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The Role of Heme in the Synthesis and Assembly of Hemoglobin

From the ¹ From the Department of Medicine, Albert Einstein College of Medicine, and Bronx Municipal Hospital Center, New York, New York 10461

The incorporation of labeled amino acids into the and ß chains of hemoglobin has been studied in rabbit reticulocytes. Relative rates of synthesis were assessed by measurement of the specific activities of globin and of constituent and ß chains prepared from unpurified, ribosomefree hemolysates. The :ß specific activity ratio of approximately 1.0 indicated that labeled and ß chains were produced in nearly equal numbers. However, in hemoglobin which was purified from these hemolysates two features were noted that indirectly suggested the presence of a pool of chains and a smaller pool of ß chains: (a) the :ß specific activity ratio was less than 1.0, and (b) a loss of chain radioactivity and a smaller loss of ß chain radioactivity were observed following purification of the hemoglobin. More direct evidence for these previously formed pools has been found in (a) the radioactivity elution pattern observed on gel filtration of unpurified hemolysate, which revealed a radioactive protein "minor peak" of high specific activity, and (b) further analysis of this minor peak by ion exchange chromatography, peptide mapping, and spectrophotometry.

Incubation with added hemin (1 x 10^-4 m) was associated with (a) an increase in the specific activities of globin and of and ß chains, (b) the disappearance of the pool of ß chains, which probably existed as ß dimers, (c) persistence of a pool of chains during the periods of incubation used, and (d) an increase in the :ß specific activity ratio in purified hemoglobin. A model of hemoglobin biosynthesis is suggested which could account for the observed effects of added hemin in terms of (a) stimulation of the synthesis of and ß chains, (b) combination of hemin with ß dimers (globin) to form hemoglobin, and (c) promotion of the assembly of newly synthesized and ß chains, thus largely bypassing the pool of chains. In this manner, heme may be said to coordinate as well as stimulate the synthesis of hemoglobin.

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