Harpers

614 / CHAPTER 52Mutations in the gene for G6PDDecreased activity of G6PDDecreased levels of NADPHDecreased regeneration of GSH from GSSG byglutathione reductase (which uses NADPH)Oxidation, due to decreased levels of GSH andincreased levels of intracellular oxidants (eg, O 2– • ),of SH groups of Hb (forming Heinz bodies), and ofmembrane proteins, altering membrane structureand increasing susceptibility to ingestionby macrophages (peroxidative damage to lipidsin the membrane also possible)HemolysisFigure 52–2. Summary of probable events causinghemolytic anemia due to deficiency of the activity ofglucose-6-phosphate dehydrogenase (G6PD) (MIM305900).back to the Fe 2+ state by the action of reduced cytochromeb 5 :3+ 2+Hb - Fe + Cyt b5red → Hb - Fe + Cyt b5 oxReduced cytochrome b 5 is then regenerated by the actionof cytochrome b 5 reductase:Cyt b5 ox + NADH → Cyt b5 red + NADMethemoglobinemia Is Inheritedor AcquiredMethemoglobinemia can be classified as either inheritedor acquired by ingestion of certain drugs and chemicals.Neither type occurs frequently, but physicians must beaware of them. The inherited form is usually due to deficientactivity of methemoglobin reductase, transmittedin an autosomal recessive manner. Certain abnormal hemoglobins(Hb M) are also rare causes of methemoglobinemia.In Hb M, mutation changes the amino acidresidue to which heme is attached, thus altering its affinityfor oxygen and favoring its oxidation. Ingestion ofcertain drugs (eg, sulfonamides) or chemicals (eg, aniline)can cause acquired methemoglobinemia. Cyanosis(bluish discoloration of the skin and mucous mem-branes due to increased amounts of deoxygenated hemoglobinin arterial blood, or in this case due to increasedamounts of methemoglobin) is usually the presentingsign in both types and is evident when over 10%of hemoglobin is in the “met” form. Diagnosis is madeby spectroscopic analysis of blood, which reveals thecharacteristic absorption spectrum of methemoglobin.Additionally, a sample of blood containing methemoglobincannot be fully reoxygenated by flushing oxygenthrough it, whereas normal deoxygenated blood can.Electrophoresis can be used to confirm the presence ofan abnormal hemoglobin. Ingestion of methylene blueor ascorbic acid (reducing agents) is used to treat mildmethemoglobinemia due to enzyme deficiency. Acutemassive methemoglobinemia (due to ingestion of chemicals)should be treated by intravenous injection ofmethylene blue.MORE IS KNOWN ABOUT THE MEMBRANEOF THE HUMAN RED BLOOD CELL THANABOUT THE SURFACE MEMBRANE OFANY OTHER HUMAN CELLA variety of biochemical approaches have been used tostudy the membrane of the red blood cell. These includeanalysis of membrane proteins by SDS-PAGE,the use of specific enzymes (proteinases, glycosidases,and others) to determine the location of proteins andglycoproteins in the membrane, and various techniquesto study both the lipid composition and disposition ofindividual lipids. Morphologic (eg, electron microscopy,freeze-fracture electron microscopy) and othertechniques (eg, use of antibodies to specific components)have also been widely used. When red bloodcells are lysed under specific conditions, their membraneswill reseal in their original orientation to formghosts (right-side-out ghosts). By altering the conditions,ghosts can also be made to reseal with their cytosolicaspect exposed on the exterior (inside-outghosts). Both types of ghosts have been useful in analyzingthe disposition of specific proteins and lipids inthe membrane. In recent years, cDNAs for many proteinsof this membrane have become available, permittingthe deduction of their amino sequences and domains.All in all, more is known about the membraneof the red blood cell than about any other membrane ofhuman cells (Table 52–5).Analysis by SDS-PAGE Resolvesthe Proteins of the Membraneof the Red Blood CellWhen the membranes of red blood cells are analyzedby SDS-PAGE, about ten major proteins are resolved