Maleylacetoacetate isomerase (MAAI/GSTZ)-deficient mice reveal a glutathione-dependent nonenzymatic bypass in tyrosine catabolism

In mammals, the catabolic pathway of phenylalanine and tyrosine is found in liver (hepatocytes) and kidney (proximal tubular cells). There are well-described human diseases associated with deficiencies of all enzymes in this pathway except for maleylacetoacetate isomerase (MAAI), which converts maleylacetoacetate (MAA) to fumarylacetoacetate (FAA). MAAI is also known as glutathione transferase zeta (GSTZ1). Here, we describe the phenotype of mice with a targeted deletion of the MAAI (GSTZ1) gene. MAAI-deficient mice accumulated FAA and succinylacetone in urine but appeared otherwise healthy. This observation suggested that either accumulating MAA is not toxic or an alternate pathway for MAA metabolism exists. A complete redundancy of MAAI could be ruled out because substrate overload of the tyrosine catabolic pathway (administration of homogentisic acid, phenylalanine, or tyrosine) resulted in renal and hepatic damage. However, evidence for a partial bypass of MAAI activity was also found. Mice doubly mutant for MAAI and fumarylacetoacetate hydrolase (FAH) died rapidly on a normal diet, indicating that MAA could be isomerized to FAA in the absence of MAAI. Double mutants showed predominant renal injury, indicating that this organ is the primary target for the accumulated compound(s) resulting from MAAI deficiency. A glutathione-mediated isomerization of MAA to FAA independent of MAAI enzyme was demonstrated in vitro. This nonenzymatic bypass is likely responsible for the lack of a phenotype in nonstressed MAAI mutant mice.     

Degradation of homogentisate by strains of Bacillus and Moraxella.

The metabolic pathways whereby strains of Moraxella and Bacillus degrade homogentisate (2,5-dihydroxyphenylacetate) are delineated. The Moraxella (strain OA3) is shown to degrade homogentisate via the pathway previously described in liver: homogentisate is cleaved by a 1,2-dioxygenase (E.C 1.13.11.5) yielding maleylacetoacetate which is isomerized by a GSH-dependent isomerase to fumarylacetoacetate before hydrolysis to acetoacetate and fumarate. A strain of Bacillus (B11c) is shown to catabolize homogentisate via a previously undescribed version of the above sequence: homogentisate is cleaved by a 1,2-dioxygenase (E.C 1.13.11.5) yielding maleylacetoacetate which is hydrolyzed directly to acetoacetate and maleate.     

Inhibition of glutathione S-transferase zeta and tyrosine metabolism by dichloroacetate: a potential unifying mechanism for its altered biotransformation and toxicity.

Dichloroacetate (DCA) inhibits its own metabolism and is converted to glyoxylate by glutathione S-transferase zeta (GSTz). GSTz is identical to maleylacetoacetate isomerase, an enzyme of tyrosine catabolism that converts maleylacetoacetate (MAA) to fumarylacetoacetate and maleylacetone (MA) to fumarylacetone. MAA and MA are alkylating agents. Rats treated with DCA for up to five days had markedly decreased hepatic GSTz activity and increased urinary excretion of MA. When dialyzed cytosol obtained from human liver was incubated with DCA, GSTz activity was unaffected. In contrast, DCA incubation inhibited enzyme activity in dialyzed hepatic cytosol from rats. Incubation of either rat or human hepatic cytosol with MA led to a dose dependent inhibition of GSTz. These data indicate that humans or rodents exposed to DCA may accumulate MA and/or MAA which inhibit(s) GSTz and, consequently, DCA biotransformation. Moreover, DCA-induced inhibition of tyrosine catabolism may account for the toxicity of this xenobiotic in humans and other species.     

Quantitative analysis of DNA binding by the Escherichia coli arginine repressor

The cis-trans isomerisation of maleylacetoacetate to fumarylacetoacetate is the penultimate step in the tyrosine/phenylalanine catabolic pathway and has recently been shown to be catalysed by glutathione S-transferase enzymes belonging to the zeta class. Given this primary metabolic role it is unsurprising that zeta class glutathione S-transferases are well conserved over a considerable period of evolution, being found in vertebrates, plants, insects and fungi. The structure of this glutathione S-transferase, cloned from Arabidopsis thaliana, has been solved by single isomorphous replacement with anomalous scattering and refined to a final crystallographic R-factor of 19.6% using data from 25.0 A to 1.65 A. The zeta class enzyme adopts the canonical glutathione S-transferase fold and forms a homodimer with each subunit consisting of 221 residues. In agreement with structures of glutathione S-transferases from the theta and phi classes, a serine residue (Ser17) is present in the active site, at a position that would allow it to stabilise the thiolate anion of glutathione. Site-directed mutagenesis of this residue confirms its importance in catalysis. In addition, the role of a highly conserved cysteine residue (Cys19) present in the active site of the zeta class glutathione S-transferase enzymes is discussed.     

Crystal structure of maleylacetoacetate isomerase/glutathione transferase zeta reveals the molecular basis for its remarkable catalytic promiscuity

Maleylacetoacetate isomerase (MAAI), a key enzyme in the metabolic degradation of phenylalanine and tyrosine, catalyzes the glutathione-dependent isomerization of maleylacetoacetate to fumarylacetoacetate. Deficiencies in enzymes along the degradation pathway lead to serious diseases including phenylketonuria, alkaptonuria, and the fatal disease, hereditary tyrosinemia type I. The structure of MAAI might prove useful in the design of inhibitors that could be used in the clinical management of the latter disease. Here we report the crystal structure of human MAAI at 1.9 A resolution in complex with glutathione and a sulfate ion which mimics substrate binding. The enzyme has previously been shown to belong to the zeta class of the glutathione S-transferase (GST) superfamily based on limited sequence similarity. The structure of MAAI shows that it does adopt the GST canonical fold but with a number of functionally important differences. The structure provides insights into the molecular bases of the remarkable array of different reactions the enzyme is capable of performing including isomerization, oxygenation, dehalogenation, peroxidation, and transferase activity.     

Characterisation of a zeta class glutathione transferase from Arabidopsis thaliana with a putative role in tyrosine catabolism

A glutathione transferase (GST) similar to zeta GSTs in animals and fungi has been cloned from Arabidopsis thaliana using RT-PCR. The Arabidopsis zeta GST (AtGSTZ1) was expressed in Escherichia coli as his-tagged polypeptides, which associated together to form the 50-kDa AtGSTZ1-1 homodimer. Following purification, AtGSTZ1-1 was assayed for a range of activities and compared with other purified recombinant plant GSTs from the phi, tau, and theta classes. AtGSTZ1-1 differed from the other GSTs in showing no glutathione conjugating activity toward xenobiotics and no glutathione peroxidase activity toward organic hydroperoxides. Uniquely among the plant GSTs, AtGSTZ1-1 showed activity as a maleylacetone isomerase (MAI). This glutathione-dependent reaction is analogous to the cis-trans isomerization of maleylacetoacetate to fumarylacetoacetate, which occurs in the course of tyrosine catabolism to acetoacetate and fumarate. Thus, rather than functioning as a conventional GST, AtGSTZ1-1 appears to be involved in tyrosine degradation. In addition to the MAI activity, the AtGSTZ1-1 also catalyzed the glutathione-dependent dehalogenation of dichloroacetic acid to glyoxylic acid. This latter activity was used to demonstrate the presence of functional AtGSTZ1-1 inplanta.     

Purification and Some Properties of Maleylpyruvate Hydrolase and Fumarylpyruvate Hydrolase from Pseudomonas alcaligenes

Hydrolysis of the gentisate ring-cleavage product, maleylpyruvate (cis-2,4-diketohept-5-enedioic acid), was shown to be catalyzed by an enzyme, maleylpyruvate hydrolase 11, in Pseudomonas alcaligenes (P25X1) after growth with 3-hydroxybenzoate. This activity was separated from fumarylpyruvate hydrolase activity during the course of its purification which accomplished an approximately 50-fold increase in specific activity. An apparent molecular weight of 77,000 was assigned on the basis of Sephadex G-200 chromatography. Despite the presence of up to three similarly migrating bands of protein on polyacrylamide-gel electrophoresis of the purified enzyme, at least two of these bands possessed maleylpyruvate hydrolase activity. Electrophoresis on sodium dodecyl sulfate-polyacrylamide before and after reduction with mercaptoethanol gave a principal band of molecular weight of 33,000 (and a minor band of molecular weight 50,000). A number of substituted maleylpyruvates also served as substrates for maleylpyruvate hydrolase 11, but maleylacetoacetate and fumarylpyruvate were not attacked. Fumarylpyruvate hydrolase was purified approximately 40-fold to give a single band on polyacrylamide gels and with an apparent molecular weight of 73,000 by Sephadex G-200 chromatography. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis before or after reduction with mercaptoethanol, a subunit molecular weight of 25,000 was obtained. Neither maleylpyruvate nor fumarylacetoacetate served as substrates for fumarylpyruvate hydrolase. The activities of both maleyl- and fumarylpyruvate hydrolases were stimulated by Mn²⁺ ions. Reasons are discussed for the presence of both enzyme activities, one of which appears to be redundant.     

Glutathione-independent maleylacetoacetate isomerase in gram-positive bacteria.

Nocardia globerula CL1 produced a glutathione-independent maleylacetoacetate isomerase after growth on L-tyrosine. Partial purification of this isomerase demonstrated its independence of low-molecular-weight cofactors such as glutathione. Similar glutathione-independent maleylacetoacetate isomerases were present in three other gram-positive bacteria grown on tyrosine.     

Recruitment of a double bond isomerase to serve as a reductive dehalogenase during biodegradation of pentachlorophenol

Tetrachlorohydroquinone dehalogenase catalyzes the replacement of chlorine atoms on tetrachlorohydroquinone and trichlorohydroquinone with hydrogen atoms during the biodegradation of pentachlorophenol by Sphingomonas chlorophenolica. The sequence of the active site region of tetrachlorohydroquinone dehalogenase is very similar to those of the corresponding regions of maleylacetoacetate isomerases, enzymes that catalyze the glutathione-dependent isomerization of a cis double bond in maleylacetoacetate to the trans configuration during the catabolism of phenylalanine and tyrosine. Furthermore, tetrachlorohydroquinone dehalogenase catalyzes the isomerization of maleylacetone (an analogue of maleylacetoacetate) at a rate nearly comparable to that of a bona fide bacterial maleylacetoacetate isomerase. Since maleylacetoacetate isomerase is involved in a common and presumably ancient pathway for catabolism of tyrosine, while tetrachlorohydroquinone dehalogenase catalyzes a more specialized reaction, it is likely that tetrachlorohydroquinone dehalogenase arose from a maleylacetoacetate isomerase. The substrates and overall transformations involved in the dehalogenation and isomerization reactions are strikingly different. This enzyme provides a remarkable example of Nature's ability to recruit an enzyme with a useful structural scaffold and elaborate upon its basic catalytic capabilities to generate a catalyst for a newly needed reaction.     

Cold sensitivity in rice (Oryza sativa L.) is strongly correlated with a naturally occurring I99V mutation in the multifunctional glutathione transferase isoenzyme GSTZ2

GSTZs [Zeta class GSTs (glutathione transferases)] are multifunctional enzymes that belong to a highly conserved subfamily of soluble GSTs found in species ranging from fungi and plants to animals. GSTZs are known to function as MAAIs [MAA (maleylacetoacetate) isomerases], which play a role in tyrosine catabolism by catalysing the isomerization of MAA to FAA (fumarylacetoacetate). As tyrosine metabolism in plants differs from animals, the significance of GSTZ/MAAI is unclear. In rice (Oryza sativa L.), a major QTL (quantitative trait locus) for seedling cold tolerance has been fine mapped to a region containing the genes OsGSTZ1 and OsGSTZ2. Sequencing of tolerant (ssp. japonica cv. M-202) and sensitive (ssp. indica cv. IR50) cultivars revealed two SNPs (single nucleotide polymorphisms) in OsGSTZ2 that result in amino acid differences (I99V and N184I). Recombinant OsGSTZ2 containing the Val99 residue found in IR50 had significantly reduced activity on MAA and DCA (dichloroacetic acid), but the Ile184 residue had no effect. The distribution of the SNP (c.295A>G) among various rice accessions indicates a significant association with chilling sensitivity in rice seedlings. The results of the present study show that naturally occurring OsGSTZ2 isoforms differ in their enzymatic properties, which may contribute to the differential response to chilling stress generally exhibited by the two major rice subspecies.     

Gene Structure, Chromosomal Location, and Expression Pattern of Maleylacetoacetate Isomerase

The gene for maleylacetoacetate isomerase (MAAI) (EC 5.2.1.2) was the last gene in the mammalian phenylalanine/tyrosine catabolic pathway to be cloned. We have isolated the human and murine genes and determined their genomic structure. The human gene spans a genomic region of approximately 10 kb, has 9 exons ranging from 50 to 528 bp in size, and was mapped to 14q24.3-14q31.1 using fluorescence in situ hybridization. The complete catabolic pathway of phenylalanine/tyrosine is normally restricted to liver and kidney, but the maleylacetoacetate isomerase gene is expressed ubiquitously. This suggests a possible second role for the MAAI protein different from phenylalanine/tyrosine catabolism. We have searched for mutations in the maleylacetoacetate isomerase gene in four cases of unexplained severe liver failure in infancy with clinical similarities to hereditary tyrosinemia type I (pseudotyrosinemia). Several amino acid changes were identified, but all were found to retain MAAI activity and thus represent protein polymorphisms. We conclude that MAAI deficiency is not a common cause of the pseudotyrosinemic phenotype.     

Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis.

Hereditary tyrosinemia type I is the most severe metabolic disease of the tyrosine catabolic pathway mainly affecting the liver. It is caused by deficiency of fumarylacetoacetate hydrolase, which prevents degradation of the toxic metabolite fumarylacetoacetate (FAA). We report here that FAA induces common effects (i.e., cell cycle arrest and apoptosis) in both human (HepG2) and rodent (Chinese hamster V79) cells, effects that seem to be temporally related. Both the antiproliferative and apoptosis-inducing activities of FAA are dose dependent and enhanced by glutathione (GSH) depletion with L-buthionine-(S,R)-sulfoximine (BSO). Short treatment (2 h) with 35 microM FAA/+BSO or 100 microM FAA/-BSO induced a transient cell cycle arrest at the G2/M transition (20% and 37%, respectively) 24 h post-treatment. In cells treated with 100 microM FAA/-BSO, an inactivation, followed by a rapid over-induction of cyclin B-dependent kinase occurred, which peaked 24 h post-treatment. Maximum levels of caspase-1 and caspase-3 activation were detected at 3 h and 32 h, respectively, whereas release of mitochondrial cytochrome c was maximal at 24-32 h post-treatment. The G2/M peak declined 24 h later, concomitantly with the appearance of a sub-G1, apoptotic population showing typical nucleosomal-sized DNA fragmentation and reduced mitochondrial transmembrane potential (Deltapsi(m)). These events were prevented by the general caspase inhibitor z-VAD-fmk, whereas G2/M arrest and subsequent apoptosis were abolished by GSH-monoethylester or N-acetylcysteine. Other tyrosine metabolites, maleylacetoacetate and succinylacetone, had no antiproliferative effects and induced only very low levels of apoptosis. These results suggest a modulator role of GSH in FAA-induced cell cycle disturbance and apoptosis where activation of cyclin B-dependent kinase and caspase-1 are early events preceding mitochondrial cytochrome c release, caspase-3 activation, and Deltapsi(m) loss. -Jorquera, R., Tanguay, R. M. Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis.     

A missense mutation (Q279R) in the Fumarylacetoacetate Hydrolase gene, responsible for hereditary tyrosinemia, acts as a splicing mutation

Background  

Tyrosinemia type I, the most severe disease of the tyrosine catabolic pathway is caused by a deficiency in fumarylacetoacetate hydrolase (FAH). A patient showing few of the symptoms associated with the disease, was found to be a compound heterozygote for a splice mutation, IVS6-1g->t, and a putative missense mutation, Q279R. Analysis of FAH expression in liver sections obtained after resection for hepatocellular carcinoma revealed a mosaic pattern of expression. No FAH was found in tumor regions while a healthy region contained enzyme-expressing nodules.     

Cucumis sativus secretes 4′-ketoriboflavin under iron-deficient conditions

A new compound in cucumber, Cucumis sativus, nutrient solution that appears under iron-deficient conditions, but not under ordinary culture conditions, has been revealed by HPLC analysis. The chemical structure of this compound was identified using LC-MS and NMR techniques as that of 4′-ketoriboflavin. This is the first report to show that 4′-ketoriboflavin can be found in metabolites from organisms.     

Survey of enzyme activity responsible for phenolic off-flavour production by <Emphasis Type="Italic">Dekkera</Emphasis> and <Emphasis Type="Italic">Brettanomyces</Emphasis> yeast

Volatile phenols are produced by Dekkera yeasts and are of organoleptic importance in alcoholic beverages. The key compound in this respect is 4-ethylphenol, responsible for the medicinal and phenolic aromas in spoiled wines. The microbial synthesis of volatile phenols is thought to occur in two steps, beginning with naturally occurring hydroxycinnamic acids (HCAs). The enzyme phenolic acid decarboxylase (PAD) converts HCAs to vinyl derivatives, which are the substrates of a second enzyme, postulated to be a vinylphenol reductase (VPR), whose activity results in the formation of ethylphenols. Here, both steps of the pathway are investigated, using cell extracts from a number of Dekkera and Brettanomyces species. Dekkera species catabolise ferulic, caffeic and p-coumaric acids and possess inducible enzymes with similar pH and temperature optima. Brettanomyces does not decarboxylate HCAs but does metabolise vinylphenols. Dekkera species form ethylphenols but the VPR enzyme appears to be highly unstable in cell extracts. A partial protein sequence for PAD was determined from Dekkera anomala and may indicate the presence of a novel enzyme in this genus.     

Construction of Chemical Libraries of Volatile Compounds by Combinatorial Synthesis of Homologous Mixtures: Alk-4-en-1-ols,Alk-4-enals and Methyl Alk-4-enoates

A compound library of sixty six linear compounds, eleven representatives of six molecular families: (E)- and (Z)-isomers of alk-4-en-1-ols, alk-4-enals, and methyl alk-4-enoates, was prepared by combinatorial syntheses to allow the creation of a mass spectral database directly usable for their identification in GC/MS analyses. We demonstrate here that compound libraries can be prepared by combinatorial syntheses using long linear synthetic sequences, i. e., eight step in the case of 4-enals. The resulting mixtures of homologues are still perfectly exploitable to deliver the requested information such as clean mass spectra and good gas chromatographic retention indices.     

Maleylacetoacetate cis-trans isomerase: One-step double cis-trans isomerization of monomethyl muconate and the enzyme's probable role in benzene metabolism

Maleylacetoacetate cis-trans isomerase together with glutathione has been found to isomerize cis-trans isomers of monomethyl muconate. Isomerization about a single double bond and concerted double isomerization of the diene unit occurs. In addition to the variations in substrate structure previously identified the current results demonstrate that a cis,cis diene skeleton and a conjugated ester function are accepted by the enzyme. The present work and the fiding of trans,trans-muconic acid in the urine of benzene-fed mice ([16.] Xenobiotica 15, 211) suggest that maleylacetoacetate cis-trans isomerase may be responsible for the geometrical isomerization. However, cis,cis-muconaldehydic acid rather than cis,cis-muconic acid is suggested to be the early intermediate in benzene metabolism capable of rapid enzyme-catalyzed cis-trans isomerization.