Ferrochelatase heme biosynthesis

Lead-induced anemia results from impairment of heme biosynthesis and acceleration of red blood cell destmction (10,13). Lead-induced inhibition of heme biosynthesis is caused by inhibition of S-aminolevulinic acid dehydratase and ferrochelatase which starts to occur at blood lead levels of 10 to 20 pu gjdL and 25 to 30 //g/dL, respectively (10,13). Anemia, however, is not manifested until higher levels are reached. 

In summary, lead inhibits the activity of certain enzymes involved in heme biosynthesis, namely, 5-aminolevulinic acid dehydratase (ALAD), and ferrochelatase. As a consequence of these changes, heme biosynthesis is decreased and the activity of the rate limiting enzyme of the pathway,... 

ALAD, and ferrochelatase. Lead indirectly stimulates the mitochondrial enzyme ALAS, which catalyzes the condensation of glycine and succinyl-coenzyme A to form ALA. The activity of ALAS is the rate-limiting step in heme biosynthesis increase of ALAS activity occurs through feedback derepression. Lead... 

The incorporation of iron(II) into the protoporphyrin IX ring, the final step in heme biosynthesis, is catalyzed by ferrochelatase. Kinetic parameters were reported both for this process and for the reaction of Fe " with ferrochelatase, whose kinetics were used to characterize the latter. ... 

Ferrochelatase, the terminal enzyme of the heme biosynthesis pathway, catalyzes the metalization ofmesoporphyrin with Zn11 with a kcat value of800 h-1. An antibody was found whose reaction according to Figure 18.6 features a kcat value of 80 IT1, the relatively highest value for antibodies in comparison to enzymes (Cochran, 1990). 

Within the past few years, there has been considerable progress in understanding the role played by the mitochondria in the cellular homeostasis of iron. Thus, erythroid cells devoid of mitochondria do not accumulate iron (7, 8), and inhibitors of the mitochondrial respiratory chain completely inhibit iron uptake (8) and heme biosynthesis (9) by reticulocytes. Furthermore, the enzyme ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) which catalyzes the insertion of Fe(II) into porphyrins, appears to be mainly a mitochondrial enzyme (10,11,12,13, 14) confined to the inner membrane (15, 16, 17). Finally, the importance of mitochondria in the intracellular metabolism of iron is also evident from the fact that in disorders with deranged heme biosynthesis, the mitochondria are heavily loaded with iron (see Mitochondrial Iron Pool, below). It would therefore be expected that mitochondria, of all mammalian cells, should be able to accumulate iron from the cytosol. From the permeability characteristics of the mitochondrial inner membrane (18) a specialized transport system analogous to that of the other multivalent cations (for review, see Ref. 19) may be expected. The relatively slow development of this field of study, however, mainly reflects the difficulties in studying the chemistry of iron. 

Fertilized 16-day-old chick embryos are kept in an incubator at 37 °C with 70 % humidity. Drugs are dissolved in a small volume (0.1-0.3 ml) of 0.15 M NaCl. According to Anderson (1978) a small dose of DDC (= l,4-dihydro-3,5-dicarbethoxycollidine) is added that leads to the formation of N-methylprotoporphyrin, which is the inhibitor of ferrochelatase, the last enzyme in heme biosynthesis. Sterile injections are made when the eggs are 18 days old. After 24 h, the embryos are killed by decapitation, the livers removed, separated from the gall bladder and rinsed with saline prior to homogenization with 3 vol of 0.25 M sucrose/0.02 M Tris buffer, pH 7.4. At least six embryo livers are pooled for each determination. 

Anderson KE (1978) Effects of antihypertensive drugs on hepatic heme biosynthesis, and evaluation of ferrochelatase inhibitors to simplify testing of drugs for heme pathway induction. Biochem Biophys Acta 543 313-327 Bayar C, Sumer N (1995) The effect of some local anesthetics on methemoglobin levels and erythrocyte enzymes. Turk J Med Sci 26 439-443... 

The hematopoietic system is affected by both short- and long-term arsenic exposure. Arsenic is known to cause a wide variety of hematological abnormalities like anemia, absolute neutropenia, leucopenia, thrombocytopenia, and relative eosinophilia - more common than absolute esino-philia, basophilic stippling, increased bone marrow vascularity, and rouleau formation (Rezuke et al, 1991). These effects may be due to a direct hemolytic or cytotoxic effect on the blood cells and a suppression of erythropoiesis. The mechanism of hemolysis involves depletion of intracellular GSH, resulting in the oxidation of hemoglobin (Saha et al, 1999). Arsenic exposure is also known to influence the activity of several enzymes of heme biosynthesis. Arsenic produces a decrease in ferrochelatase, and decrease in COPRO-OX and increase in hepatic 5-aminolevulinic acid synthetase activity (Woods and Southern, 1989). Subchronic... 

Finally, it is widely known that Pb impairs the formation of red blood cells. The mechanism involved in the impairment is that Pb inhibits both 5-aminolevulinic acid dehydratase (ALA-D) (Hernberg et al. 1970) and ferrochelatase (Tephly et al. 1978). These are two key enzymes involved in heme biosynthesis. ALA-D catalyzes the conversion of 5-aminolevulinic acid into porphobilinogen (PBG), whereas ferrochelatase is responsible for catalyzing the incorporation of Fe2+ into protoporphyrin IX to form heme (Figure 9.1). Lead inhibition of the two enzymes appears to be due to its interaction with Zn and Fe required in the process. 

The genes for all the enzymes of human heme biosynthesis have been characterized (Table 32-2), and the structures of 5-aminolevulinic acid dehydratase (ALAD), hydroxymethyl-bilane synthase (HMBS), uroporphyrinogen-III synthase (UROS), uroporphyrinogen decarboxylase (UROD), and ferrochelatase (FECH) have been determined by x-ray crys-tallography. - - ... 

We devised a screen for isolating mutants defective in iron-dependent regulation of heme biosynthesis that did not require prior knowledge of the mechanism or of the rate-limiting steps [83]. We speculated that if the pathway as a whole were regulated by iron, a mutant defective in that control would accumulate protoporphyrin under iron limitation. Mutants defective in the heme synthesis enzymes ferrochelatase [75] or protoporphyrinogen oxidase would likely have a similar phenotype, but porphyrin accumulation would likely be independent of iron in the structural gene mutants, and those strains would also be expected to be heme auxotrophs. 

Frataxin (Figure 2) has recently been identified as a likely physiological iron donor to ISU during assembly of the [2Fe-2S] cluster core 10, 21-23), and to ferrochelatase for the final step of heme biosynthesis 24, 25), Dysfunction of fi-ataxin results in mitochondrial iron accumulation and oxidative damage to mitochondrial DNA 26, 27), as well as the potential to disrupt other cellular processes dependent on such iron cofactors. This is the first characterized example of an iron-delivery protein and it remains to be seen how relevant its functional chemistry will be, relative to cell metabolism across a spectrum of organisms. As noted later, the absence of sequence homologs in certain other organisms does not preclude the presence of structural and functional... 

Iron insertion is the final step of heme biosynthesis, and this is controlled by the enzyme ferrochelatase. This enzyme is located on the inner face of the mitochondrial inner wall and its failure to function properly leads to a condition called erythropoietic protoporphyria, which, because protoporphyrin IX builds up in the skin, leads to light sensitivity. Protoporphyrin is also found in the red blood cells, bile, and feces. The three porphyrias that arise due to genetic malfunction of the last three enzymes in heme biosynthesis have not yet been tracked down to specific genetic abnormalities. 

Hematological effects Decreased heme biosynthesis by inhibiting d-aminolevulinic acid dehydratase (ALAD) and ferrochelatase activity, an increase in blood and plasma d-aminolevulinic acid (ALA) and free erythrocyte protoporphyrins, hemolytic anemia and Frank anemia... 

The final step of heme synthesis is catalyzed by ferrochelatase. This enzyme is firmly bound to the inner mitochondrial membrane (Jones and Jones, 1968). Incorporation of iron into protoporphyrin ring systems supplies heme for the electron transport system in cytochromes and for oxygen transport and storage in hemoglobin and myoglobin, respectively. Increased heme biosynthesis in vivo may stimulate mitochondrial cytochrome formation (Beattie, 1971). 

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