because the values obtained could simulate a normal condition.
