Ferrochelatase, inhibition

P.R. Ortiz de Montellano (1988). 2,2-Dialkyl-1,2-dihydroquinolines Cytochrome P-450 catalyzed N-alkylporphyrin formation, ferrochelatase inhibition, and induction of 5-aminolevulinic acid synthase activity. Chem. Res. Toxicol. 1, 208—215. 

Tephly, T.R., A.H. Gibbs, G. Ingall, and F. De Matteis (1980). Studies on the mechanism of experimental porphyria and ferrochelatase inhibition produced by 3,5-diethoxycarbonyl-l,4-dihy-drocollidine. Int. J. Biochem. 12, 993-998. 

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,... 

Inhibition of ferrochelatase in the heme pathway causes accumulation of protoporphyrin in erythrocytes (CDC 1985). Most protoporphyrin in erythrocytes (about 90%) exists as zinc protoporphyrin (ZnPP). This fraction is preferentially measured by hematofluorometers. Extraction methods measure all the protoporphyrin present, but strip the zinc from the ZnPP during the extraction process. For this reason,... 

Answer C. Lead inhibits both ferrochelatase (increasing the zinc protoporphyrin) and ALA dehydrase (increasing 5-ALA). 

Lead ions also inhibit ferrochelatase-cataiyzed insertion ofFe into protoporphyrin iX in the final step of heme synthesis. 

Other photosensitisers in clinical or pre-clinical trials include zinc phthalocya-nine, aluminium sulphonated phthalocyanines, benzoporphyrins, benzochlorins and purpurin-lS-iV-alkylamides, all of which absorb strongly in the 675-700 nm region. An alternative approach to the photosensitisation in PDT involves the use of 5-aminolaevulinic acid (ALA). This compound itself is not a sensitiser but in human cells it is the key metabolic precursor in the biosynthesis of protoporphyrin IX, which can act as a photosensitiser. Normally the biosynthetic process would continue beyond protoporphyrin IX to the iron containing haem. However, by adding extra ALA and iron chelators, the ferrochelatase action is inhibited and the normal feedback mechanism by-passed resulting in a build up of protoporphyrin IX in the cell. The mechanism is illustrated in Figure 4.24. ... 

Formation of heme Uroporphyrinogen III is converted to heme by a series of decarboxylations and oxidations summarized in Figure 21.4. The introduction of Fe2+into protoporphyrin IX occurs spontaneously, but the rate is enhanced by the enzyme ferrochelatase—an enzyme that is inhibited by lead (see p. 279). 

Ferrochelatase and ALA dehydrase are particularly sensitive to inhibition by lead... 

Committed step in heme synthesis, its coenzyme, and inhibitor The committed step in heme synthesis is the formation of 5-amlnolevulinic acid (ALA). The reaction, which requires pyridoxal phosphate as a coenzyme, is catalyzed by ALA synthase. The reaction is inhibited by hemin (the oxidized form of heme that accumulates in the cell when it is being under-used). The conversion of protoporphyrin IX to heme, catalyzed by ferrochelatase, is inhibited by lead. 

Ferrochelatase (protoheme ferro-lyase)401 403 inserts Fe2+ into protoporphyrin IX to form heme. The enzyme is found firmly bound to the inner membrane of mitochondria of animal cells, chloroplasts of plants, and chromatophores of bacteria. While Fe2+ is apparently the only metallic ion ordinarily inserted into a porphyrin, the Zn2+ protoporphyrin chelate accumulates in substantial amounts in yeast, and Cu2+-heme complexes are known (p. 843). Ferrochelatase, whose activity is stimulated by Ca2+, appears to be inhibited by lead ions, a fact that may account for some of the acute toxicity of lead.404... 

The other therapeutic strategy is based on selective inhibition of iron-containing enzymes [260]. The inhibition of ferrochelatase by iron chelating agent l,2-diethyl-3-hydroxy-4-pyridione increases the level of its substrate protoporphyrin IX, which acts as an endogenous photosensitizer formed during PDT from 5-aminolevulinic acid [5,261]. 

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. 

A second major lead-induced toxicity involves interruption of heme synthesis. Lead interacts at several steps in the heme biosynthetic pathway (Figure 21.13). As mentioned above, Pb inhibits the enzyme 8-aminolevulinic acid dehydratase (ALA-D), which catalyzes the second step of heme synthesis involving the condensation of two molecules of aminolevulinic acid (ALA) to form porphobilinogen. The result of this inhibition is the accumulation of aminolevulinic acid in the serum and increased excretion of ALA in the urine. A second major disruption of the heme biosynthetic pathway is Pb inhibition of ferrochelatase. This enzyme is responsible for the incorporation of the ferrous ion (Fe2+) into protoporphrin IX to produce heme (Figure 21.2). Accumulated protoporphrin is incorporated into red blood cells and chelates zinc as the cells circulate. This zinc-protoporphrin complex is fluorescent and used to diagnose Pb poisoning. 

Chronic exposure to methyl mercury results in increased urinary excretion of uro- and coproporphyrins in rats, mediated via inhibition of ferrochelatase. Acute... 

Dailey HA, Fleming JE. Bovine ferrochelatase. Kinetic analysis of inhibition by N -methylprotoporphyrin, manganese, and heme. J. Biol. Chem. 1983 258(19) 11453-11459. 62. 

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. 

Glyceraldehyde-3-phosphate dehydrogenase, an enzyme in the glycolytic pathway (Chapter 8), is inactivated by alkylation with iodoacetate. Enzymes that use sulfhydryl groups to form covalent bonds with metal cofactors are often irreversibly inhibited by heavy metals (e.g., mercury and lead). The anemia in lead poisoning is caused in part because of lead binding to a sulfhydryl group of fer-rochelatase. Ferrochelatase catalyzes the insertion of Fe2+ into heme. 

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