Clinical Information

Hemolytic anemia may be associated with deficiency of erythrocyte enzymes. Red blood cell (RBC) enzymes linked to hemolysis are those important in the energy generation of glycolysis or protection from oxidative stress such as the hexose monophosphate shunt.

The hexose monophosphate pathway depends primarily upon the glucose 6-phosphate dehydrogenase (G6PD) enzyme for the generation of reduced nicotinamide-adenine dinucleotide phosphate (NADPH) with 6-phosphogluconate dehydrogenase (6PGD) providing an additive effect. Both reactions require adequate levels of reduced glutathione (GSH). Because RBCs lack the citric acid cycle, this is an important source of NADPH, and a deficiency of G6PD or GSH results in the inability to neutralize oxidative insults. GSH is synthesized from amino acids by two enzymatic steps and is present in liver, kidney, brain, muscle, and RBCs. It plays widely versatile and important roles in the synthesis of proteins and DNA, the processing of medications and toxins, and other redox reactions.

Similar to G6PD deficiency, glutathione deficiency can have an episodic acute time course of hemolysis or jaundice, be triggered by fava beans, and cause neonatal hyperbilirubinemia. Five enzymes impact GSH availability and therefore are potential candidates for abnormalities leading to glutathione deficiency:

-Two enzymes, gamma-glutamylcysteine synthetase (GCLC) and glutathione synthetase (GSS), are required for GSH synthesis

-Two enzymes, glutathione reductase (GSR) and glutathione peroxidase (GPX1), are required for reduction-oxidation cycling of oxidized glutathione (GSSG) to reduced glutathione (GSH)

-A family of enzymes, glutathione S-transferases (GSTs), utilizes GSH in the detoxification and preparation of substances for excretion into the bile or urine

Enzyme deficiencies have been reported in all of these enzymes, albeit very rarely. The best characterized are GSS and GCLC deficiencies. GSS deficiency is associated with two clinical presentations; a mild form causing isolated chronic hemolytic anemia, and a more severe form marked by urinary excretion of 5-oxoproline, metabolic acidosis, hemolytic anemia, and central nervous system disorders (5-oxoprolinuria). GCLC deficiency is associated with moderate to severe chronic hemolytic anemia present from neonatal or early childhood, or compensated hemolysis with sporadic but recurrent anemia or jaundice. Some cases have shown learning disabilities, severe and progressive ataxia with myopathy and spinocerebellar degeneration. GSR deficiency has been confirmed in three siblings with favism (episodic hemolysis after fava bean ingestion) and cataracts in early adulthood, and an unrelated infant with marked neonatal hyperbilirubinemia. GSR activity can be decreased in riboflavin deficiency, but whether this results in hemolysis is not clear. Although patients have been reported with anemia in the context of decreased GPX1 activity and decreased GST activity was found in a person with hemolytic anemia, splenomegaly, hyperbilirubinemia, and cholelithiasis, neither have been characterized sufficiently as the definitive cause of hemolysis. All described cases have shown autosomal recessive inheritance pattern.

A deficiency of either of the synthetic enzymes, GCLC or GSS, results in GSH levels less than 25%, but many show a virtual absence of measurable GSH. Heterozygotes usually show normal GSH levels. Elevated concentrations of GSH are found in patients with myelofibrosis and in those with pyrimidine-5'-nucleotidase deficiency.