A rapid spectrophotometric assay for ferrochelatase activity in preparations containing much endogenous hemoglobin and its application to soybean root-nodule preparations

Bacterial delta-aminolevulinic acid synthase activity is not essential for leghemoglobin formation in the soybean/Bradyrhizobium japonicum symbiosis

Previous studies of legume nodules have indicated that formation of the heme moiety of leghemoglobin is a function of the bacterial symbiont. We now show that a hemA mutant of Bradyrhizobium japonicum that cannot carry out the first step in heme biosynthesis forms fully effective nodules on soybeans. The bacterial mutant strain was constructed by first isolating the wild-type hemA gene encoding delta-aminolevulinic acid synthase (EC 2.3.1.37) from a cosmid library, using a fragment of the Rhizobium meliloti hemA gene as a hybridization probe. A deletion of the hemA gene region, generated in vitro, then was used to construct the analogous chromosomal mutation by gene-directed mutagenesis. The mutant strain had no delta-aminolevulinic acid synthase activity and was unable to grow in minimal medium unless delta-aminolevulinic acid was added. Despite its auxotrophy, the mutant strain incited nodules that appeared normal, contained heme, and were capable of high levels of acetylene reduction. These results rule out bacterial delta-aminolevulinic acid synthase activity as the exclusive source of delta-aminolevulinic acid for heme formation in soybean nodules.

Characterization of a Bradyrhizobium japonicum ferrochelatase mutant and isolation of the hemH gene

A Tn5-induced mutant of Bradyrhizobium japonicum, strain LORBF1, was isolated on the basis of the formation of fluorescent colonies, and stable derivatives were constructed in backgrounds of strains LO and I110. The stable mutant strains LOek4 and I110ek4 were strictly dependent upon the addition of exogenous hemin for growth in liquid culture and formed fluorescent colonies. The fluorescent compound was identified as protoporphyrin IX, the immediate precursor of protoheme. Cell extracts of strains LOek4 and I110ek4 were deficient in ferrochelatase activity, the enzyme which catalyzes the incorporation of ferrous iron into protoporphyrin IX to produce protoheme. Mutant strain I110ek4 could take up 55Fe from the growth medium, but, unlike the parent strain, no significant incorporation of radiolabel into heme was found. This observation shows that heme was not synthesized in mutant strain I110ek4 and that the heme found in those cells was derived from exogenous hemin in the growth medium. The putative protein encoded by the gene disrupted in strain LORBF1 and its derivatives was homologous to ferrochelatases from eukaryotic organisms. This homology, along with the described mutant phenotype, provides strong evidence that the disrupted gene is hemH, that which encodes ferrochelatase. Mutant strain I110ek4 incited nodules on soybean that did not fix nitrogen, contained few viable bacteria, and did not express leghemoglobin heme or apoprotein. The data show that B. japonicum ferrochelatase is essential for normal nodule development.

Cloning of murine ferrochelatase

Ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) catalyzes the last step in the heme biosynthetic pathway, the chelation of ferrous iron and protoporphyrin to form heme. The activity of ferrochelatase is deficient in the inherited disease protoporphyria. In this study, murine ferrochelatase cDNAs were obtained by screening cDNA libraries with an oligonucleotide probe. The derived amino acid sequence of murine ferrochelatase has 47% identity with the recently cloned Saccharomyces cerevisiae ferrochelatase, but it is not significantly similar to other published sequences. Results of Southern blotting are consistent with a single murine ferrochelatase gene, while Northern blotting demonstrates two ferrochelatase transcripts in all tissues examined. The ferrochelatase protein and mRNAs have different relative concentrations in different tissues. The cloning of murine ferrochelatase cDNAs provides the basis for future studies on ferrochelatase gene expression and on the identification of the molecular defect in protoporphyria.

Effects of the photobleaching herbicide, acifluorfen-methyl, on protoporphyrinogen oxidation in barley organelles, soybean root mitochondria, soybean root nodules, and bacteria

The photobleaching herbicide, acifluorfen-methyl (AFM), has been reported to be an inhibitor of the heme and chlorophyll biosynthetic enzyme protoporphyrinogen oxidase (Protox) in several plant species. However, AFM had no effect on the levels of Protox activity measured in a mitochondrial fraction from soybean roots. In contrast, AFM inhibited Protox activity in etioplasts from barley leaves and in mitochondria from barley roots, but the extent of inhibition varied depending upon the assay conditions and was maximal only in the presence of 5 mM dithiothreitol (DTT). AFM inhibition was enhanced by preincubation of barley organelle extract in the presence of DTT. Preincubation of barley extract with DTT and AFM together (but not with AFM alone) caused extensive enzyme inhibition which was not reversible by dialysis. These findings have implications for the mechanism of AFM action and for the differential effect of these herbicides on crop and weed species. AFM had no effect on the Protox activity of membranes from free-living bacterial cell of Bradyrhizobium japonicum or Escherichia coli, or on the high levels of Protox activity associated with the plant-derived membrane surrounding the symbiotic bacteria within the soybean root nodule.

Protoporphyrinogen oxidation, a step in heme synthesis in soybean root nodules and free-living rhizobia

Extracts of the crude bacteroid fraction of symbiotically grown Bradyrhizobium japonicum were much more active in oxidizing protoporphyrinogen to protoporphyrin than were extracts of cells grown under free-living conditions, especially when assayed in atmospheres containing only traces of oxygen. This correlates with the higher heme content of the microaerophilic nodules. Furthermore, the high level of oxidative activity in the crude bacteroid fraction was associated with an uncharacterized membrane fraction, probably of plant origin, that was separable from the bacteroids by Percoll gradient centrifugation.

Bacterial heme synthesis is required for expression of the leghemoglobin holoprotein but not the apoprotein in soybean root nodules

In Bradyrhizobium japonicum/soybean symbiosis, the leghemoglobin (legume hemoglobin) apoprotein is a plant product, but the origin of the heme prosthetic group is not known. B. japonicum strain LO505 is a transposon Tn5-induced cytochrome-deficient mutant; it excreted the oxidized heme precursor coproporphyrin III into the growth medium. Mutant strain LO505 was specifically deficient in protoporphyrinogen oxidase (protoporphyrinogen-IX:oxygen oxidoreductase, EC 1.3.3.4) activity, and thus it could not catalyze the penultimate step in heme biosynthesis. Soybean root nodules formed from this mutant did not contain leghemoglobin, but the apoprotein was synthesized nevertheless. Data show that bacterial heme synthesis is required for leghemoglobin expression, but the heme moiety is not essential for apoleghemoglobin synthesis by the plant. Soybean leghemoglobin, therefore, is a product of both the plant and bacterial symbionts.

Alterations in renal heme biosynthesis during metal nephrotoxicity

The regulation of the heme biosynthetic pathway in the kidney by various metals has been reviewed. In addition, a study on the effects of lead on renal heme biosynthesis after acute treatment of rats has been reported. Chronic low-level lead exposure in rats results in relatively small effects on renal heme biosynthetic pathway enzymes. After acute treatment of rats with lead, no effects on ALAD or UROS and mild, transitory effects on ALAS and ferrochelatase are observed. The intracellular binding of lead within intranuclear inclusion bodies in the proximal tubule cells and to high-affinity cytosolic lead-binding proteins probably protects sensitive subcellular systems, such as the heme pathway, from lead toxicity. Chronic exposure to methyl mercury results in increased urinary excretion of uro- and coproporphyrins in rats, mediated via inhibition of ferrochelatase and UROS and stimulation of ALAS. A tissue-specific inhibition of ALAD occurs in the kidney after treatment of rats with indium. Acute treatment of rats with nickel, platinum, tin, antimony, bismuth, and cobalt results in induction of heme oxygenase, followed by decreased microsomal heme content and ALAS stimulation in the kidney.

Tn5-induced cytochrome mutants of Bradyrhizobium japonicum: effects of the mutations on cells grown symbiotically and in culture

Two Bradyrhizobium japonicum cytochrome mutants were obtained by Tn5 mutagenesis of strain LO and were characterized in free-living cultures and in symbiosis in soybean root nodules. One mutant strain, LO501, expressed no cytochrome aa3 in culture; it had wild-type levels of succinate oxidase activity but could not oxidize NADH or N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). The cytochrome content of LO501 root nodule bacteroids was nearly identical to that of the wild type, but the mutant expressed over fourfold more bacteroid cytochrome c oxidase activity than was found in strain LO. The Tn5 insertion of the second mutant, LO505, had a pleiotropic effect; this strain was missing cytochromes c and aa3 in culture and had a diminished amount of cytochrome b as well. The oxidations of TMPD, NADH, and succinate by cultured LO505 cells were very similar to those by the cytochrome aa3 mutant LO501, supporting the conclusion that cytochromes c and aa3 are part of the same branch of the electron transport system. Nodules formed from the symbiosis of strain LO505 with soybean contained no detectable amount of leghemoglobin and had no N2 fixation activity. LO505 bacteroids were cytochrome deficient but contained nearly wild-type levels of bacteroid cytochrome c oxidase activity. The absence of leghemoglobin and the diminished bacterial cytochrome content in nodules from strain LO505 suggest that this mutant may be deficient in some aspect of heme biosynthesis.

Effects of lead on the heme biosynthetic pathway in rat kidney

Exposure of rats to lead in drinking water at concentrations of 500 ppm and 1000 ppm for 3 and 6 months resulted in elevated blood lead levels, formation of kidney intranuclear inclusion bodies, and increased urinary excretion of uroporphyrin and coproporphyrin. The erythrocytic Zn-protoporphyrin was increased in the highest dose group. No significant effects on body weight gain or kidney weight were observed. Renal activity of delta-aminolevulinic acid synthetase (ALAS) was not significantly affected by lead treatment. The renal activity of delta-aminolevulinic acid dehydrase (ALAD) was moderately increased and ferrochelatase activity was significantly decreased. The relatively small effects of chronic lead exposure on renal heme biosynthesis suggests that intracellular complexation of lead with high-affinity renal lead binding proteins (PbBP) and formation of intranuclear inclusions in proximal tubule cells protects this highly sensitive pathway in kidney from lead inhibition in vivo. These data also suggest that the observed increases in urinary porphyrin excretion are primarily due to lead effects on the erythropoietic system.

Separation of haem compounds by reversed-phase ion-pair high-performance liquid chromatography and its application in the assay of ferrochelatase activity

The separation of haems and porphyrins was achieved in a reversed-phase ion-pair high-performance liquid chromatography system using tetrabutylammonium hydrogen sulphate as the pairing ion. The concentration of methanol and pH in the mobile phase were determinative parameters for the elution pattern of the compounds. Two isocratic systems--one for the assay of protohaem IX and one for deuterohaem IX--were developed. The chromatographic systems were applied to the assay of ferrochelatase activity in mitochondria using either protoporphyrin or deuteroporphyrin as the substrate. The ferrochelatase activity was also measured in reticulocytes, which contain high levels of endogenous haem.

Studies on the mechanisms of thallium-mediated inhibition of hepatic mixed function oxidase activity. Correlation with inhibition of NADPH-cytochrome c (P-450) reductase

Thallium (TlCl3) administration to rats produced a dose-dependent loss of hepatic NADPH-cytochrome c (P-450) reductase and microsomal mixed function oxidase activities within 2-4 hr following treatment. These changes occurred independently of apparent effects on microsomal heme or cytochrome P-450 content, both of which remained unchanged with respect to control levels despite transient inhibition of delta-aminolevulinic acid (ALA) synthetase and induction of heme oxygenase. These results are consistent with the recognized properties of thallium as both a flavoprotein antagonist and sulfhydryl inhibitor and differ uniquely from the action of other metals which impair mixed function oxidase activity through compromise of heme biosynthesis and heme depletion. The potential utility of thallium compounds in further evaluating the functional characteristics of NADPH-cytochrome c (P-450) reductase and its role in microsomal oxidative processes is suggested from these observations.

Sex differences in haem biosynthesis and porphyrin content in the Harderian gland of the golden hamster

Methods are described for the measurement of seven haem biosynthetic enzymes in Harderian gland tissue from male and female golden hamsters. Sex differences were found in five of the seven enzymes. In each case, female tissue exhibited higher activity than male tissue. These differences in enzyme activity are sufficient to account for the major sex difference in porphyrin content in the Harderian gland of this species.

Deficient heme synthesis as the cause of noninducibility of hemoglobin synthesis in a Friend erythroleukemia cell line

Friend cells of the line Fw are not induced to accumulate substantial amounts of hemoglobin and to become benzidine-positive when treated with butyric acid or other inducers, except in the presence of exogenous hemin. The cells are shown to have a deficiency in heme synthesis since they require exogenous hemin during the period of maximal hemoglobin synthesis; since endogenous heme synthesis cannot be induced to the level found in normal inducible Friend cells, even after hemoglobin synthesis has been induced by hemin and butyric acid and the hemin has then been withdrawn; since they are not inducible for ferrochelatase (heme synthetase) activity; and since they accumulate free globin chains after stimulation with butyric acid in the absence of hemlin. Comparison of globin synthesis and globin mRNA content of the cells shows that globin synthesis is not controlled by the hemin-controlled repressor of protein synthesis (HCR) nor by any specific translational control of globin synthesis by hemlin.

Heme synthetase activity in normal human and rat erythroid cells and in sideroblastic anemia

The enzyme heme synthetase, involved in the final step of the biosynthesis of heme, has been assessed in rat and human bone marrow and peripheral blood. The pH optimum of the enzyme in bone marrow appeared to be pH 7.6, whereas the Michaelis constant in human and rat bone marrow was found to be 1.6 micrometer and 0.6 micrometer, respectively. Rat reticulocytes showed approximately 100-times higher heme synthetase activities than did rat erythrocytes. By contrast, human reticulocytes did not show significantly higher activities than human erythrocytes. This difference between rat and human reticulocytes could be confirmed by in vitro experiments with intact cells in which iron uptake and heme synthesis of human and rat cells were compared. Finally, heme synthetase activity was assessed in bone marrow cells of two patients with sideroblastic anemia. In both cases the enzyme activities were found to be comparable to those in control bone marrow.

Studies on the ferrochelatase activity of isolated rat liver mitochondria with special reference to the effect of oxidizable substrates and oxygen concentration

The mitochondrial ferrochelatase activity has been studied in coupled rat liver mitochondria using deuteroporphyrin IX (incorporated into liposomes of lecithin) and Fe(III) or Co(II) as the substrates. 1. It was found that respiring mitochondria catalyze the insertion of Fe(II) and Co(II) into deuteroporphyrin. When Fe(III) was used as the metal donor, the reaction revealed an absolute requirement for a supply of reducing equivalents supported by the respiratory chain. 2. A close correlation was found between the disappearance of porphyrin and the formation of heme which allows an accurate estimate of the extinction coefficient for the porphyrin to heme conversion. The value deltae (mM-1 - cm-1) = 3.5 for the wavelength pair 498 509 nm, is considerably lower than previously reported. 3. The maximal rate of deuteroheme synthesis was found to be approx. 1 nM - min-1 - mg-1 of protein at 37 degrees C, PH 7.4 and optimal substrate concentrations, i.e. 75 muM Fe(III) and 50 muM deuteroporphyrin. 4. Provided the mitochondria are supplemented with an oxidizable substrate, the presence of oxygen has no effect on the rate of deuteroheme synthesis.