Homologous NF-YC2 subunit from Arabidopsis and tobacco is activated by photooxidative stress and induces flowering

The transcription factor NF-Y consists of the three subunits A, B and C, which are encoded in Arabidopsis in large gene families. The multiplicity of the genes implies that NF-Y may act in diverse combinations of each subunit for the transcriptional control. We aimed to assign a function in stress response and plant development to NF-YC subunits by analyzing the expression of NF-Y genes and exploitation of nf-y mutants. Among the subunit family, NF-YC2 showed the strongest inducibility towards oxidative stress, e.g. photodynamic, light, oxidative, heat and drought stress. A tobacco NF-YC homologous gene was found to be inducible by photooxidative stress generated by an accumulation of the tetrapyrrole metabolite, coproporphyrin. Despite the stress induction, an Arabidopsis nf-yc2 mutant and NF-YC2 overexpressors did not show phenotypical differences compared to wild-type seedlings in response to photooxidative stress. This can be explained by the compensatory potential of other members of the NF-YC family. However, NF-YC2 overexpression leads to an early flowering phenotype that is correlated with increased FLOWERING LOCUS T-transcript levels. It is proposed that NF-YC2 functions in floral induction and is a candidate gene among the NF-Y family for the transcriptional activation upon oxidative stress.

Characterization of the heme synthesis enzyme coproporphyrinogen oxidase (CPO) in zebrafish erythrogenesis

Hemoglobin consists of heme and globin proteins and is essential for oxygen transport in all vertebrates. Although biochemical features of heme synthesis enzymes have been well characterized, the function of these enzymes in early embryogenesis is not fully understood. We found that the sixth heme synthesis enzyme, coproporphyrinogen oxidase (CPO), is predominantly expressed in the intermediate cell mass (ICM) that is a major site of zebrafish primitive hematopoiesis. Knockdown of zebrafish CPO using anti-sense morpholinos (CPO-MO) leads to a significant suppression of hemoglobin production without apparent reduction of blood cells. Injection of human CPO RNA, but not a mutant CPO RNA that is similar to a mutant responsible for a hereditary coproporphyria (HCP), restores hemoglobin production in the CPO-MO-injected embryos. Furthermore, expression of CPO in the ICM is severely suppressed in both vlad tepes/gata1 mutants and in biklf-MO-injected embryos. In contrast, over-expression of biklf and gata1 significantly induces ectopic CPO expression. The function of CPO in heme biosynthesis is apparently conserved between zebrafish and human, suggesting that CPO-MO-injected zebrafish embryos might be a useful in vivo assay system to measure the biological activity of human CPO mutations.

A cascade analysis of the interaction of mercury and coproporphyrinogen oxidase (CPOX) polymorphism on the heme biosynthetic pathway and porphyrin production

Mercury (Hg) exposure in various forms remains a persistent public health concern in many parts of the world. In previous studies, we have described a biomarker of mercury exposure characterized by increased urinary concentrations of specific porphyrins, pentacarboxyporphyrin (5-CP) and coproporphyrin (4-CP), and the atypical keto-isocoproporphyrin (KICP), based on selective interference with the fifth (uroporphyrinogen decarboxylase, UROD) and sixth (coproporphyrinogen oxidase, CPOX) enzymes of the heme biosynthetic pathway. Whereas this response occurs in a predictable manner among approximately 85% of subjects with Hg exposure, an atypical porphyrinogenic response (APR) has been observed in approximately 15% of Hg-exposed persons, in which the three porphyrins that are affected by Hg, i.e., 5-CP, 4-CP and, KICP, are excreted in substantial excess of that predicted on the basis of Hg exposure alone. This APR has been attributed to a specific polymorphism in exon 4 of the CPOX gene (CPOX4). In the present study, we sought to further confirm the hypothesis that the observed changes in porphyrin excretion patterns might serve as a biomarker of Hg exposure and potential toxicity by statistically modeling the cascading effects on porphyrin concentrations within the heme biosynthetic pathway of Hg exposure and CPOX4 polymorphism in a human population with long-term occupational exposure to elemental mercury. Our results are highly consistent with this hypothesis. After controlling for precursor porphyrin concentrations, we demonstrated that 5-CP and 4-CP are independently associated with Hg concentration, while KICP is associated only with the CPOX4. An unpredicted association of Hg with heptacarboxyporphyrin (7-CP) may indicate a previously unidentified point of mercury inhibition of UROD. These findings lend further support to the proposed utility of urinary porphyrin changes as a biomarker of exposure and potential toxicity in subjects with mercury exposure. Additionally, these findings demonstrate the successful application of a computational model for characterizing complex metabolic responses and interactions associated with both toxicant exposure and genetic variation in human subjects.

Normal and Abnormal Heme Biosynthesis. 3.1Synthesis and Metabolism of Tripropionate Analogues of Coproporphyrinogen-III: Novel Probes for the Active Site of Coproporphyrinogen Oxidase

Coproporphyrinogen oxidase (copro'gen oxidase) catalyses the oxidative decarboxylation of two propionate side chains on coproporphyrinogen-III to produce protoporphyrinogen-IX. This process is very poorly understood at a molecular level, and copro'gen oxidase remains one of the least well-characterized enzymes in the heme biosynthetic pathway. To provide a rigorous test for a proposed model for substrate recognition and binding by this enzyme, two tripropionate analogues of copro'gen-III were prepared where an ethyl group replaced one of the usual propionate residues on positions 13 or 17. Although the required substrate probes are porphyrinogens (hexahydroporphyrins), the corresponding porphyrin methyl esters were initially synthesized via tripyrrene and a,c-biladiene intermediates. These were hydrolyzed and reduced with 3% sodium-amalgam to give the unstable porphyrinogens needed for the biochemical investigations. The modified structure with a 13-ethyl moiety was metabolized by avian preparations of copro'gen oxidase to give a monovinylic product, but the isomeric 17-ethylporphyrinogen afforded a divinylic product, albeit with poorer overall conversion. These results strongly support the proposed model for substrate binding at the active site of copro'gen oxidase.

Effect of acute lead treatment on coproporphyrinogen oxidase activity in HepG2 cells

Acute toxic effects of lead were evaluated on porphyrin synthesis and coproporphyrinogen oxidase (CO) activity in an in vitro model, using HepG2 cells, a hepatoma cell line of human origin. Lead concentrations for exposure treatments were 0.5, 1.0, 2.5, 5.0 microM. No significant changes were found in treated cells with respect to uroporphyrin cellular or media concentrations. Cellular protoporphyrin increased in dose response shape, but no changes in extracellular content were found. Extracellular coproporphyrin concentration increased in a dose response manner without changes in cellular content. The CO activity was depressed in dose response shape, reaching 62% of control activity at 5.0 microM of lead treatment. The CO activity in Pb-treated cells was recovered after dithiothreitol (DTT) treatment, suggesting that sulphydryl groups play an essential role in the enzyme activity. The dose-response increase of coproporphyrin secretion accompanied by the depression of CO activity supports the suggestion that lead causes CO inhibition, as observed in this cellular model.

Time-dependent porphyric response in mice subchronically exposed to arsenic

1. A time-course study was carried out in mice subchronically exposed to As III (as sodium arsenite) or As V (as sodium arsenate), via drinking water, relating the pattern of urinary porphyrin excretion to the renal and hepatic enzyme activities of porphobilinogen deaminase (PBGD), uroporphyrinogen III synthetase (URO III-S), uroporphyrinogen decarboxylase (URO-D) and coproporphyrinogen oxidase (COPRO-O), as well as to the hepatic porphyrin accumulation in the treated animals. 2. A time-dependent, wave-like porphyric response was found in mice exposed to As V, and the increases seen in total urinary porphyrins (at 3 weeks of exposure) corresponded to an increased activity of PBGD and Uro III-S in liver. 3. Significant decreases in renal URO-D and hepatic and renal COPRO-O activities were found in treated mice; these inhibitions were more pronounced in animals exposed to As III. 4. The combination of these enzymic effects may explain the time-dependent porphyric response of mice subchronically exposed to As. Finally, the relative magnitudes of URO-D and COPRO-O inhibitions may determine the pattern of porphyrin concentration observed in urine and tissues. 5. The decrease in renal URO-D activity may help to explain the inversion in the coproporphyrin/uroporphyrin ratio previously reported in humans chronically exposed to As; however, there were differences between the urinary porphyrin profiles found in both species. The possible reasons for the similarities and differences are briefly discussed.

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.

Inhibition of human lymphocyte coproporphyrinogen oxidase activity by metals, bilirubin and haemin

Coproporphyrinogen III oxidase activity in human lymphocytes was found to be inhibited by cadmium and mercury but not by lead. The organo-metal compounds tributyltin and methylmercury were effective inhibitors of this haem biosynthetic pathway enzyme. Haemin (the ultimate product of the pathway) and bilirubin (a product of haem catabolism) were also shown to be inhibitory. Kinetic studies performed under initial velocity conditions showed that bilirubin was a non-competitive inhibitor and that one bilirubin molecule was bound to both the enzyme and enzyme substrate complex. The analysis also showed haemin to be a non-competitive inhibitor in which two haemin molecules bind to the enzyme whereas the enzyme substrate complex accepts only one haemin molecule. The possible physiological significance of the inhibition of coproporphyrinogen III oxidase activity by haemin and bilirubin is discussed.

Studies on the etiology of trace metal-induced porphyria: effects of porphyrinogenic metals on coproporphyrinogen oxidase in rat liver and kidney

Studies were conducted on the etiology of trace metal-induced porphyria in rats, with particular emphasis on the action of metals on hepatic and renal coproporphyrinogen oxidase. Prolonged exposure of rats to methyl mercury hydroxide or sodium arsenate at subtoxic dose levels in drinking water resulted in a progressive coproporphyrinuria, reaching highest rates of coproporphyrin excretion 5 weeks after initiation of exposure. The development of coproporphyrinuria was accompanied by substantial metal accumulation in the kidney and a significant decrease in renal, but not hepatic, coproporphyrinogen oxidase activity. During prolonged exposure to either metal, the rates of coproporphyrin excretion and metal accumulation by the kidney continued to increase for 2 to 3 weeks following maximal inhibition of renal coproporphyrinogen oxidase. Acute treatment studies and studies in vitro support the conclusion that the kidney is the principal source of excess urinary coproporphyrin during metal exposure. These observations demonstrate that metal-induced coproporphyrinuria is predominantly of renal etiology and that impairment of renal coproporphyrinogen oxidase is a principal cause of this effect.