Synthesis of acetylated human fetal hemoglobin
The in vitro biosynthesis of acetylated human fetal hemoglobin (Hb F 1) was investigated in umbilical cord blood from two normal newborns and in peripheral blood from five adults with different hematologic disorders accompanied by elevated levels of fetal hemoglobin. Hemoglobin biosynthesis was measured by [ 3H]leucine incorporation into hemoglobin components which were separated by chromatography on Bio Rex 70 cation-exchange resin. Following incubation of the samples for times ranging from 2 1/2 to 120 min, the specific radioactivities of newly synthesized Hb F(I) ranged from 2.1 to 3.9 times those corresponding values for the major fetal hemoglobin component (Hb F(o)) which is not acetylated. For both umbilical cord blood and the δβ/β + thalassemia blood, the ratio of specific radioactivities (Hb F(I)/Hb F(o) decreased significantly during the initial 20 min of incubation. Pulse-chase experiments revealed that the newly synthesized Hb F(I) was stable. These results indicate that in the circulating reticulocytes of the newborn and in those of adults with elevated fetal hemoglobin, the rate of biosynthesis of acetylated fetal hemoglobin is higher than that expected from the amounts of this hemoglobin existing in the blood. Radiolabeled blood cells from an adult patient with δβ/β + thalassemia were separated into five zones by centrifugation in a density gradient. Although each of the five zones contained similar amounts of cellular hemoglobin, about 84% of the protein 3H-radioactivity was present in the lightest (top) zone with the remainder distributed almost equally among the other zones. The Hb F(I)/Hb F(o) specific radioactivity ratio was about 1.5 in the lightest gradient zone and ranged from 11.1 to 18.4 in the four heaviest zones. These data suggest that much of the Hb F(I) synthesis occurred in a separate population of erythroid cells heavier than, and probably more mature than, the cells responsible for most of the hemoglobin synthesis. This result may occur because of increased activity of an acetyltransferase enzyme which could serve as a marker for erythroid cell maturation.