Heme formation

In summary, the capacity to synthesize both hemoproteins and iron sulfur proteins appears to be a ubiquitous attribute in organisms attacking reduced inorganic substrates. If the reactions by which these cells obtain energy represent relics of ancient forms of metabolism, it can only be concluded that heme formation and iron-sulfur coordination must have been invented at a very early stage in evolution. 

In many of these conditions, inhibition of heme formation leads to up-regulation of ALA synthase and exacerbates the accumulation of toxic intermediates. 

D of protoporphyrin-IX depends on the steric constraints of the substrate binding pocket. Ortiz de Montellano (59) has used this selectivity to probe the active site structure of several heme enzymes. The structure of phenyl-cyt P-450cam has been determined by X-ray crystallography and indicates that N-phenyl heme formation is an accurate, low-resolution probe of active site structure. 

The first step in heme formation is the rate-limiting condensation reaction between succinyl-CoA and glycine to form 5-aminolevulinic acid (ALA). This reaction is catalyzed by a mitochondrial matrix enzyme, ALA synthase (ALAS). 

Ortiz de Montellano, P.R. and L.A. Grab (1986). Inactivation of cytochrome P-450 during catalytic oxidation of a 3-[(arylthio)ethyl]sydnone N-vinyl heme formation via insertion into the Fe-N bond. J.Am. Chem. Soc. 108, 5584-5589. 

Previously, Tangeras [52] had proposed that a pool of iron in the inner mitochondrial compartment (1 nmol mg of protein) is available to ferrochelatase for heme formation. A soluble component of that compartment could maintain sufficient ferrous iron in equilibrium with ferric iron, allowing the observed rate of 0.3 nmol of heme formation per hour, which corresponds to an amount of about five times that necessary for the turnover of hemoproteins in hepatocytes [53]. 

The cellular levels of iron and ferrochelatase are important determinants of PpIX yield under exogenous ALA stimulation. The use of iron chelators is based on the relative inefficiency of ferrochelatase compared to other enzymes, which causes PpIX build-up when heme precursors are produced at an increased rate. In some cancer cell lines lower levels of ferrochelatase have been found than in normal cells and this may contribute to tumor selectivity in certain cancers [197]. In an attempt to further reduce or totally abrogate heme formation, exogenous chelating agents were used to remove iron. It was shown in vitro that iron chelation caused both increased PpIX formation and improved PDT efficacy and in vivo applications in animals and humans have confirmed the concept [191,198,199]. In lymphocytes that express the transferrin receptor (CD71), which is interpreted as an indication of low intracellular iron levels, higher PpIX concentrations were reached under ALA stimulation [200]. In addition, an analysis of iron availability at the molecular level... 

Methionine deficiency leads to coproporphyrin accumulation. Lascelles and Hatch [145] suggested that heme formation may be inhibited under these conditions, perhaps at the iron insertion step because methionine is required for the synthesis of phosphatidyl choline and the latter appears to be needed for ferrochelatase activity. Tait [147a] reported that under anaerobic conditions the conversion of coproporphyrinogen to protoporphyrinogen required methionine, ATP, and ferrous ions. 

Human hemoglobin oxidation with Fe(lll)-heme formation as a result of the vanadyl ion effect was observed using Mossbauer spectroscopy [ 105]. Partial oxidation of rat hemoglobin after 70 days of rats feeding with cerium chloride was detected by Mossbauer spectroscopy [106]. An attempt to study hemoglobin from patients with diabetes was done in Ref. 107. However, poor statistical rates, a low signal-to-noise ratio, and a low velocity resolution in these studies did not permit the authors to extract more detailed and exact information. 

Hematotoxic effects of lead have triggered debates as to the nature and severity of lead hematotoxicity versus lead toxicity associated with brain, cardiovascular, and kidney injuries. Part of the debate goes to the basic question of what comprises a toxicologically adverse effect, a functionally adverse effect, and a biochemically adverse effect. Effects of Pb on heme formation from precursors are produced through effects on mitochondrial function. 

With the use of labeled succinate-1,4-C S succinate-2,3-C labeled a-ketoglutarate, and citrate as substrates for heme formation it was demonstrated that a-ketoglutarate gave rise to the succinyl derivative (27). By blocking succinic dehydrogenase with malonate (28) the succinyl derivative was found to be formed via the citric acid cycle, or to a lesser extent more directly from succinate. The predicted carbon atoms in heme contained the C . 

Attempts to isolate an iron-inserting enzyme are hampered by the relative ease of nonenzymic iron incorporation into porphyrins under appropriate conditions. Extracts of erythrocytes (148-161) or of mitochondria (168) are found to form heme from ferrous iron and PROTO. However if PROTO is solubilized, i.e., not colloidal and if ferrous iron is kept reduced and is not tightly tied up by buffers, amines, etc., then heme formation readily occurs in the absence of any enzyme. Possibly kinetics and specificity tests will help to isolate the enzymic process. Some steps along these lines have been reported (163). 

Immature red blood cells can use inorganic Fe++ for heme synthesis (164). However this is not a normal physiological process. Two mechanisms appear to be used for iron incorporation into immature red cells of the bone marrow. One is the adsorption of Fe-transferrin (a jS-globulin of the serum specific for iron transport) onto specific sites of the erythroblast or reticulocyte then the iron is transferred into the cell (as an iron complex or by pinoc3dosis of transferrin ) and is used directly for heme formation or... 

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