An h.p.l.c. assay for protoporphyrinogen oxidase activity in rat liver

Flumioxazin metabolism in pregnant animals and cell-based protoporphyrinogen IX oxidase (PPO) inhibition assay of fetal metabolites in various animal species to elucidate the mechanism of the rat-specific developmental toxicity

Flumioxazin, an N-phenylimide herbicide, inhibits protoporphyrinogen oxidase (PPO), a key enzyme in heme biosynthesis in mammals, and causes rat-specific developmental toxicity. The mechanism has mainly been clarified, but no research has yet focused on the contribution of its metabolites. We therefore conducted in vivo metabolism studies in pregnant rats and rabbits, and found 6 major known metabolites in excreta. There was no major rat-specific metabolite. The most abundant component in rat fetuses was APF, followed by flumioxazin and 5 identified metabolites. The concentrations of flumioxazin and these metabolites in fetuses were lower in rabbits than in rats. In vitro PPO inhibition assays with rat and human liver mitochondria showed that flumioxazin is a more potent PPO inhibitor than the metabolites. There were no species differences in relative intensity of PPO inhibition among flumioxazin and these metabolites. Based on the results of these in vivo and in vitro experiments, we concluded that flumioxazin is the causal substance of the rat-specific developmental toxicity. As a more reliable test system for research on in vitro PPO inhibition, cell-based assays with rat, rabbit, monkey, and human hepatocytes were performed. The results were consistent with those of the mitochondrial assays, and rats were more sensitive to PPO inhibition by flumioxazin than humans, while rabbits and monkeys were almost insensitive. From these results, the species difference in the developmental toxicity was concluded to be due to the difference in sensitivity of PPO to flumioxazin, and rats were confirmed to be the most sensitive of these species.

Mechanism of Developmental Effects in Rats Caused by an N-Phenylimide Herbicide: Transient Fetal Anemia and Sequelae during Mid-to-Late Gestation

BACKGROUND

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects [VSDs] and wavy ribs) was produced by an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. Major characteristics of the developmental toxicity included species difference between rats and rabbits, compound-specific difference among structurally similar herbicides, and sensitive period. Protoporphyrin accumulation in treated fetuses closely correlated with the major characteristics. Iron deposits in erythroblastic mitochondria and degeneration of erythroblasts were observed in treated rat fetuses. In this study we investigated fetal anemia and subsequent developmental effects in rats, and inhibition of PPO in rats, rabbits, and humans by the herbicides in vitro.

METHODS

Fetuses were treated on gestational day (GD) 12 and removed on GDs 13 through 20. All litters were examined externally. One half of litters were examined for blood and skeletal development, and the other half for interventricular foramen closure. Effects on PPO were determined in mitochondria from embryos and adult livers.

RESULTS

Fetal anemia in rats was evident on GDs 13 through 16. Subsequently, enlarged heart, delayed closure of the foramen, reduced serum protein, and retarded rib ossification were observed. In vitro PPO inhibition exhibited species- and compound-specific differences corresponding to the developmental toxicity.

CONCLUSION

We propose that developmental toxicity results from PPO inhibition in primitive erythroblasts, causing transient fetal anemia followed by death. Compensatory enlargement of the fetal heart results in failure of interventricular foramen closure and VSD. Reduced serum protein leads to delayed ossification and wavy ribs.

Impairment of heme biosynthesis induces short circadian period in body temperature rhythms in mice

It has been demonstrated that the function of mammalian clock gene transcripts is controlled by the binding of heme in vitro. To examine the effects of heme on biological rhythms in vivo, we measured locomotor activity (LA) and core body temperature (Tb) in a mouse model of porphyria with impaired heme biosynthesis by feeding mice a griseofulvin (GF)-containing diet. Mice fed with a 2.0% GF-containing diet (GF2.0) transiently exhibited phase advance or phase advance-like phenomenon by 1–3 h in terms of the biological rhythms of Tbor LA, respectively (both, P < 0.05) while mice were kept under conditions of a light/dark cycle (12 h:12 h). We also observed a transient, ∼0.3 h shortening of the period of circadian Tbrhythms in mice kept under conditions of constant darkness ( P < 0.01). Interestingly, the observed duration of abnormal circadian rhythms in GF2.0 mice lasted between 1 and 3 wk after the onset of GF ingestion; this finding correlated well with the extent of impairment of heme biosynthesis. When we examined the effects of therapeutic agents for acute porphyria, heme, and hypertonic glucose on the pathological status of GF2.0 mice, it was found that the intraperitoneal administration of heme (10 mg·kg−1·day−1) or glucose (9 g·kg−1·day−1) for 7 days partially reversed (50%) increases in urinary δ-aminolevulinic acids levels associated with acute porphyria. Treatment with heme, but not with glucose, suppressed the phase advance (-like phenomenon) in the diurnal rhythms ( P < 0.05) and restored the decrease of heme ( P < 0.01) in GF2.0 mice. These results suggest that impairments of heme biosynthesis, in particular a decrease in heme, may affect phase and period of circadian rhythms in animals.

Site-directed mutagenesis and computational study of the Y366 active site in Bacillus subtilis protoporphyrinogen oxidase

Protoporphyrinogen IX oxidase (PPO), the last common enzyme of heme and chlorophyll biosynthesis, catalyses the oxidation of protoporphyrinogen IX to protoporphyrin IX, with FAD as cofactor. Among PPO, Bacillus subtilis PPO (bsPPO) is unique because of its broad substrate specificity and resistance to inhibition by diphenylethers. Identification of the activity of bsPPO would help us to understand the catalysis and resistance mechanisms. Based on the modeling and docking studies, we found that Y366 site in bsPPO was adjacent to substrate and FAD. In order to evaluate the functional role of this site, three mutants Y366A Y366E and Y366H were cloned and kinetically characterized. The efficiency of catalysis for Y366A and Y366H reduced to 10% of the wild-type enzyme's activity, while Y366E just retained 1%. Y366E shows large resistance (K (i) = 153.94 microM) to acifluorfen. Molecular docking was carried out to understand the structure and functional relationship of PPO. The experimental results from the site-directed mutagenesis are consistent with the computational studies. The residue at position 366 is seemed to be responsible for substrate binding and catalysis and involved in herbicide resistance of bsPPO.

A capillary electrophoresis assay for recombinant Bacillus subtilis protoporphyrinogen oxidase

Protoporphyrinogen oxidase (PPO) is a flavin adenine dinucleotide (FAD)-containing enzyme in the tetrapyrrole biosynthetic pathway that leads to the formation of both heme and chlorophylls, which has been identified as one of the most important action targets of commercial herbicides. The literature reports gave different PPO-catalytic kinetic parameters for the substrate protoporphyrinogen IX (K(m) of 0.1 to 10.4 miocroM) with different sources of PPO using fluorescent or HPLC methods. Herein we assayed the enzymatic activity of recombinant Bacillus subtilis PPO by using capillary electrophoresis (CE), a method with high separation efficiency, easy automation, and low sample consumption. The Michaelis constant and maximum reaction velocity were determined as 7.0+/-0.6 miocroM and 0.38+/-0.02 miocromol min(-1)miocrog(-1), respectively. The interaction between PPO and acifluorfen, a commercial PPO-inhibiting herbicide, was measured as the inhibition constant 186.9+/-9.3 miocroM EM, Cyrillic. The relationship between cofactor FAD and PPO activity can also be quantitatively studied by this CE method. The CE method used here should also be a convenient, reliable method for PPO study.

Measurement of protoporphyrinogen oxidase activity

Protoporphyrinogen oxidase catalyzes the oxidation of protoporphyrinogen to protophyrin. It is a membrane-bound mitochondrial enzyme and it is the target of photobleaching herbicides. The basic assay presented in this unit for measuring oxidase activity is based on oxidation of the colorless, nonfluorescent substrate, protoporphyrinogen, to the colored, fluorescent protophyrin. Alternate protocols are provided for the measuring the accumulation of protoporphyrinogen resulting from a decrease in oxidase activity due to treatment with diphenyl ether herbicides or oxidase inhibitor.

Sensitive isocratic liquid chromatographic assay for the determination of 5,10,15,20-tetra(m-hydroxyphenyl)chlorin in plasma and tissue with electrochemical detection

A simple extraction procedure and a sensitive high-performance liquid chromatographic (HPLC) method are described for the determination of the photodynamic therapeutic agent 5,10,15,20-tetra(m-hydroxyphenyl)chlorin (mTHPC) in plasma and tumour tissue. Reversed-phase high-performance liquid chromatography was performed on a C18 column (70 x 4.6 mm I.D.) with a mobile phase of 0.01 M potassium dihydrogenphosphate buffer, pH 2.5-acetonitrile (55:45, v/v) and a coulometric detection (+0.80 V). The mean recoveries of mTHPC in the concentration ranges (5-2000 and 10-1000 ng/ml) were 90 and 89% for plasma and tumour samples, respectively. The procedure for plasma and tissue preparation involved solvent precipitation using methanol combined with ammonia solution and dimethyl sulphoxide (4, 0.2, 0.1, v/v/v) and (2, 0.1, 0.1, v/v/v) for plasma and tissue, respectively. For mTHPC at concentrations ranging from 5 to 2000 ng/ml, the within-day relative standard deviations, based on triplicate determinations were less than 8% and the between-day relative standard deviations calculated by performing extraction procedure of plasma samples on three different days ranged from 3 to 18%. This highly sensitive method, 5 and 10 ng/ml for plasma and tissue respectively, was applied successfully to the determination of mTHPC in mouse tumours for pharmacokinetic studies.

Preparation and separation of hydroxy derivatives of uroporphyrinogen I by high-performance liquid chromatography with electrochemical detection

The preparation and high-performance liquid chromatography (HPLC) separation of meso-hydroxyuroporphyrinogen I, hydroxyacetic acid uroporphyrinogen I and beta-hydroxypropionic acid uroporphyrinogen I is described. meso-Hydroxyuroporphyrin I, hydroxyacetic acid uroporphyrin I and beta-hydroxypropionic acid uroporphyrin I were isolated from the urine of a patient with congenital erythropoietic porphyria. The porphyrins were reduced to the corresponding porphyrinogens with 3% (w/w) Na/Hg amalgam. The hydroxy porphyrinogens were separated on a Hypersil ODS column with 4% (v/v) acetonitrile in 1 M ammonium acetate buffer, pH 5.16, containing EDTA (0.27 mM) as the mobile phase, and detected electrochemically. Reduction of meso-hydroxyuroporphyrin I and hydroxyacetic acid uroporphyrin I, followed by HPLC analysis, showed that, in addition to the expected formation of meso-hydroxyuroporphyrinogen I and hydroxyacetic uroporphyrinogen I, respectively, uroporphyrinogen I was also produced. Reduction of beta-hydroxypropionic acid uroporphyrin I, however, gave beta-hydroxypropionic acid uroporphyrinogen I, acrylic acid uroporphyrinogen I and uroporphyrinogen I as the products. The peaks were identified by conversion into the porphyrin methyl esters and analysed by liquid secondary-ion mass spectrometry.

Determination of 5,10,15,20-tetra-(m-hydroxyphenyl)chlorin in human plasma by high performance liquid chromatography

A high performance liquid chromatographic method for the determination of the photodynamic chemotherapeutic agent 5,10,15,20-tetra(m-hydroxyphenyl)chlorin (m-THPC) in human plasma following intravenous infusion is described. The procedure involves extraction of the drug in plasma with methanol/dimethyl sulphoxide (4:1 v/v) containing 5,10,15,20-tetra(p-hydroxyphenyl)chlorin as the internal standard and separation on a C18 reversed phase column with acetonitrile:0.1% trifluoroacetic acid (77:23 v/v) as the mobile phase. The drug was detected specifically and sensitively at its absorption maximum of 423 nm with a detection limit of 15 ng/mL (signal-to-noise ratio of 5). The intra- and inter-assay coefficients of variation (CV) on analysis of a plasma spiked with m-THPC (1 micrograms/mL) were 2.3 and 3.4% (n = 6), respectively.

Lead and the terminal mitochondrial enzymes of haem biosynthesis

Lead exposure causes increases in urinary coproporphyrin excretion and the accumulation of zinc protoporphyrin in red cells. In the conventional view of the effect of lead on haem biosynthesis, the accumulation of these metabolites results from lead inhibition of two of the mitochondrial enzymes of haem biosynthesis, coproporphyrinogen oxidase (EC 1.3.3.3) and ferrochelatase (EC 4.99.1.1). This review critically assesses the evidence for the inhibition of these enzymes. We consider this evidence to be inconclusive and alternative explanations for the increased concentrations of coproporphyrin and zinc protoporphyrin are proposed.

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.

Coproporphyrinogen oxidase, protoporphyrinogen oxidase and ferrochelatase activities in iron-overloaded and ethanol-treated rats

The activities of coproporphyrinogen oxidase, protoporphyrinogen oxidase and ferrochelatase in liver homogenates of iron-overloaded and acutely or chronically ethanol-treated rats were assayed by high performance liquid chromatographic methods. Decreased coproporphyrinogen oxidase activity was found in chronically carbonyliron-fed rats while the activities of protoporphyrinogen oxidase and ferrochelatase remained unchanged. Coproporphyrinogen oxidase and protoporphyrinogen oxidase activities in both chronically and acutely ethanol-treated animals were not significantly different from controls. In chronically ethanol-fed rats normal ferrochelatase activity was found.

Homozygous variegate porphyria: a case report

Homozygous variegate porphyria is described in a 14-year-old girl with a unique clinical presentation of photosensitivity from the second year of life, mental retardation, clinodactyly, and normal growth rate. The erythrocyte protoporphyrin concentration was raised with the protoporphyrin being predominantly zinc-chelated, which appears to be characteristic for all homozygous hepatic porphyrias. Protoporphyrinogen oxidase activity in lymphoblasts was decreased in both patient and parents despite the latter having normal porphyrin excretion.

Coproporphyrinogen oxidase, protoporphyrinogen oxidase and ferrochelatase activities in human liver biopsies with special reference to alcoholic liver disease

The activities of coproporphyrinogen oxidase, protoporphyrinogen oxidase and ferrochelatase have been assayed in human liver biopsies using recently developed highly sensitive specific enzyme assays. The specific activities (nmol/min/mg protein) in controls were 0.010 +/- 0.003 (mean +/- S.D., n = 11), 0.18 +/- 0.07 (n = 9) and 0.062 +/- 0.022 (n = 8), respectively. The total activities (mumol/min/liver) were determined, using ultrasound to determine liver volumes, and were 2.6 +/- 0.6 (n = 5), 36.6 +/- 13.9 (n = 6) and 14.2 +/- 5.4 (n = 3), respectively. Both specific and total enzyme activities in alcoholics with fatty liver were not significantly different from normal controls. Decreased protoporphyrinogen oxidase activity (0.08 nmol/min/mg protein or 20.2 mumol/min/liver) was found in two patients with variegate porphyria. In a patient with erythrohepatic protoporphyria a reduction of the ferrochelatase activity (0.01 nmol/min/mg protein) was demonstrated.

High-performance liquid chromatography of porphyrins

Techniques for the analysis of porphyrins in the biomedical fields are reviewed. The emphasis is on high-performance liquid chromatography and its aspplications in: (1) the quantitative analysis of porphyrins in blood, urine and faeces; (2) qualitative porphyrin profiles in normal subjects and in the porphyrias; (3) assay of haem biosynthetic enzyme activities and (4) resolution of type isomers of porphyrins and porphyrinogens. Detection systems, quantitation methods, peak identification and sample preparation procedures are discussed.

An HPLC assay for rat liver ferrochelatase activity

A rapid, reliable, sensitive and reproducible HPLC method was developed for the assay of ferrochelatase activity in rat liver. The assay was carried out aerobically with Zn2+ and mesoporphyrin or protoporphyrin IX as substrates. Zn-porphyrins formed were extracted with dimethyl sulphoxide/methanol (30:70, v/v) containing Zn-deuteroporphyrin as the internal standard for separation and quantification by reversed-phase chromatography. The Km for mesoporphyrin was 5.9 microM, for protoporphyrin IX 8.8 microM and for zinc 6.0 microM. The specific activities were 33.1 +/- 5.0 nmol Zn-mesoporphyrin or 13.4 +/- 2.0 nmol Zn-protoporphyrin formed per hour per mg of protein for mitochondria and 12.3 +/- 2.2 nmol Zn-mesoporphyrin or 4.6 +/- 0.9 nmol Zn-protoporphyrin per hour per mg of protein for liver homogenate.