Deficiency of the red blood cell (RBC) enzyme glucose-6-phosphate dehydrogenase (G-6-PD) is the most common human enzyme defect, affecting nearly one-tenth of the world's population. In this inherited abnormality, the activity of this enzyme is markedly diminished. As a result, the RBC is unable to protect itself against oxidant stress. Normally, reduced glutathione is generated to protect the sulfhydryl groups of hemoglobin and the red cell membrane from oxidation. When the enzyme is deficient, the hemoglobin sulfhydryl groups become oxidized and the hemoglobin precipitates within the RBC, forming Heinz bodies that are readily removed from the circulation.
Clinically, the resulting hemolytic anemia varies greatly in severity. It is an X-linked disorder, and clinical manifestations are seen in male heterozygotes and female homozygotes. In the United States, 15 percent of African-American males have a mild form of this deficiency. Acute hemolytic crises occur that are incited by bacterial and viral infections, exposure to oxidant drugs, metabolic acidosis (such as diabetic ketoacidosis), and ingestion of fava beans in some patients. Within 1 to 3 days following oxidant stress, the patient can develop hemoglobinuria and the potential for vascular collapse. These hemolytic crises are generally well tolerated and self-limited because only the older RBCs will hemolyze. In more severe but less common variants of this disease, the patient can have severe chronic hemolytic anemia. The diagnosis of G-6-PD deficiency can be established by the demonstration of decreased enzyme activity through quantitative assay. There is no specific treatment for this disease. Prevention of hemolytic episodes is crucial, with prompt treatment of infections and the avoidance of oxidant drugs. The drugs most commonly associated with oxidant stress are sulfa drugs, antimalarials, pyridium, and nitrofurantoin. HIV-positive patients are commonly screened for this defect because of the common use of the sulfa drugs for the treatment and prophylaxis of Pneumocystis carinii pneumonia.
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