Argyrophilic grain disease in individuals younger than 75 years: clinical variability in an under-recognized limbic tauopathy

BACKGROUND AND PURPOSE

Argyrophilic grain disease (AGD) is a limbic-predominant 4R-tauopathy. AGD is thought to be an age-related disorder and is frequently detected as a concomitant pathology with other neurodegenerative conditions. There is a paucity of data on the clinical phenotype of pure AGD. In elderly patients, however, AGD pathology frequently associates with cognitive decline, personality changes, urine incontinence and cachexia. In this study, clinicopathological findings were analysed in individuals younger than 75.

METHODS

Patients were identified retrospectively based on neuropathological examinations during 2006-2017 and selected when AGD was the primary and dominant pathological finding. Clinical data were obtained retrospectively through medical records.

RESULTS

In all, 55 patients (2% of all examinations performed during that period) with AGD were identified. In seven cases (13%) AGD was the primary neuropathological diagnosis without significant concomitant pathologies. Two patients were female, median age at the time of death was 64 years (range 51-74) and the median duration of disease was 3 months (range 0.5-36). The most frequent symptoms were progressive cognitive decline, urinary incontinence, seizures and psychiatric symptoms. Brain magnetic resonance imaging revealed mild temporal atrophy.

CONCLUSIONS

Argyrophilic grain disease is a rarely recognized limbic tauopathy in younger individuals. Widening the clinicopathological spectrum of tauopathies may allow identification of further patients who could benefit from tau-based therapeutic strategies.

Plasma and cerebrospinal fluid tau and neurofilament concentrations in rapidly progressive neurological syndromes: a neuropathology-based cohort

BACKGROUND AND PURPOSE

Cerebrospinal fluid (CSF) tau and neurofilament light chain (NF-L) proteins have proved to be reliable biomarkers for neuronal damage; however, there is a strong need for blood-based tests.

METHODS

The present study included 132 autopsy cases with rapidly progressive neurological syndromes, including Alzheimer disease (AD) (21), sporadic (65) and genetic (21) Creutzfeldt-Jakob disease (CJD), 25 cases with vascular, neoplastic and inflammatory alterations, and additionally 18 healthy control individuals. CSF tau and NF-L concentrations were measured by enzyme-linked immunosorbent assay. Plasma tau and NF-L concentrations were measured using ultra-sensitive single molecule array technology.

RESULTS

Plasma and CSF tau (R = 0.59, P < 0.001) and NF-L (R = 0.69, P < 0.001) levels correlated significantly (Spearman test). Plasma tau and NF-L levels were significantly higher in all disease groups compared to healthy controls (P < 0.001). Receiver operating characteristic curves were used and area under the curve values for comparisons with controls were 0.82 (AD), 0.94 (sporadic CJD), 0.92 (genetic CJD) and 0.83 (other neurological disorders) for plasma tau and 0.99, 0.99, 1.00 and 0.96 for plasma NF-L, respectively. Molecular subtyping of sporadic CJD showed a strong effect (linear logistic regression) on plasma tau (P < 0.001) but not NF-L levels (P = 0.19).

CONCLUSION

Plasma tau and NF-L concentrations are strongly increased in CJD and show similar diagnostic performance to the corresponding CSF measure. Molecular subtypes of sporadic CJD show different levels of plasma tau. Although not disease-specific, these findings support the use of plasma tau and NF-L as tools to identify, or to rule out, neurodegeneration.

Danon disease: case report and detection of new mutation

Danon disease is an X-linked disorder resulting from mutations in the lysosome-associated membrane protein-2 (LAMP2) gene. We report a male patient with skeletal myopathy, mental retardation, and massive hypertrophic obstructive cardiomyopathy necessitating heart transplantation. Immunohistochemistry of skeletal muscle and leukocytes, western blot analysis of leukocytes and cardiac muscle, flow cytometry, and DNA sequencing were performed. Muscle biopsy revealed autophagic vacuolar myopathy and lack of immunohistochemically detectable LAMP-2. Diagnosis of Danon disease was confirmed by western blot analysis of myocardial tissue and peripheral blood sample of the patient showing deficiency of LAMP-2 in myocardium and leukocytes. Moreover, absence of LAMP-2 in lymphocytes, monocytes and granulocytes was shown by flow cytometric analysis. Genetic analysis of the LAMP2 gene revealed a novel 1-bp deletion at position 179 (c.179delC) at the 3' end of exon 2, resulting in a frameshift with a premature stop codon.

Redox properties of the couple compound I/native enzyme of myeloperoxidase and eosinophil peroxidase

The standard reduction potential of the redox couple compound I/native enzyme has been determined for human myeloperoxidase (MPO) and eosinophil peroxidase (EPO) at pH 7.0 and 25 degrees C. This was achieved by rapid mixing of peroxidases with either hydrogen peroxide or hypochlorous acid and measuring spectrophotometrically concentrations of the reacting species and products at equilibrium. By using hydrogen peroxide, the standard reduction potential at pH 7.0 and 25 degrees C was 1.16 +/- 0.01 V for MPO and 1.10 +/- 0.01 V for EPO, independently of the concentration of hydrogen peroxide and peroxidases. In the case of hypochlorous acid, standard reduction potentials were dependent on the hypochlorous acid concentration used. They ranged from 1.16 V at low hypochlorous acid to 1.09 V at higher hypochlorous acid for MPO and from 1.10 V to 1.03 V for EPO. Thus, consistent results for the standard reduction potentials of redox couple compound I/native enzyme of both peroxidases were obtained with all hydrogen peroxide and at low hypochlorous acid concentrations: possible reasons for the deviation at higher concentrations of hypochlorous acid are discussed. They include instability of hypochlorous acid, reactions of hypochlorous acid with different amino-acid side chains in peroxidases as well as the appearance of a compound I-chloride complex.

Catalase-peroxidase from synechocystis is capable of chlorination and bromination reactions

Catalase-peroxidases (KatGs) are multifunctional heme peroxidases exhibiting an overwhelming catalase activity and a substantial peroxidase activity of broad specificity. Here, we show that catalase-peroxidases are also haloperoxidases capable of oxidizing chloride, bromide, and iodide in a peroxide- and enzyme-dependent manner. Recombinant KatG and the variants R119A, W122F, and W122A from the cyanobacterium Synechocystis PCC 6803 have been tested for their halogenation activity. Halogenation of monochlorodimedon (MCD), formation of triiodide and tribromide, and bromide- and chloride-mediated oxidation of glutathione have been tested. Halogenation of MCD by chloride, bromide, and iodide was shown to be catalyzed by wild-type KatG and the variant R119A. Generally, rates of halogenation increased in the order Cl(-) < Br(-) < I(-) and/or by decreasing pH. The halogenation activity of R119A was about 7-9% that of the wild-type enzyme. Upon exchange of the distal Trp122 by Phe and Ala, both the catalase and halogenation activities were lost but the overall peroxidase activity was increased. The findings suggest that the same redox intermediate is involved in H(2)O(2) and halide oxidation and that distal Trp122 is involved in both two-electron reactions. That halides compete with H(2)O(2) for the same redox intermediate is also emphasized by the fact that the polarographically measured catalase activity is influenced by halides, with bromide being more effective than chloride.

The role of distal tryptophan in the bifunctional activity of catalase-peroxidases

Catalase-peroxidases are bifunctional peroxidases exhibiting an overwhelming catalase activity and a substantial peroxidase activity. Here we present a kinetic study of the formation and reduction of the key intermediate compound I by probing the role of the conserved tryptophan at the distal haem cavity site. Two wild-type proteins and three mutants of Synechocystis catalase-peroxidase (W122A and W122F) and Escherichia coli catalase-peroxidase (W105F) have been investigated by steady-state and stopped-flow spectroscopy. W122F and W122A completely lost their catalase activity whereas in W105F the catalase activity was reduced by a factor of about 5000. However, the mutations did not influence both formation of compound I and its reduction by peroxidase substrates. It was demonstrated unequivocally that the rate of compound I reduction by pyrogallol or o-dianisidine sometimes even exceeded that of the wild-type enzyme. This study demonstrates that the indole ring of distal Trp in catalase-peroxidases is essential for the two-electron reduction of compound I by hydrogen peroxide but not for compound I formation or for peroxidase reactivity (i.e. the one-electron reduction of compound I).

A transient kinetic study on the reactivity of recombinant unprocessed monomeric myeloperoxidase

Spectral and kinetic features of the redox intermediates of human recombinant unprocessed monomeric myeloperoxidase (recMPO), purified from an engineered Chinese hamster ovary cell line, were studied by the multi-mixing stopped-flow technique. Both the ferric protein and compounds I and II showed essentially the same kinetic behavior as the mature dimeric protein (MPO) isolated from polymorphonuclear leukocytes. Firstly, hydrogen peroxide mediated both oxidation of ferric recMPO to compound I (1.9 x 10(7) M(-1) s(-1), pH 7 and 15 degrees C) and reduction of compound I to compound II (3.0 x 10(4) M(-1) s(-1), pH 7 and 15 degrees C). With chloride, bromide, iodide and thiocyanate compound I was reduced back to the ferric enzyme (3.6 x 10(4) M(-1) s(-1), 1.4 x 10(6) M(-1) s(-1), 1.4 x 10(7) M(-1) s(-1) and 1.4 x 10(7) M(-1) s(-1), respectively), whereas the endogenous one-electron donor ascorbate mediated transformation of compound I to compound II (2.3 x 10(5) M(-1) s(-1)) and of compound II back to the resting enzyme (5.0 x 10(3) M(-1) s(-1)). Comparing the data of this study with those known from the mature enzyme strongly suggests that the processing of the precursor enzyme (recMPO) into the mature form occurs without structural changes at the active site and that the subunits in the mature dimeric enzyme work independently.

Spectral and kinetic studies on eosinophil peroxidase compounds I and II and their reaction with ascorbate and tyrosine

Eosinophil peroxidase, the major granule protein in eosinophils, is the least studied human peroxidase. Here, we have performed spectral and kinetic measurements to study the nature of eosinophil peroxidase intermediates, compounds I and II, and their reduction by the endogenous one-electron donors ascorbate and tyrosine using the sequential-mixing stopped-flow technique. We demonstrate that the peroxidase cycle of eosinophil peroxidase involves a ferryl/porphyrin radical compound I and a ferryl compound II. In the absence of electron donors, compound I is shown to be transformed to a species with a compound II-like spectrum. In the presence of ascorbate or tyrosine compound I is reduced to compound II with a second-order rate constant of (1.0+/-0.2)x10(6) M(-1) s(-1) and (3.5+/-0.2)x10(5) M(-1) s(-1), respectively (pH 7.0, 15 degrees C). Compound II is then reduced by ascorbate and tyrosine to native enzyme with a second-order rate constant of (6.7+/-0.06)x10(3) M(-1) s(-1) and (2.7+/-0.06)x10(4) M(-1) s(-1), respectively. This study revealed that eosinophil peroxidase compounds I and II are able to react with tyrosine and ascorbate via one-electron oxidations and therefore generate monodehydroascorbate and tyrosyl radicals. The relatively fast rates of the compound I reduction demonstrate that these reactions may take place in vivo and are physiologically relevant.

Mechanism of reaction of melatonin with human myeloperoxidase

Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving myeloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myeloperoxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MPO, we investigated the oxidation of melatonin by its redox intermediates compounds I and II using transient-state spectral and kinetic measurements at 25 degrees C. Spectral and kinetic analysis revealed that both compound I and compound II oxidize melatonin via one-electron processes. The second-order rate constant measured for compound I reduction at pH 7 and pH 5 are (6.1 +/- 0.2) x 10(6) M(-1) s(-1) and (1.0 +/- 0.08) x 10(7) M(-1) s(-1), respectively. The rates for the one-electron reduction of compound II back to the ferric enzyme are (9.6 +/- 0.3) x 10(2) M(-1) s(-1) (pH 7) and (2.2 +/- 0.1) x 10(3) M(-1) s(-1) (pH 5). Thus, melatonin is a much better electron donor for compound I than for compound II. Steady-state experiments showed that the rate of oxidation of melatonin is dependent on the H(2)O(2) concentration, is not affected by superoxide dismutase, and is quickly terminated by sodium cyanide. Melatonin can markedly inhibit the chlorinating activity of MPO at both pH 7 and pH 5. The implication of these findings in the activated neutrophil is discussed.

The molecular peculiarities of catalase-peroxidases

In developing ideas of how protein structure modifies haem reactivity, the activity of Class I of the plant peroxidase superfamily (including cytochrome c peroxidase, ascorbate peroxidase and catalase-peroxidases (KatGs)) is an exciting field of research. Despite striking sequence homologies, there are dramatic differences in catalytic activity and substrate specificity with KatGs being the only member with substantial catalase activity. Based on multiple sequence alignment performed for Class I peroxidases, we present a hypothesis for the pronounced catalase activity of KatGs. In their catalytic domains KatGs are shown to possess three large insertions, two of them are typical for KatGs showing highly conserved sequence patterns. Besides an extra C-terminal copy of the ancestral hydroperoxidase gene resulting from gene duplication, these two large loops are likely to control the orientation of both the haem group and of essential residues in the active site. They seem to modulate the access of substrates to the prosthetic group at the distal side as well as the flexibility and character of the bond between the proximal histidine and the ferric iron. The hypothesis presented opens new possibilities in the rational engineering of peroxidases.

The reactivity of myeloperoxidase compound I formed with hypochlorous acid

The reaction of human myeloperoxidase (MPO) with hypochlorous acid (HOCl) was investigated by conventional stopped-flow spectroscopy at pH 5, 7, and 9. In the reaction of MPO with HOCl, compound I is formed. Its formation is strongly dependent on pH. HOCl (rather than OCl-) reacts with the unprotonated enzyme in its ferric state. Apparent second-order rate constants were determined to be 8.1 x 10(7) M(-1)s(-1) (pH 5), 2.0 x 10(8) M(-1)s(-1) (pH 7) and 2.0 x 10(6) M(-1)s(-1) (pH 9) at 15 degrees C. Furthermore, the kinetics and spectra of the reactions of halides and thiocyanate and of physiologically relevant one-electron donors (ascorbate, nitrite, tyrosine and hydrogen peroxide) with this compound I were investigated using the sequential-mixing technique. The results show conclusively that the redox intermediates formed upon addition of either hydrogen peroxide or hypochlorous acid to native MPO exhibit the same spectral features and reactivities and thus are identical. In stopped-flow investigations, the MPO/HOCl system has some advantage since: (i) in contrast to H2O2, HOCl cannot function as a one-electron donor of compound I; and (ii) MPO can easily be prevented from cycling by addition of methionine as HOCl scavenger. As a consequence, the observed absorbance changes are bigger and errors in data analysis are smaller.

Oxidation of hydroquinone, 2,3-dimethylhydroquinone and 2,3,5-trimethylhydroquinone by human myeloperoxidase

Myeloperoxidase is very susceptible to reducing radicals because the reduction potential of the ferric/ferrous redox couple is much higher compared with other peroxidases. Semiquinone radicals are known to reduce heme proteins. Therefore, the kinetics and spectra of the reactions of p-hydroquinone, 2,3-dimethylhydroquinone and 2,3,5-trimethylhydroquinone with compounds I and II were investigated using both sequential-mixing stopped-flow techniques and conventional spectrophotometric measurements. At pH 7 and 15 degrees C the rate constants for compound I reacting with p-hydroquinone, 2,3-dimethylhydroquinone and 2,3,5-trimethylhydroquinone were determined to be 5.6+/-0.4 x 10(7) M(-1)s(-1), 1.3+/-0.1 x 10(6) M(-1)s(-1) and 3.1+/-0.3 x 10(6) M(-1)s(-1), respectively. The corresponding reaction rates for compound II reduction were calculated to be 4.5+/-0.3 x 10(6) M(-1)s(-1), 1.9+/-0.1 x 10(5) M(-1)s(-1) and 4.5+/-0.2 x 10(4) M(-1)s(-1), respectively. Semiquinone radicals, produced by compounds I and II in the classical peroxidation cycle, promote compound III (oxymyeloperoxidase) formation. We could monitor formation of ferrous myeloperoxidase as well as its direct transition to compound II by addition of molecular oxygen. Formation of ferrous myeloperoxidase is shown to depend strongly on the reduction potential of the corresponding redox couple benzoquinone/semiquinone. With 2,3-dimethylhydroquinone and 2,3,5-trimethylhydroquinone as substrate, myeloperoxidase is extremely quickly trapped as compound III. These MPO-typical features could have potential in designing specific drugs which inhibit the production of hypochlorous acid and consequently attenuate inflammatory tissue damage.

Spectral and kinetic studies on the formation of eosinophil peroxidase compound I and its reaction with halides and thiocyanate

Compound I of peroxidases takes part in both the peroxidation and the halogenation reaction. This study for the first time presents transient kinetic measurements of the formation of compound I of human eosinophil peroxidase (EPO) and its reaction with halides and thiocyanate, using the sequential-mixing stopped-flow technique. Addition of 1 equiv of hydrogen peroxide to native EPO leads to complete formation of compound I. At pH 7 and 15 degrees C, the apparent second-order rate constant is (4.3 +/- 0.4) x 10(7) M(-1) s(-1). The rate for compound I formation by hypochlorous acid is (5.6 +/- 0.7) x 10(7) M(-1) s(-1). EPO compound I is unstable and decays to a stable intermediate with a compound II-like spectrum. At pH 7, the two-electron reduction of compound I to the native enzyme by thiocyanate has a second-order rate constant of (1.0 +/- 0. 5) x 10(8) M(-1) s(-1). Iodide [(9.3 +/- 0.7) x 10(7) M(-1) s(-1)] is shown to be a better electron donor than bromide [(1.9 +/- 0.1) x 10(7) M(-1) s(-1)], whereas chloride oxidation by EPO compound I is extremely slow [(3.1 +/- 0.3) x 10(3) M(-1) s(-1)]. The pH dependence studies suggest that a protonated form of compound I is more competent in oxidizing the anions. The results are discussed in comparison with those of the homologous peroxidases myeloperoxidase and lactoperoxidase and with respect to the role of EPO in host defense and tissue injury.

Two-electron reduction and one-electron oxidation of organic hydroperoxides by human myeloperoxidase

The reaction of native myeloperoxidase (MPO) and its redox intermediate compound I with hydrogen peroxide, ethyl hydroperoxide, peroxyacetic acid, t-butyl hydroperoxide, 3-chloroperoxybenzoic acid and cumene hydroperoxide was studied by multi-mixing stopped-flow techniques. Hydroperoxides are decomposed by MPO by two mechanisms. Firstly, the hydroperoxide undergoes a two-electron reduction to its corresponding alcohol and heme iron is oxidized to compound I. At pH 7 and 15 degrees C, the rate constant of the reaction between 3-chloroperoxybenzoic acid and ferric MPO was similar to that with hydrogen peroxide (1.8x10(7) M(-1) s(-1) and 1.4x10(7) M(-1) s(-1), respectively). With the exception of t-butyl hydroperoxide, the rates of compound I formation varied between 5.2x10(5) M(-1) s(-1) and 2.7x10(6) M(-1) s(-1). Secondly, compound I can abstract hydrogen from these peroxides, producing peroxyl radicals and compound II. Compound I reduction is shown to be more than two orders of magnitude slower than compound I formation. Again, with 3-chloroperoxybenzoic acid this reaction is most effective (6. 6x10(4) M(-1) s(-1) at pH 7 and 15 degrees C). Both reactions are controlled by the same ionizable group (average pK(a) of about 4.0) which has to be in its conjugated base form for reaction.

Effect of distal cavity mutations on the formation of compound I in catalase-peroxidases

Catalase-peroxidases have a predominant catalase activity but differ from monofunctional catalases in exhibiting a substantial peroxidase activity and in having different residues in the heme cavity. We present a kinetic study of the formation of the key intermediate compound I by probing the role of the conserved distal amino acid triad Arg-Trp-His of a recombinant catalase-peroxidase in its reaction with hydrogen peroxide, peroxoacetic acid, and m-chloroperbenzoic acid. Both the wild-type enzyme and six mutants (R119A, R119N, W122F, W122A, H123Q, H123E) have been investigated by steady-state and stopped-flow spectroscopy. The turnover number of catalase activity of R119A is 14.6%, R119N 0.5%, H123E 0.03%, and H123Q 0.02% of wild-type activity. Interestingly, W122F and W122A completely lost their catalase activity but retained their peroxidase activity. Bimolecular rate constants of compound I formation of the wild-type enzyme and the mutants have been determined. The Trp-122 mutants for the first time made it possible to follow the transition of the ferric enzyme to compound I by hydrogen peroxide spectroscopically underlining the important role of Trp-122 in catalase activity. The results demonstrate that the role of the distal His-Arg pair in catalase-peroxidases is important in the heterolytic cleavage of hydrogen peroxide (i.e. compound I formation), whereas the distal tryptophan is essential for compound I reduction by hydrogen peroxide.

Nucleotide sequence analysis, overexpression in Escherichia coli and kinetic characterization of Anacystis nidulans catalase-peroxidase

Bifunctional catalase-peroxidases are the least understood type of peroxidases. A high-level expression in Escherichia coli of a fully active recombinant form of a catalase-peroxidase (KatG) from the cyanobacterium Anacystis nidulans (Synechococcus PCC 6301) is reported. Since both physical and kinetic characterization revealed its identity with the wild-type protein, the large quantities of recombinant KatG allowed the examination of both the spectral characteristics and the reactivity of its redox intermediates by using the multi-mixing stopped-flow technique. The homodimeric acidic protein (pI = 4.6) contained high catalase activity (apparent K(m) = 4.8 mM and apparent k(cat) = 8850 s(-1)). Cyanide is shown to be an effective inhibitor of the catalase reaction. The second-order rate constant for cyanide binding to the ferric protein is (6.9 +/- 0.2) x 10(5) M(-1 )s(-1) at pH 7.0 and 15 degrees C and the dissociation constant of the cyanide complex is 17 microM. Because of the overwhelming catalase activity, peroxoacetic acid has been used for compound I formation. The apparent second-order rate constant for formation of compound I from the ferric enzyme and peroxoacetic acid is (1.3 +/- 0.3) x 10(4 )M(-1 )s(-1) at pH 7.0 and 15 degrees C. The spectrum of compound I is characterized by about 40% hypochromicity, a Soret region at 406 nm, and isosbestic points between the native enzyme and compound I at 355 and 428 nm. Rate constants for reduction of KatG compound I by o-dianisidine, pyrogallol, aniline and isoniazid are shown to be (7.3 +/- 0.4) x 10(6) M(-1 )s(-1), (5.4 +/- 0.3) x 10(5) M(-1 )s(-1), (1.6 +/- 0.3) x 10(5) M(-1 )s(-1) and (4.3 +/- 0.2) x 10(4) M(-1 )s(-1), respectively. The redox intermediate formed upon reduction of compound I did not exhibit the classical red-shifted peroxidase compound II spectrum which characterizes the presence of a ferryl oxygen species. Its spectral features indicate that the single oxidizing equivalent in KatG compound II is contained on an amino acid which is not electronically coupled to the heme.

Catalase-peroxidases in cyanobacteria--similarities and differences to ascorbate peroxidases

Cyanobacteria (blue-green algae) are oxygenic phototrophic bacteria carrying out plant-type photosynthesis. The only hydrogen peroxide scavenging enzymes in at least two unicellular species have been demonstrated to be bifunctional cytosolic catalase-peroxidases (CatPXs) having considerable homology at the active site with plant ascorbate peroxidases (APXs). In this paper we examined optical and kinetic properties of CatPXs from the cyanobacteria Anacystis nidulans and Synechocystis PCC 6803 and discuss similarities and differences to plant APXs. Both CatPXs and APX showed similar spectra of the ferric enzyme, the redox intermediate Compound I and the cyanide complex, whereas the spectrum of CatPX Compound II had characteristics reminiscent of the spectrum of the native enzyme. Both steady-state and multi-mixing transient-state kinetic studies were performed in order to characterize the kinetic behaviour of CatPXs. Bimolecular rate constants of both formation and reduction of a CatPX Compound I are presented. Because of its intrinsic high catalase activity (which cannot be found in APXs), the rate constants for Compound I formation were measured with peroxoacetic acid and are shown to be 5.9 x 10(4) M(-1) s(-1) for CatPX from A. nidulans and 8.7 x 10(3) M(-1) s(-1) for the Synechocystis enzyme. Compared with o-dianisidine (2.7-6.7 x 10(6) M(-1) s(-1)) and pyrogallol (8.6 x 10(4)-1.6 x 10(5) M(-1) s(-1)), the rate constant for Compound I reduction by ascorbate was extremely low (5.4 x 10(3) M(-1) s(-1) at pH 7.0 and 15 degrees C), in marked contrast to the behaviour of APXs.

Catalase-peroxidase from the cyanobacterium Synechocystis PCC 6803: cloning, overexpression in Escherichia coli, and kinetic characterization

The Synechocystis PCC 6803 katG gene encodes a dual-functional catalase-peroxidase (EC 1.11.1.7). We have established a system for the high level expression of a fully active recombinant form of this enzyme. Its entire coding DNA was extended using a synthetic oligonucleotide encoding a hexa-histidine tag at the C-terminus and expressed in Escherichia coli [BL21-(DE3)pLysS] using the pET-3a vector. Hemin was added to the culture medium to ensure its proper association with KatG upon induction. The expressed protein was purified to homogeneity by two chromatography steps including a metal chelate affinity and hydrophobic interaction chromatography. The homodimeric acidic protein (pl = 5.4) had a molecular mass of 170 kDa and a Reinheitszahl (A406/A280) of 0.64. The recombinant protein contained high catalase activity (apparent Km = 4.9 +/- 0.25 mM and apparent kcat = 3500 s(-1)) and an appreciable peroxidase activity with o-dianisidine, guaiacol and pyrogallol, but not with NAD(P)H, ferrocytochrome c, ascorbate or glutathione as electron donors. By using both conventional and sequential stopped-flow spectroscopy, formation of compound I with peroxoacetic acid was calculated to be (8.74 +/- 0.26) x 10(3) M(-1) s(-1), whereas compound I reduction by o-dianisidine, pyrogallol and ascorbate was determined to be (2.71 +/- 0.03) x 10(6) M(-1) S(-1), (8.62 +/- 0.21) x 10(4) M(-1) S(-1), and (5.43 +/- 0.19) x 10(3) M(-1) S(-1), respectively. Cyanide binding studies on native and recombinant enzyme indicated that both have the same heme environment. An apparent second-order rate constant for cyanide binding of (4.8 +/- 0.1) x 10(5) M(-1) S(-1) was obtained.

Spectral and kinetic studies of the oxidation of monosubstituted phenols and anilines by recombinant Synechocystis catalase-peroxidase compound I

A high-level expression in Escherichia coli of a fully active recombinant form of a catalase-peroxidase (KatG) from the cyanobacterium Synechocystis PCC 6803 is reported. Since both physical and kinetic characterization revealed its identity with the wild-type protein, the large quantities of recombinant KatG allowed the first examination of second-order rate constants for the oxidation of a series of aromatic donor molecules (monosubstituted phenols and anilines) by a bifunctional catalase-peroxidase compound I using the sequential-mixing stopped-flow technique. Because of the overwhelming catalase activity, peroxoacetic acid has been used for compound I formation. A >/=50-fold excess of peroxoacetic acid is required to obtain a spectrum of relatively pure and stable compound I which is characterized by about 40% hypochromicity, a Soret maximum at 406 nm, and isosbestic points between the native enzyme and compound I at 357 and 430 nm. The apparent second-order rate constant for formation of compound I from ferric enzyme and peroxoacetic acid is (8.74 +/- 0.26) x 10(3) M(-)(1) s(-)(1) at pH 7. 0. Reduction of compound I by aromatic donor molecules is dependent upon the substituent effect on the benzene ring. The apparent second-order rate constants varied from (3.6 +/- 0.1) x 10(6) M(-)(1) s(-)(1) for p-hydroxyaniline to (5.0 +/- 0.1) x 10(2) M(-)(1) s(-)(1) for p-hydroxybenzenesulfonic acid. They are shown to correlate with the substituent constants in the Hammett equation, which suggests that in bifunctional catalase-peroxidases the aromatic donor molecule donates an electron to compound I and loses a proton simultaneously. The value of rho, the susceptibility factor in the Hammett equation, is -3.4 +/- 0.4 for the phenols and -5.1 +/- 0.8 for the anilines. The pH dependence of compound I reduction by aniline exhibits a relatively sharp maximum at pH 5. The redox intermediate formed upon reduction of compound I has spectral features which indicate that the single oxidizing equivalent in KatG compound II is contained on an amino acid which is not electronically coupled to the heme.

Purification and characterization of a hydroperoxidase from the cyanobacterium Synechocystis PCC 6803: identification of its gene by peptide mass mapping using matrix assisted laser desorption ionization time-of-flight mass spectrometry

A cytosolic catalase-peroxidase from the cyanobacterium Synechocystis PCC 6803 was purified to homogeneity by a six-step purification procedure. It is a homodimeric enzyme with a subunit molecular mass of 85 kDa. The isoelectric point of the protein is at pH 5.5; Michaelis constant, turnover number, and catalytic efficiency of the catalase activity for H2O2 were measured to be 4.8 mM, 3450 s-1, and 7.2 x 10(5) M-1 s-1, respectively. Preparation and spectroscopy of the pyridine ferrohemochrome identified an iron protoporphyrin IX as the prosthetic group. The enzyme was shown to exhibit both catalase and peroxidase activities, both of which were inhibited by cyanide, leading to a high-spin to low-spin transition of the heme iron center as detected by a shift of the Soret peak from 405 to 421 nm. The catalase-specific inhibitor 3-amino-1,2,4-triazole proved ineffective. o-Dianisidine, pyrogallol and guaiacol functioned as a peroxidatic substrate, but no reaction was detected with NADH, NADPH, glutathione, and ascorbate. Peptide mass mapping using matrix assisted laser desorption ionization time-of-flight mass spectrometry showed the identity between the purified protein and a putative katG gene derived from the genome of Synechocystis PCC 6803. A comparison of amino acid sequences of the catalase-peroxidase from Synechocystis PCC 6803 and those from other bacteria showed a high homology around the assumed distal and proximal histidine residues, suggesting a highly conserved histidine as the fifth ligand of the heme iron.

Purification and characterization of a homodimeric catalase-peroxidase from the cyanobacterium Anacystis nidulans

Cytosolic extracts of the cyanobacterium Anacystis nidulans exhibit both catalase and o-dianisidine peroxidase activity. Native polyacrylamide gel electrophoresis demonstrates one distinct enzyme, which has been purified to essential homogeneity and found to be composed of two identical subunits of equal size (80.5 kDa). The isoelectric point is at pH 4.7. It is a very efficient catalase with a broad pH optimum between 6.5 and 7.5 and a Km for H2O2 of 4.3 mM, a calculated turnover number of 7200 s(-1), and an overall-rate constant of 3.5 x 10(6) M(-1) s(-1). The behaviour of this protoheme-enzyme is typical of the class of prokaryotic catalase-peroxidases, which is sensitive to cyanide (Ki = 27.2 microM) and insensitive to the eukaryotic catalase inhibitor 3-amino-1,2,4-triazole. The enzyme accepts electrons from o-dianisidine, but not from ascorbate, glutathione, and NADH. With hydrogen peroxide in steady-state conditions the enzyme is mainly in the ferric state indicating that Compound I is much faster reduced by H2O2 than it is formed. The native enzyme is in the high-spin state, which is transformed to low-spin upon addition of cyanide. With peroxoacetic acid Compound I is formed at a rate of 5.9 x 10(4) M(-1) s(-1) at pH 7.0 and 25 degrees C with about 50% hypochromicity, a Soret-maximum at 405 nm and isosbestic points at 354 and 427 nm.

Deletion of the structural gene for the NADH-dehydrogenase subunit 4 of Synechocystis 6803 alters respiratory properties

Chloroplasts and cyanobacteria contain genes encoding polypeptides homologous to some subunits of the mitochondrial respiratory NADH-ubiquinol oxidoreductase complex (NADH dehydrogenase). Nothing is known of the role of the NADH dehydrogenase complex in photosynthesis, respiration, or other functions in chloroplasts, and little is known about the specific roles of the perhaps 42 subunits of this complex in the mitochondrion. Inactivation of a gene for subunit 4 (ndhD-2, ndh4) of this complex in the cyanobacterium Synechocystis 6803 has no effect on photosynthesis, judging from the rate of photoautotrophic growth of mutant cells, but the mutant's respiratory rate is about 6 times greater than that of wild-type cells. Respiratory electron transport activity in cyanobacteria is associated both with photosynthetic thylakoid membranes and with the outer cytoplasmic membrane of the cell. Cytoplasmic membranes of mutant cells have much greater NADH-dependent cytochrome reductase activity than preparations from wild-type cells; this activity remains at wild-type levels in isolated thylakoid membranes. It is suggested that the 56.6-kD product of ndhD-2 is not essential for the activity of a cytoplasmic membrane-bound NADH dehydrogenase but that it regulates the rate of electron flow through the complex, establishing a link between this ndh gene and respiration. The activity of the molecularly distinct thylakoid-bound NADH dehydrogenase is apparently unaffected by the loss of ndhD-2.