Enhanced carboxyl methylation of membrane-associated hemoglobin in human erythrocytes.
The alpha- and beta-chains of hemoglobin (Hb) are methylated in intact erythrocytes and in cellular extracts by a protein D-aspartate methyltransferase (EC 2.1.1.77) specific for D-aspartyl and L-isoaspartyl residues. During an 18-h incubation of intact erythrocytes with L-[methyl-3H]methionine, the subfraction of Hb molecules associated with the membrane becomes progressively enriched with methyl esters, reaching a specific activity 10-fold that of cytosolic Hb. The enhanced methylation of membrane Hb in intact cells appears not to result from its methylation at sites with inherently greater stability, since salt-extracted membrane Hb 3H-methyl esters and cytosolic Hb 3H-methyl esters are hydrolyzed at similar rates at pH 8.4 in vitro. Oxidative treatment of column-purified Hb with acetylphenylhydrazine produces an immediate 4-fold increase in its specific methyl-accepting activity coincident with the production of hemichrome forms known to possess a higher affinity for membrane binding sites. Together, the results suggest that the methyltransferase preferentially recognizes partially denatured Hb molecules which possess a higher affinity for membrane binding sites, similar to Hb forms observed in senescent erythrocytes.
O'Connor
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CM
Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545.
Yutzey
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KE
D004591:Electrophoresis, Polyacrylamide Gel; D004912:Erythrocytes / Q000378:metabolism*; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008745:Methylation; D010659:Phenylhydrazines / Q000494:pharmacology; D026601:Protein D-Aspartate-L-Isoaspartate Methyltransferase; D011496:Protein Methyltransferases / Q000378:metabolism
D006454:Hemoglobins; D010659:Phenylhydrazines; C006384:N(1)-acetylphenylhydrazine; D011496:Protein Methyltransferases; D026601:Protein D-Aspartate-L-Isoaspartate Methyltransferase