IRON SUPPLEMENTATION IN VITRO AND THE STATE OF AGGREGATION AND FUNCTION OF RETICULOCYTE RIBOSOMES IN HEMOGLOBIN SYNTHESIS

Regulation of protein synthesis by the heme-regulated eIF2alpha kinase: relevance to anemias

During erythroid differentiation and maturation, it is critical that the 3 components of hemoglobin, alpha-globin, beta-globin, and heme, are made in proper stoichiometry to form stable hemoglobin. Heme-regulated translation mediated by the heme-regulated inhibitor kinase (HRI) provides one major mechanism that ensures balanced synthesis of globins and heme. HRI phosphorylates the alpha-subunit of eukaryotic translational initiation factor 2 (eLF2alpha) in heme deficiency, thereby inhibiting protein synthesis globally. In this manner, HRI serves as a feedback inhibitor of globin synthesis by sensing the intracellular concentration of heme through its heme-binding domains. HRI is essential not only for the translational regulation of globins, but also for the survival of erythroid precursors in iron deficiency. Recently, the protective function of HRI has also been demonstrated in murine models of erythropoietic protoporphyria and beta-thalassemia. In these 3 anemias, HRI is essential in determining red blood cell size, number, and hemoglobin content per cell. Translational regulation by HRI is critical to reduce excess synthesis of globin proteins or heme under nonoptimal disease states, and thus reduces the severity of these diseases. The protective role of HRI may be more common among red cell disorders.

Iron requirement for isolated rat liver mitochondrial protein synthesis

Isolated rat liver mitochondrial protein synthesis was severly inhibited by alpha, alpha-dipyridyl (a ferrous iron-chelating agent), chloramphenicol and hemin (10(-7) M or greater). In contrast, gamma, gamma-dipyridyl (a non-iron-chelating analogue of alpha, alpha-dipyridyl), cycloheximide and lower concentrations of hemin were non-inhibitory. The inhibitory action of alpha, alpha-dipyridyl was reversed by addition of Fe(NH4)2(SO4)2 while ZnCl2, CuCl2 and CoCl2 were ineffective. Hemin, however, did not protect against the alpha, alpha-dipyridyl inhibition of mitochondrial protein synthesis. These results indicate that ferrous iron is required for mitochondrial protein synthesis and suggests that it is through a mechanism independent of hemin concentration.

On the nature of sickle-cell disease in the Arabian Peninsula

The sickle-cell gene contributes substantially to the presentation of anaemia in certain areas of the Arabian Peninsula. However, the clinical presentation of the homozygous state of Hb S is less severe than that observed in other ethnic groups, such as American negroes. In the present paper, biosynthesis studies performed on reticulocytes from heterozygotes and homozygotes for the Hb S give further indications of the mild nature of sickle-cell disease in Arabia. Comparison of two affected families, from Saudi Arabia and Jordan, showed that clinical manifestation of the disease is mirrored by the biochemical and haematological findings in affected individuals. The results are discussed in terms of the effect of co-existing thalassaemia and/or iron deficiency with Hb S. It is suggested that both genetic and acquired conditions play a role in the clinical features of the disease. The mechanisms responsible for regulation of alpha-chain synthesis by iron (haem) deficiency are discussed.

Ethanol inhibition of reticulocyte protein synthesis: the role of haem

Ethanol, in concentrations of 0.05-0.8 M, inhibited intact human and rabbit reticulocyte protein synthesis in the presence of iron-transferrin for endogenous haem synthesis. Associated with this effect there was a conversion of polyribosomes to monoribosomes and a decreased incorporation of radioactive leucine into nascent globin chains. When physiological levels of ethanol (0.05-0.1 M) were used, these effects were prevented by incubation with 50 muM haemin and reversed by removing the alcohol and reincubating with iron-transferrin or haemin. The polyribosomal disaggregation was also prevented by stopping ribosomal movement with 5 mM cycloheximide. Neither ATP nor GSH levels were altered in the presence of ethanol. When non-physiological levels of 0.8 M ethanol were used, haemin did not prevent the inhibition of protein synthesis. Likewise, in the rabbit reticulocyte cell-free lysate system containing haemin inhibition was noted at concentrations greater than 0.05 M ethanol. The polyribosomal disaggregation in reticulocytes incubated with 0.8 M ethanol was associated with decreased dissociation of monoribosomes into subunits. Similarly, when ribosomes were directly suspended cell-free in 0.1 or 0.8 M ethanol there was a decreased percentage of subunits. These results indicate that physiological concentrations of ethanol inhibit initiation of reticulocyte protein synthesis secondary to a block in haem synthesis. When intact cells are exposed to high non-physiological concentrations of ethanol the inhibition is secondary to decreased ribosomal dissociation. The cell-free lysate inhibition is also through this effect on ribosomal dissociation. This study supports the view that alcohol is a direct toxin to developing red cell precursors via its effect on mitochondrial haem synthesis. The physiological role of the decreased dissociation of monoribosomes into subunits is not yet clear.

Control of globin synthesis in cell-free preparations of reticulocytes by formation of a translational repressor that is inactivated by hemin

The control of globin synthesis by hemin in cell-free preparations of rabbit reticulocytes is mediated by an inhibitor (translational repressor) of globin chain initiation that is inactivated by hemin. When the ribosome-free supernatant fraction is warmed at 34 degrees without hemin, it quickly acquires the ability to inhibit a fresh cell-free preparation. If the warmed supernatant fraction is further incubated with hemin, its inhibitory activity is lost. This reversible inhibitor, which is observable only during the early period of incubation without hemin, is distinct from an irreversible inhibitor that becomes prevalent during longer incubations. The reversible inhibitor may be an intermediate in the formation of the irreversible inhibitor. The sensitivity of the reversible inhibitor to inactivation by hemin, which permits resumption of globin synthesis in both intact cells and lysates, indicates that the inhibitor is a physiological regulator.

Stimulation of globin synthesis: relative responsiveness of reticulocytes and nucleated erythroid cells

The effects of iron, cobalt, hemin, and plasma on hemoglobin synthesis by suspensions of rabbit reticulocytes and nucleated bone marrow cells were studied. L-Leucine-(14)C and sodium pyruvate-3-(14)C were employed to measure globin and heme synthesis, respectively. Normal plasma (or serum) was found to stimulate the rate of globin synthesis in both systems. The stimulatory effects of iron and hemin were additive to those of plasma or serum only in the reticulocytes. Cobaltous ion, at concentrations less than 1.0 mmole/liter, was found to stimulate globin synthesis by reticulocytes as effectively as ferrous ion; cobalt was inhibitory only at concentrations greater than 3.0-5.0 mmoles/liter. Heme synthesis by reticulocytes was inhibited at all concentrations employed (0.2-5.0 mmoles/liter). In bone marrow nucleated erythroid cells, globin synthesis was markedly enhanced by exogenous hemin. In contrast to reticulocytes, however, bone marrow cells were unresponsive to either cobalt or transferrin-bound iron. Possible implications of these findings on regulation of the rate and mechanism of iron uptake and hemoglobin synthesis in vivo are discussed.

The inhibitory effect of heme on heme formation in vivo: possible mechanism for the regulation of hemoglobin synthesis

1) The effect of hemin on heme synthesis was studied in vivo. Heme synthesis was measured by determining red cell (59)Fe uptake and glycine-2-(14)C incorporation into red cell hemin in normal CF(1) female mice.2) Both bovine and human hemin significantly decreased red cell (59)Fe uptake 48, 72, and 96 hours after hemin injection.3) Glycine-2-(14)C incorporation into red cell hemin was decreased to 50% of control values after the administration of hemin (90 mumoles per kg).4) There was no significant difference between the plasma iron of mice injected with hemin and the values in control groups.5) Protoporphyrin decreased the inhibitory effect of hemin on red cell (59)Fe uptake by 64%.6) There was no measurable effect of delta-aminolevulinic acid on hemin inhibition of red cell (59)Fe uptake.7) Data are presented to show that the effect of hemin is not by a dilution mechanism nor by a decrease in red cell maturation and subsequent release of reticulocytes into the peripheral blood. The findings are discussed in regard to the possible site of a negative feedback control of heme synthesis by heme.