Heme, translation control

Regulation of catalase expression in eukaryotes takes place as part of a generalized response mechanism. In yeast, promoter elements of the peroxisomal catalase CTA-1 respond to glucose repression and activation by fatty acids as part of organelle synthesis. The cytosolic catalase CTT-1 responds as part of a generalized stress response to starvation, heat, high osmolarity, and H2O2, and there is even evidence of translational control mediated by heme availability 26). 

The rabbit reticulocyte, which synthesizes about 90 of its protein as hemoglobin, offers one of the best examples of translational control of protein synthesis in animal cells. In the complete absence of a nucleus (which is extruded from the cell during its maturation) it coordinates the production of the a and R chains of globin, both with each other and with the supply of heme. However, the exact mechanism of this translational control is not yet fully understood, and only a partial description of what appears to be a surprisingly complex process can be given below. 

One of the clearest instances of translational control that can be studied in vitro is the block in initiation of translation in reticulocyte lysates observed in the presence of dsRNA (Ehrenfeld and Hunt, 1971). Initiation of translation becomes inhibited with kinetics virtually identical to those seen in the absence of heme (Hunter et aL, 1975) and as in that case, the block involves inactivation of an initiation factor (Kaempfer and Kaufman, 1973) identified as eIF-2 (Kaempfer, 1974ft Clemens et al., 1975) addition to this factor is sufficient to overcome the dsRNA-mediated inhibition (Kaempfer, 1974ft). 

While the final explanation of the phenomena connected with translational control by dsRNA and by heme deprivation may turn out different from that considered here, it is clear from the observations discussed in Section 7.2.1 that the steps leading to phosphorylation of eIF-2 may be under more subtle and complex control than first thought, even though the effect of phosphorylation on elF-2 activity has now been clarified. 

One of the most heavily labeled proteins was later isolated from parasite grown in the presence of [ Hjdihydroartemismin (300 mM) and identified as a 25-kDa translationally controlled tumor protein (TCTP) homolog, that is able to bind heme with modest affinity (17). In vitro, the reaction of dihydroartemisinin with recombinant TCTP is clearly dependent on the presence of heme the single cysteine of the protein also appears to be necessary for the reaction, probably serving as a source of electrons for the heme-mediated activation of e drug. 

The translation of globin mRNA is dependent on free heme. The absence of heme leads to phosphorylation of eIF2 through a hemin-controlled inhibitor. In the absence of heme, the hemin-controlled inhibitor protein is an active protein kinase, which phosphorylates... 

Control of heme biosynthesis revolves around the initial enzyme, ALA-synthase. Heme, the end product of this pathway, exerts an inhibitory effect on ALA-synthase through several mechanisms. Heme, or its oxidation product hematin, activates a repressor protein that turns off ALA-synthase biosynthesis at the translation level. Erythropoietin, a protein produced by the kidneys and found in larger than normal amounts in high-altitude dwellers, counteracts the effects of the repressor protein. Erythropoietin deficiency exists in chronic kid-... 

Hemoglobin synthesis is controlled at the translational level by the availability of heme. It declines when heme concentration falls and increases when heme concentration rises. Heme deficiency activates an inhibitor. 

Microsomal NADH-cytochrome 6b reductase and its acceptor substrate cytochrome 6s are amphipathic proteins, that is, they are each composed of a hydrophobic domain and a soluble domain (74, SS9, 340). The hydrophobic domains serve to anchor the proteins by strong nonco-valent interaction with the lipid bilayer of the microsome. The soluble domains, containing the active sites, FAD or heme, project into the surrounding cytosol. The two domains, in each case, are connected by what is presumed (because of their proteolytic lability) to be rather fl ible sections of polypeptide, imparting considerable mobility to the projecting catalytic domains. The proteins have been shown also to have translational mobility. Interaction between the reductase and the cytochrome is controlled by both types of mobility (341-344)-... 

In animals the control of heme biosynthesis appears to be primarily a control on the rate of biosynthesis of the enzyme ALA-synthetase. Recent experiments [Sassa and Granick, 21] suggest that control of this enzyme occurs at both the transcription and the translation levels. Because these controls have been studied mainly in the liver, we shall discuss the evidence for the control mechanisms in this tissue, particularly the more recent work using chick embyro liver cells grown in primary culture. In later sections we shall discuss the ALA-synthetase control mechanism in the red cells for heme and hemoglobin synthesis, and the control mechanisms for chlorophyll synthesis in plants. 

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