Glucose-6-phosphate dehydrogenase deficiency and methaemoglobin reduction in sickle cell anaemia - abstract only

ABSTRACT

Red cell glucose-6-phosphate dehydrogenase (G6PD) deficiency is inherited as an X-linked condition with full expression in males and homozygous females, who can be detected by assay of their red cell enzyme activity and by various simple tests. G6PD converts glucose-6-phosphate (GSP) to 6-phosphogluconate (6PG), thereby making hydrogen available for the reduction of the nucleotide nicotine-adenine dinucleotide phosphate (here referred to as TPN) to its reduced form (TPNH), which may be measured by its absorbance at 340 millimicrons. The TPNH formed as a result of G6PD activity is available for the reduction of oxidised glutathione (GSSG) and methaemoglobin and thus protects the red cell from the effects of oxidants and oxidant drugs. In erythrocytes with normal G6PD levels the pathway is strongly stimulated by the presence of small quantities of methylene blue. Brewer's test, which takes advantage of this stimulation, has been widely used to detect G6PD deficiency. It was reported recently from West Africa that there is an association between sickling and G6PD deficiency; Brewer's test indicates that, in that area, some 28 percentof sickle cell traits (Hb AS) and 48 percentof sickle cell anaemias (Hb SS) are deficient, whilst only 14 percentof controls (Hb AA) are deficient. Such an association requires the existence of strong forces of natural selection in favour of the double genetic defect and the workers in West Africa suggest that G6PD-deficient sickle cell anaemia patients have such an advantage. In Jamaica, 13 percentof males are G6PD-deficient and, as sickle cell anaemia is common, it was decided to investigate the incidence of the deficiency in sickle cell patients. Three methods for detecting G6PD deficiency were employed, viz.(1) Sass' method, involving the reduction of methylene blue, (2) the G6PD assay by TPN reduction, (3) the nitrite-free reduction of methaemoglobin in the presence of methylene blue (modified Brewer's test). The deficiency in males and homozygous females was detected by the first two methods. As expected, faster methaemoglobin reduction rates and higher G6PD assay levels were found for sickle cell anaemia blood than for normal blood. With the modified Brewer's test, the initial methaemoglobin reduction rate was found to be independent of the haemoglobin type, but dependent on the G6PD assay levels. The reaction showed approximately zero-order kinetics when sufficient G6PD was present but tended to first-order behaviour when G6PD was low. Blood from some females had methaemoglobin reduction behaviour with a tendency towards first-order kinetics but a rate and G6PD level within the normal range. These are probably heterozygous, with uneven distribution of the enzyme in their red cell. They can be detected with the Sass m ethylene blue test if the dividing line between normal and deficient blood is lowered to allow for the effect of increased G6PD in sickle cell anaemia generally. By these tests, 7 1/2 per cent of males and 12 per cent of females with sickle cell anemia are relatively G6PD deficient, which is a lower incidence than expected and does not support the hypothesis that there is a natural selection of these two mutant genes. (AU)