Chelation of iron and zinc by protoporphyrin catalyzed by mitochondrial preparations

Overexpression in Escherichia coli, and one-step purification of the human recombinant ferrochelatase

Ferrochelatase (EC 4.99.1.1), a mitochondrial inner membrane-bound protein, is the terminal enzyme of heme biosynthesis. The cDNA encoding the human mature ferrochelatase was placed under transcriptional control of T7 RNA polymerase in an Escherichia coli expression system. The bacteria produced large amounts of 42 kDa protein which reacted with anti-ferrochelatase antibodies. Expressed ferrochelatase exhibited iron- and zinc-chelating activities, and was found as a soluble protein. The recombinant enzyme has been purified to apparent homogeneity with a high yield, by one-step purification involving Blue-Sepharose chromatography. The purified enzyme which showed a molecular weight of about 40,000 by gel-filtration, functioned in a monomeric form. Km value for both mesoporphyrin IX and protoporphyrin IX with zinc was 12.5 microM. Km values for iron and zinc with mesoporphyrin IX were 6.7 microM and 11.8 microM, respectively. Zinc-chelating activity was markedly stimulated by palmitic acid, but iron-chelating activity remained unchanged. The above results were similar to those reported previously for mammalian ferrochelatase. The overexpression and the simple purification of a functional ferrochelatase exhibiting the same properties as natural enzyme will allow us to elucidate the mechanism of the enzyme reaction and structural changes of the mutated enzyme.

High-performance liquid chromatographic assays for protoporphyrinogen oxidase and ferrochelatase in human leucocytes

Rapid, sensitive and specific high-performance liquid chromatographic assays are described for protoporphyrinogen oxidase and ferrochelatase in human leucocytes. The enzyme reaction products were separated and quantitated by reversed-phase high-performance liquid chromatography with fluorescence detection. The optimal pH for the protoporphyrinogen oxidase assay was 8.6 and the Michaelis constant for protoporphyrinogen IX was 9.78 +/- 0.96 microM (mean +/- S.D.). The mean (+/- S.D.) activity of protoporphyrinogen oxidase in fourteen apparently healthy subjects was 0.146 +/- 0.023 nmol protoporphyrin IX per min per mg protein. In one patient with variegate porphyria, the activity was 0.028 nmol protoporphyrin IX per min per mg protein. The optimal pH for ferrochelatase was 7.4 and with protoporphyrin and Zn2+ as substrates, the Michaelis constants were 1.49 and 8.33 microM, respectively. The mean activity of ferrochelatase in ten control subjects was 0.24 nM Zn-protoporphyrin or 2.05 nM Zn-mesoporphyrin formed per h per mg protein.

Elevation of blood levels of zinc protoporphyrin by radiomimetic drugs and Friend leukemia virus

Sublethal doses of whole-body irradiation induced the elevation of zinc protoporphyrin (ZPP). Experiments were conducted to determine if recovery from radiomimetic drugs also resulted in elevation of ZPP. Daily injections with hydroxyurea and other cytotoxic drugs for 10 days caused ZPP elevation and a dose of radiation too low to cause ZPP elevation by itself caused ZPP elevation when hydroxyurea was administered prior to irradiation. Friend leukemia virus also brought about an elevation of ZPP. However, not all factors that increased erythropoiesis brought about ZPP elevation. The elevated erythropoiesis in response to hypoxia and the enhanced erythropoiesis that followed administration of folic acid to folic acid-deficient mice was not accompanied by ZPP elevation.

Characterization of deficient heme synthase activity in protoporphyria with cultured skin fibroblasts

Heme synthase (ferrochelatase) activity, as determined by the chelation of ferrous iron to protoporphyrin or deuteroporphyrin, is reduced to 10-25% of normal in tissues of patients with protoporphyria. With cultured skin fibroblasts from seven patients with protoporphyria and six normal individuals, the present studies examined the enzymatic defect.Heme synthase activity in normal and protoporphyria fibroblasts had the same pH optimum, showed similar inhibition by divalent metals, and had the highest specific activity in the mitochondrial-enriched fraction. The ultrastructural features and other biochemical parameters of mitochondria were normal in protoporphyria cells, excluding a general mitochondrial defect. Measurement of the rate of deuteroheme formation at different concentrations of substrate demonstrated a significant reduction in the apparent K(m) for deuteroporphyrin in detergent-treated sonicates of protoporphyria fibroblasts compared to normal (7.5 +/- 0.9 muM, mean +/- SEM, vs. 17.4 +/- 1.8), as well as a decrease in the velocity of reaction (mean level was 21% of normal). Studies with intact cells, in which heme synthase activity was estimated indirectly, also indicated that the apparent K(m) for porphyrin substrate was significantly lower in protoporphyria lines. These data show that heme synthase in protoporphyria fibroblasts has markedly reduced catalytic activity despite an increased affinity for porphyrin substrate. This could be caused by either a change in the enzyme protein, or an alteration of its micro-environment.

Purification and characterization of the membrane-bound ferrochelatase from Spirillum itersonii

The membrane-bound enzyme ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) was purified from isolated membrane fragments of Spirillum itersonii approximately 490-fold. Purification was achieved by solubilization with chaotropic salts followed by ammonium sulfate fractionation, diethylaminoethyl-cellulose chromatography, and gel filtration on Sephadex G-200. The purified enzyme has an apparent minimum molecular weight of approximately 50,000, as determined by gel filtration in the presence of 0.1% Brij 35 and 1 mM dithiothreitol but forms high-molecular-weight aggregates in the absence of detergent. Purified ferrochelatase is strongly stimulated in the presence of copper. The apparent Km for Fe2+ is 20 micrometer in the absence of copper and 9.5 micrometer in the presence of 20 micrometer CuCl2. The apparent Km for protoporphyrin is 50 micrometer, and it is unaltered by copper. Ferrochelatase has a single pH optimum of 7.50, and it is inhibited 50% by 20 micrometer heme. Certain divalent cations and sulfhydryl reagents also inhibit the enzyme.