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Reduction of methaemoglobin to haemoglobin via the primary pathway, catalysed by cytochrome-B5 reductase (below), in contrast to the methylene blue-dependent alternative pathway via NADPH-methaemoglobin reductase (above). Activation of the alternative pathway requires methylene blue as a cofactor, in the presence of adequate glucose-6-phosphate dehydrogenase (G6PD). Figure created by authors.
