Mechanism of the tpnh-linked reduction of methemoglobin by methylene blue

Reduction and uptake of methylene blue by human erythrocytes

“…Whereas activation of the pentose phosphate cycle by NADP ϩ generated by NADPH oxidation is clearly required to sustain MB ϩ reduction (5, 38), a role for NADH has been suggested only indirectly in studies using glycolytic inhibitors in erythrocytes (37) and in pulmonary artery endothelial cells (26). Our results, showing nearly complete oxidation of NADH by low concentrations of MB ϩ (Fig.…”

“…The presence of MB ϩ in erythrocytes in this and a previous study (32), as well as in endothelial cells (27), shows that intracellular MBH undergoes oxidation with trapping of MB ϩ in the cells. Oxidation of MBH by Fe 3ϩ in hemoglobin (37) and in Fe 3ϩ -containing enzymes is responsible for the beneficial effects of MB ϩ , such as reduction of methemoglobin and activation of guanylate or nitric oxide synthases. If these pathways are saturated, however, MBH will react with molecular oxygen to generate superoxide and other reactive oxygen species (ROS) that can be damaging to the cells (25,39) (Fig.…”

“…MBH that has entered or has been generated within cells can be oxidized by molecular oxygen (9,25), but its major fate in cells is to reduce Fe 3ϩ in prosthetic groups of numerous cellular proteins and enzymes (25). In erythrocytes, the major acceptor for electrons from MBH is Fe 3ϩ in (met)hemoglobin (37), whereas in other cells, MBH donates electrons to heme groups of cytochrome c (25), guanylate cyclase (8), and nitric oxide synthase (24). Although MB ϩ accumulates in certain as yet unidentified organelles in endothelial cells (27), whether MB ϩ or MBH accumulates in erythrocytes is unclear.…”

Anaesthesia and glucose‐6‐phosphate dehydrogenase deficiency

“…40 In the presence of G6PD deficiency the blocked hexose monophosphate shunt does not regenerate NADPH and hence methylene blue is not effective in treating methaemoglobinaemia. In addition, methylene blue is an oxidant which has been incriminated as a trigger of haernolysis in a G6PD-deficient i n d i~i d u a l .~' Thus, apart from being ineffectivc.…”

Defective molecular variants of glucose-6-phosphate dehydrogenase and methaemoglobin reductase

“…The mechanism of the spectacular action of methylene blue-which may be administered orally or intravenously-is explained by the intervention of an NADPH-dependent diaphorase (Sass, Caruso, and Axelrod, 1969). The latter enzyme, whose physiological role remains obscure, is an entirely different protein.…”