Studies on verdohemochrome

Suicide-peroxide inactivation of horseradish peroxidase in the presence of sodium n-dodecyl sulphate: a study of the enzyme deactivation kinetics

In the presence of the anionic surfactant sodium n-dodecyl sulphate (SDS), horseradish peroxidase (HRP) undergoes a deactivation process. Suicide inactivation of horseradish peroxidase by hydrogen peroxide(3 mM) was monitored by the absorbance change in product formation in the catalytic reaction cycle. The progress curve of the catalytic reaction cycle was obtained at 27degrees C and phosphate buffer 2.5 mM (pH = 7.0). The corresponding kinetic parameters i.e., intact enzyme activity (alpha i); the apparent rate constant of suicide inactivation by peroxide (ki); and the apparent rate constants of enzyme deactivation by surfactant (kd) were evaluated from the obtained kinetic equations. The experimental data are accounted for by the equations used in this investigation. Addition of SDS to the reaction mixture intensified the inactivation process. The deactivation ability of denaturant could be resolved from the observed inactivation effect of the suicide substrate by applying the proposed model. The results indicate that the deactivation and the inactivation processes are independent of each other.

Separation and identification of the regioisomers of verdoheme by reversed-phase ion-pair high-performance liquid chromatography, and characterization of their complexes with heme oxygenase

We report an HPLC method for separating the four regioisomers of verdoheme formed in the coupled oxidation of hemin with oxygen and ascorbate in aqueous pyridine. The reversed-phase ion-pair system uses hexafluoroacetone and pyridine as ion-pair agents. The regiochemistry of the separated isomers was established both by HPLC of the corresponding biliverdin IX derivatives and by 1H NMR of each isomer. Optical spectra of the pyridine verdohemochrome isomers were similar to each other, but showed differences in the absorption maxima in the red region, which appear at 680, 663, 648 and 660 nm for the alpha, beta, gamma, and delta-isomers, respectively. Each of the four isomers was incorporated anaerobically into heme oxygenase-1, yielding the corresponding verdoheme-enzyme complex. The ferrous forms had absorption maxima at 690, 667, 655, and 663 nm, and their CO-bound forms had maxima at 638, 624, 616, and 626 nm for alpha, beta, gamma, and delta-isomer, respectively. Addition of ferricyanide to the alpha-verdoheme-heme oxygenase complex brought about a ferric low-spin heme-like signal, which is identical with the ferric alpha-verdoheme complexed with the heme oxygenase that was observed in the heme oxygenase reaction.

On the mechanism of the chemical and enzymic oxygenations of alpha-oxyprotohemin IX to Fe.biliverdin IX alpha

alpha-Oxyprotohemin IX, an early intermediate in heme catabolism, was synthesized and its autoxidation to biliverdin IX alpha was studied. In anaerobic aqueous pyridine, alpha-oxyprotohemin (hexacoordinated) underwent autoreduction to yield an Fe(II) alpha-oxyprotoporphyrin pi-neutral radical bis(pyridine) complex, which reacted with an equimolar amount of dioxygen to give pyridine.verdohemochrome IX alpha and CO in 75-80% yield via an intermediate with an absorption maximum at 893 nm. Verdohemochrome IX alpha did not react with further dioxygen. Reconstituted apomyoglobin.alpha-oxyprotohemin IX complex (pentacoordinated) reacted with an equimolar amount of dioxygen to form an Fe(II) oxyporphyrin pi-neutral radical intermediate, which rearranged to a green compound (lambda max 660 and 704 nm) with elision of CO. The green product, which is probably an apomyoglobin.verdoheme pi-radical complex, reacted with another equimolar amount of dioxygen to give Fe(III).biliverdin IX alpha. Demetallation of this gave biliverdin IX alpha in overall yield of 70-75%. These results indicate that the sequence of oxyheme autoxidation in the presence of apomyoglobin is alpha-oxyprotoheme IX O2----CO----verdohemochrome IX alpha pi-radical O2----Fe(III).biliverdin IX alpha. A similar mechanism may prevail in vivo. The hexa- and pentacoordinated Fe(II) pi-radical form of the oxyporphyrin is crucial in triggering the autoxidation of the complex to verdohemochrome IX alpha. Further oxygenation of verdohemochrome IX alpha to Fe(III).biliverdin IX alpha occurred only in the pentacoordinated apomyoglobin.verdoheme Fe(II) complex.

Structure and apoprotein linkages of phycourobilin

R-Phycoerythrin contains two covalently bound bilin prosthetic groups, phycoerythrobilin and phycourobilin. The two chromophore types were separated as their peptide-bound derivatives by subjecting tryptic digests of R-phycoerythrin to adsorption chromatography on Sephadex G-25. The structure and apoprotein linkages of the bound phycoerythrobilin were found to be identical with those previously reported for this phycobilin [Killilea, O'Carra & Murphy (1980) Biochem. J. 187, 311-320]. Phycourobilin is a tetrapyrrole, containing no oxo bridges and has the same order of side chains as IX alpha bilins. The chromophore is linked to the peptide through two and possibly three of its pyrrole rings. One linkage possibly consists of an ester bond between the hydroxy group of a serine residue and the propionic acid side chain of one of the inner rings. The second linkage is a labile thioether bond between a cysteine residue and the C2 side chain of pyrrole ring A. The third linkage is a stable thioether bond between a cysteine residue and the alpha-carbon atom of the C2 side chain of pyrrole ring D. Ring D is unsaturated and is attached to ring C through a saturated carbon bridge. Rings B and C have a conjugated system of five bonds, as found in other urobilinoid pigments. Ring A is attached to ring B via a saturated carbon bridge. Both of the alpha-positions of ring A are in the reduced state, but the ring does contain an unsaturated centre (probably a double bond between the beta-carbon and the ring nitrogen atom). The presence of this double bond and its isomerization into the bridge position between rings A and B would explain the extension of the conjugated system of phycourobilin to that of a phycoerythrobilinoid/rhodenoid pigment in acid or alkali.

The structure of verdohemochrome and its implications for the mechanism of heme catabolism

The synthesis, purification as a tetrafluoroborate salt and structural elucidation of the verdohemochrome 2a derived from the coupled oxidation of octaethylhemochrome 1 is described. Based on elemental analyses, spectroscopic studies (visible and infrared absorption, 1H-NMR) and fast atom bombardment mass spectrometry, the assignment of the iron(II) oxaporphyrin structure for the verdohemochrome 2a and the blue monocarbonyl species 2b, obtained upon treatment of 2a with carbon monoxide, has been accomplished. This assignment raises a number of questions regarding the iron oxidation state of intermediates in the pathway of heme catabolism both in vitro and in vivo. Furthermore, the implications of the occurrence of an iron oxaporphyrin intermediate in the pathway of heme metabolism, which is suggested by the similarity of the visible absorption spectrum of the CO species 2b with that of a new intermediate recently observed in the heme oxygenase-catalyzed degradation of heme and mesoheme, is considered.

Verdohemochrome IX alpha: preparation and oxidoreductive cleavage to biliverdin IX alpha

Several studies have shown that both terminal oxygen atoms of biliverdin are derived from molecular oxygen. Since the conversion of verdohemochrome to biliverdin has been assumed to be hydrolytic, these findings seemed to exclude verdohemochrome as an intermediate in the degradation of heme to biliverdin. Coupled oxidation of myoglobin and ascorbate yielded a pure preparation of verdohemochrome IX alpha. The structure and ferrous state of this product were determined from its composition, ligand reactions, 1H NMR spectrum, and conversion to biliverdin IX alpha dimethyl ester. Reaction with ascorbate and 18O2 converted this compound to biliverdin that contained an atom of 18O. Successive treatment of verdohemochrome, first oxidation with H2O2 and then reduction with phenylhydrazine, yielded the iron complex of biliverdin. These results showed that hydrolysis is not an obligatory step in the conversion of verdohemochrome to biliverdin and, moreover, indicated how heme can be converted, with verdohemochrome as an intermediate, into biliverdin in which the two terminal oxygen atoms are derived from different O2 molecules.

Non-enzymic nature of the pyridine haemochrome-cleaving activity of mammalian tissue extracts ('haem alpha-methenyl oxygenase')

1. The pyridine haemochrome-cleaving activity of extracts from mammalian liver and other tissues is shown conclusively to be entirely non-enzymic in nature and attributable to coupled oxidation with ascorbate. 2. Reduced glutathione probably contributes to the activity indirectly by continuously regenerating the ascorbate to the reduced form. 3. The cleavage shows no specificity for the alpha-methine bridge of pyridine haemochrome. 4. Results are presented suggesting some probable reasons for the erroneous characterization of the activity as an alpha-methine-specific haem-cleaving enzyme (;haem alpha-methenyl oxygenase') by Nakajima and co-workers (e.g. Nakajima, Takemura, Nakajima & Yamaoka, 1963; Nakajima & Gray, 1967).