Definition: a group of abnormal Hb's in which a single amino acid substitution favors the formation of methemoglobin in spite of normal quantities of methemoglobin reductase. Strictly speaking, Hb's M are Hb's with mutations at the proximal or distal histidyl residues. Other Hb's M tend to favor the Fe(III) state. Heterozygotes have congenital methemoglobinemia; the homozygous state of these genes is unknown and is presumably lethal. Specific types include: Hb MIwate, a87His?Tyr (a chain, position 87, histidine replaced by tyrosine); Hb M Hyde Park, ß92His?Tyr; Hb MBoston, a58His?Tyr; Hb MSaskatoon, ß63His?Tyr; Hb MMilwaukee-1, ß67Val?Glu.
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