PubMed · 7110413
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K te Velde. 1982-06-26. [Porphyria].. https://pubmed.ncbi.nlm.nih.gov/7110413/
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Heme, an organometallic tetrapyrrole, is widely engaged in oxygen transport, electron delivery, enzymatic reactions, and signal transduction. In plants, it is also involved in photomorphogenesis and photosynthesis. HEME OXYGENASE 1 (HO1) initiates the first committed step in heme catabolism, and it has generally been thought that this reaction takes place in chloroplasts. Here, we show that HO1 in both Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) has 2 transcription start sites (TSSs), producing long (HO1L) and short (HO1S) transcripts. Their products localize to the chloroplast and the cytosol, respectively. During early development or de-etiolation, the HO1L/HO1S ratio gradually increases. Light perception via phytochromes (Phys) and cryptochromes elevates the HO1L/HO1S ratio in the whole seedling through the functions of ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG and through the suppression of DE-ETIOLATED 1, CONSTITUTIVE PHOTOMORPHOGENESIS 1, and PHYTOCHROME INTERACTING FACTORs. HO1L introduction complements the HO1-deficient mutant; surprisingly, HO1S expression also restores the short hypocotyl phenotype and high pigment content and helps the mutant recover from the genomes uncoupled (gun) phenotype. This indicates the assembly of functional Phys within these lines. Furthermore, our findings support the hypothesis that a mobile heme signal is involved in retrograde signaling from the chloroplast. Altogether, our work clarifies the molecular mechanism of HO1 TSS regulation and highlights the presence of a cytosolic bypass for heme catabolism in plant cells.
To probe the distortions of the heme groups resulting from heme-apoprotein interaction in the isolated subunits of oxygenated human hemoglobin (i.e., alpha SH-oxyHbA and beta SH-oxyHbA), the dispersion of the depolarization ratio of the Raman lines at 1375 cm-1 (nu 4) and 1638 cm-1 (nu 10) was measured at various pHs. The data were analyzed in terms of vibronic coupling parameters which depend on symmetry-classified normal distortions of the heme groups. In the alpha-chain the nu 10 mode is not affected by symmetry-lowering distortions. In the beta-chain, however, this mode is significantly influenced by asymmetric B1g and B2g distortions. This was interpreted in terms of different interactions between the peripheral substituents and the porphyrin macrocycle in the respective chains. The nu 4 mode of both chains is subject to B1g (B2g) and A2g distortions, which are more pronounced in beta SH-oxyHbA. This is most probably due to differences in the repulsive interactions between the proximal imidazole and the pyrrole. While the depolarization ratio of both lines investigated is pH-independent in alpha SH-oxyHbA, it exhibits a significant pH dependence in beta SH-oxyHbA. This parallels the finding that the isolated beta-chains exhibit a Bohr effect whereas the alpha-chains do not. Consequently, the pH dependence of the coupling parameters and the Bohr effect of beta SH-oxyHbA could be rationalized in terms of the very same proton binding processes.(ABSTRACT TRUNCATED AT 250 WORDS)
Myeloperoxidase (MPO) is a heme-containing glycoprotein found in the primary granules (or azurophilic granules) of human polymorphonuclear leukocytes. In the present study, cultured myeloid leukemia HL-60 cells were exposed for 0-72 h to 250 microM 4,6-dioxoheptanoic acid (succinylacetone, SA), a specific inhibitor of heme biosynthesis, and the effects were evaluated using ultrastructural, immunochemical, and cytochemical methods. En bloc peroxidase staining of glutaraldehyde-fixed cells was accomplished with a 30-min exposure to 3,3'-diaminobenzidine (DAB) tetrahydrochloride. Ultrastructural examination revealed that peroxidase reactivity in the endoplasmic reticulum (ER) was relatively unchanged for 8 h and decreased between 12 and 24 h; however, ER lacked DAB-reactive peroxidase at 48-72 h. Peroxidase reactivity in the ER reappeared within 4 h after removal of SA. Seventy-two hours after exposure to SA the number of condensed cytoplasmic granules stained with DAB was significantly decreased, and many of the granules had a "target" appearance with a central DAB-reactive dense core. Staining of mitochondria was observed with overnight exposure to DAB and persisted in HL-60 cells treated 72 h with SA. Mitochondrial and nuclear morphology appeared unaltered. Immunostaining of MPO in thin sections of paraformaldehyde/glutaraldehyde-fixed unosmicated HL-60 cells, embedded in Lowicryl K4M, was accomplished with sequential exposure to an affinity-purified monospecific rabbit antibody to HL-60-MPO and protein A conjugated to 5- or 10-nm colloidal gold. Compared to untreated control HL-60 cells, cells exposed to SA for 48 h exhibited comparable to increased immunoreactive MPO in the ER, despite the absence of heme-dependent peroxidase reactivity. The data indicate that SA inhibits formation of enzymatically active MPO and that in the presence of SA, the ER contains a form(s) of MPO that lacks enzymatic reactivity.