Hemoglobin synthesis during rabbit reticulocyte maturation in vitro

Maturation-associated loss and incomplete de novo synthesis of the transferrin receptor in peripheral sheep reticulocytes: response to heme and iron

Hemin, but not iron, in the culture medium stimulates the maturation-associated loss of the transferrin receptor from sheep reticulocytes (t1/2 for loss approximately 6 hr) and its appearance in a population of externalized vesicles. A similar pattern is seen with nucleoside binding (a measure of the nucleoside transporter), where hemin increases the loss of binding activity from the cells during culture, concomitant with an increase in nucleoside binding in the externalized vesicles. Sheep reticulocytes retain the ability to synthesize the transferrin receptor, but the 35S-labeled receptors are not detected in released vesicles. Whereas hemin stimulates the loss of 35S-labeled transferrin receptors from the cell (t1/2 for loss approximately 20 hr), nonheme iron is more effective than heme. This difference in response of native and 35S-labeled receptor to hemin and iron supplements appears to be related to the differences in the two classes of receptors. Although the 35S-labeled receptor binds transferrin and both native and 35S-labeled peptides comigrate after chemical deglycosylation, the 35S-receptor is approximately 2 kD smaller than the native receptor and fails to acquire its complete size even when chased for up to 24 hr. Moreover, the 35S-labeled receptor is not expressed at the cell surface, but is retained in a nonrecycling compartment, where it is insensitive to digestion by trypsin at both 0 degrees C and 37 degrees C.

Effect of ionophores and metabolic inhibitors on protein synthesis in rabbit reticulocytes

The effect of some ionophores and metabolic inhibitors on reticulocytes protein synthesis was examined. At microM concentrations, valinomycin, nigericin and CCCP rapidly inhibit protein synthesis, while with antimycin-A or DCCD the inhibition is rather slow. The onset of the arrest of protein synthesis coincides with a 20--30% drop in the intracellular ATP content. No inhibition in protein synthesis or drop in ATP was found after 1 h incubation without glucose or in the presence of 2-deoxyglucose. It is shown that the inhibition by valinomycin, nigericin or CCCP is not due to their effect on K+ and/or H+ fluxes through the plasma membrane. Reticulocytes incubated at pH 8.2 show much lower inhibition of protein synthesis by nigericin, CCCP, DCCD or antimycin-A. On the other hand, at this alkaline pH, starvation to glucose causes high inhibition of protein synthesis. It is concluded that the ionophores inhibit protein synthesis due to their uncoupling effect on mitochondrial ATP synthesis. At high pH, the glycolytic activity is relatively high, and the ATP generated by the glycolysis can compensate to some degree for the ATP loss in the oxidative phosphorylation.

Metabolism of poly(A)(+)RNA in toad bladder epithelial cells

Previous studies have characterized the induction of poly(A)(+)RNA synthesis by aldosterone during the latent period, preceding the increased active transepithelial sodium transport (measured as short-circuit current, SCC). To assess the role of aldosterone in the maintenance of the response in general and the metabolism of this RNA in particular, the decay of the increased SCC and of the newly synthesized poly(A)(+)RNA was monitored. On removal of the hormone, the SCC decayed with a half-life of 6.5 hr after a lag period of 2-3 hr. Studies on the disappearance from the cytoplasm of poly(A)(+)RNA synthesized in the first two hours after addition of aldosterone revealed a number of RNA species with diverse size decaying at a relatively slow rate after removal of aldosterone, and RNA sedimenting in the 10-14 S region decaying at a faster rate closely related to the decay in SCC. Maintenance of aldosterone in the media resulted in a much slower rate of decay of this 10-14 S. It is concluded that the decay of the 10-14 S poly(A)(+)RNA is closely related to the decay in SCC and the stability of this RNA is influenced by the retention of aldosterone in the medium.

The stimulation of globin synthesis by cobalt in reticulocytes with inhibited heme synthesis

Cobalt was found to stimulate globin synthesis in reticulocytes the heme synthesis of which was inhibited by isonicotinic acid hydrazide or 4,6-dioxoheptanoic acid (succinyl acetone). These results suggest that in reticulocytes cobalt does not stimulate globin synthesis through the formation of cobalt protoporphyrin. They do, however, support the hypothesis that cobalt causes the formation of a pool heme from preformed hemoglobin which stimulates the synthesis of new globin chains.

Changes in energy charge and block of protein synthesis in rabbit reticulocytes under the action of valinomycin

Valinomycin blocks protein synthesis at the elongation stage in intact reticulocyte while being without action on a cell free system. We demonstrate here that this membrane mediated inhibition is accompanied by a change of the energy charge (ATP/ADP/AMP ratio) of the cells. There seems to be an accumulation of AMP without a proportional gain in ADP. The change in energy charge in itself can be the cause of the inhibition of protein synthesis. We show also that valinomycin can cause a decrease of the energy charge at concentrations where its action as an ionophore is not seen and independent of a possible activation of the Na-K dependent ATPase.

Characterization of [3H]-valinomycin binding to red blood cell membrane

Valinomycin was tritiated by exchange and its biological activity found to be similar to that of non-labeled drug. [3H]-valinomycin binds to red blood cell membranes following a biphasic pattern. High concentrations of the drug lead to an irreversible binding while low concentrations lead to a completely reversible binding. Maximum binding was obtained at acidic pH (pH 4.2) and physiological temperature (37 degrees C). We demonstrate that valinomycin binds strongly to the lipidic phase of the membrane. When binding to erythrocytes and reticulocytes was compared, it was found that the mature red blood cells had less binding capacity than the reticulocytes.

Interaction between membrane properties and protein synthesis in reticulocytes: influence of trypsinization on [3H]-valinomycin action

Using [3H]-Valinomycin we show here that two types of sites can be described for this cyclic depsipeptide. A first type is sensitive to low concentration of trypsin while the other, more internal, is uncovered by the use of the protease. Of these two kinds of sites, the more external one seems more concerned with the effect that Valinomycin has on protein synthesis in rabbit reticulocytes. However, when a high concentration of Valinomycin is used, all the sites can be occupied even those which can be revealed only by tripsinization. In this case, even prolonged trypsin action does not result in release of the protein synthesis inhibitory action of Valinomycin. It is concluded that hydrophobic sites are occupied by Valinomycin only after the cell surface has been saturated by hydrophylic bonds with the antibiotic.

Block in the elongation of protein synthesis in rabbit reticulocyte by action of the ionophore valinomycin

In this paper we show that ribosomes isolated from cells inhibited for protein synthesis by the antibiotic ionophore Valinomycin can still support incorporation of [14C]-leucine into polypeptides in a cell-free system. The extent of this functional integrity depends upon the concentration of Valinomycin used and whether it acted as an ionophore or not. We demonstrate that the antibiotic acts on protein synthesis at the level of elongation and has no action either at the initiation level or as activating an RNase. Moreover, we show that it can inhibit protein synthesis at concentrations where its action as an ionophore cannot be detected.

The rate limiting step in the reticulocyte uptake of transferrin and transferrin iron. Effects of some incubation variables

Reticulocytes incubated in an isotonic NaCl saline medium containing glucose, glutamine and amino acids, were able to detach both iron atoms from all the transferrin incorporated by them. In the absence of these metabolites, although transferrin uptake was the same, the reticulocytes failed to remove completely the iron from the transferrin which they incorporated. It has been shown before that there is unspecific as well as specific binding of transferrin to the reticulocyte. By incubating the cells in the presence of a high concentration of bovine serum albumin, we have been able to prevent the unsepcific attachment of transferrin. At least 94% of the iodinated transferrin was capable of donating its iron to the reticulocytes.

The time course of the effect of hypophysectomy and oestrogen treatment on the hepatic metabolism of androst-4-ene-3,17-dione in male and female rats

Hypophysectomy of male animals has little effect on the hepatic androst-4-ene,3,17-dione (androstenedione) metabolism, except for possible changes in the kinetics of the 16alpha-and 7alpha-hydroxylases. On the other hand, hypophysectomy of female animals leads to a "masculinization" of hepatic androstenedione metabolism, following the changes seen in Vmax. of the enzymes involved, probably due to the removal of the source of "feminizing factor" thought to maintain the "female" type of metabolism in the liver. There seems to be a temporal dissociation of the effects on the various enzymes, indicating different cellular control mechanisms for these enzymes. Oestrogen treatment of male rats causes "feminization" of the hepatic androstenedione metabolism. The time study shows an initial increase in 17-hydroxy steroid oxidoreductase and 6beta- and 16alpha-hydroxylase activities, followed by a decrease to the values in females. This biphasic effect is possibly due to an initial direct effect via the hypothalamo-pituitary system.

Different lifetimes of reticulocyte messenger RNA

About 85-90% of cytoplasmic protein synthesized by young reticulocytes is globin, and about 10% is a polypeptide (I) of molecular weight 64,000 daltons. Maturation of reticulocytes is accompanied by selective reduction in the synthesis of polypeptide I, relative to globin; mature reticulocytes synthesize globin at a high level but make no detectable polypeptide I. Studies in which RNA from young and old reticulocytes was translated in a wheat germ cell-free extract showed that reduction in synthesis of polypeptide I was correlated with a reduction in the amount of translatable mRNA for this protein. Differential destruction of mRNA thus is an important factor in determining the types of proteins made during the final stages of erythropoiesis.

Evidence supporting a physiological role for the hemin-controlled translational repressor of globin synthesis in reticulocytes

The synthesis of heme and globin in rabbit reticulocytes was compared at 35 and 25 degrees C. The lower temperature decreased heme synthesis significantly more than globin synthesis and resulted in a much greater accumulation of globin dimers. After 16 h of incubation in the absence of iron, globin synthesis in reticulocytes which were at 35 degrees C could not be stimulated by iron, whereas cells which were at 25 degrees C responded with nearly control levels of globin synthesis. Since the formation of the hemin-controlled translational repressor in reticulocyte lysates is also decreased much more than protein synthesis at reduced temperature the results provide evidence for a physiological role for the translational repressor in controlling globin synthesis in reticulocytes.

Inhibitory effect of hydroxyurea on [3-H]leucine incorporation in human peripheral lymphocytes

Hydroxyurea (HU), generally considered to be a specific inhibiter of DNA synthesis, has an inhibitory effect on the incorporation of TCA-precipitable [3-H]leucine in peripheral lymphocytes. This action is not secondary to the inhibition of DNA synthesis since incorporation of [3-H]leucine is unaffected when DNA synthesis is inhibited by 5-fluorodeoxyuridine (FUdR); it does not appear to be directly related to inhibition of RNA synthesis; and it is not mediated at the level of translation since HU has no effect on protein synthesis in rabbit reticulocytes. The relevance of these findings to the use of HU as a DNA inhibitor is discussed.

Half-life and rate of synthesis of globin messenger ribonucleic acid. Determination of half-life of messenger ribonucleic acid and its relative synthetic rate in erythroid cells

The specific radioactivity of mouse globin mRNA in blood reticulocytes was measured after injection of [(3)H]uridine into anaemic mice up to 60h before collection of reticulocytes. From these data, the decay of the acid-soluble nucleotide pool in the marrow and the relative marrow-cell composition it is possible to build models that allow the cell life-times and half-life of mRNA in the erythroid cells of the marrow to be calculated. Best fit of models to these data favour a model with either one or two cell divisions from the onset of mRNA synthesis. The single-cell-division model has cell times of 20, 13 and 7h respectively for the basophilic erythroblast, polychromatophilic erythroblast and reticulocyte. The two-cell-division model has cell times of 12, 12, 12 and 7h for the basophilic erythroblast 1 and 2, polychromatophilic erythroblast and reticulocyte respectively. Both models have an mRNA half-life of 17h and a constant rate of mRNA synthesis until enucleation at the reticulocyte stage, when synthesis stops. A declining rate of mRNA synthesis can be accommodated in a two-cell-division model, when synthesis halves at each cell division and cell times are essentially the same as above, but mRNA half-life is either 9h in the basophilic and polychromatophilic erythroblasts and 17h in the later cells, or 10h in the basophilic erythroblasts and polychromatophilic erythroblasts and 14.5h in later cells. In all cases it is clear that mRNA synthesis occurs over a time-period of only 30-36h and that mRNA cannot be pre-synthesized in precursor erythroid cells.

Efficient translation of tobacco mosaic virus RNA and rabbit globin 9S RNA in a cell-free system from commercial wheat germ

Extracts prepared from commercially available wheat germ efficiently translate messenger RNAs from either viral or eukaryotic origin. The addition of tobacco mosaic virus RNA stimulated amino-acid incorporation more than 100-times and rabbit globin 9S RNA between 20- and 30-times. The in vitro product directed by globin 9S RNA comigrated precisely with authentic rabbit globin on sodium dodecyl sulfate-polyacrylamide gels. Furthermore, during high voltage ionophoresis two [(35)S]methionine-labeled tryptic peptides synthesized in vitro comigrated in two dimensions with alphaT5 and betaT5 tryptic peptides from authentic [(35)S]methionine-labeled rabbit globin.