Utilization of red-cell FAD by methaemoglobin reductases at the expense of glutathione reductase in heterozygous beta-thalassaemia.

FAD-dependent methaemoglobin reductases (MHR) were studied in red cells in heterozygous beta-thalassaemia to investigate how they related to low FAD-dependent glutathione reductase (GR). In contrast to GR, MHR activities were usually normal or increased. In particular, whether expressed in relation to haemoglobin or number of red cells, NADPH-MHR activity was markedly increased in most subjects, probably being a response to increased oxidative stress. Oral riboflavin had no effect on MHR activities, indicating saturation with FAD even though GR was deficient. A strong correlation between percent stimulation of GR by FAD and NADPH-MHR activity indicates that FAD is utilized by MHR at the expense of GR. This could be an important influence on GR in heterozygous beta-thalassaemia. Thus, the low activity resulting from an inherited deficiency of FAD is decreased further.