Novel sources of mammalian C-S lyase activity

A novel method for the analysis of 3-mercaptopyruvate using high-performance liquid chromatography with fluorescence detection

3-Mercaptopyruvate (3-MP) is a metabolite of cysteine present in mammalian tissues and is known to be a substrate of 3-mercaptopyruvate sulfurtransferase (3MST, EC.3.4.1.2). The physiological relevance of the 3-MP pathway has not been fully recognized because the metabolic behavior of 3-MP remains unclear. Here, we describe a novel method using high-performance liquid chromatography with fluorescence detection to measure 3-MP formation from cysteine. To demonstrate the practical value of the present method, we applied it to analyze the 3-MP produced in biological samples from mouse tissue.

A novel C-S lyase from the latex-producing plant Taraxacum brevicorniculatum displays alanine aminotransferase and l-cystine lyase activity

We isolated a novel pyridoxal-5-phosphate-dependent l-cystine lyase from the dandelion Taraxacum brevicorniculatum. Real time qPCR analysis showed that C-S lyase from Taraxacum brevicorniculatum (TbCSL) mRNA is expressed in all plant tissues, although at relatively low levels in the latex and pedicel. The 1251 bp TbCSL cDNA encodes a protein with a calculated molecular mass of 46,127 kDa. It is homologous to tyrosine and alanine aminotransferases (AlaATs) as well as to an Arabidopsis thaliana carbon-sulfur lyase (C-S lyase) (SUR1), which has a role in glucosinolate metabolism. TbCSL displayed in vitrol-cystine lyase and AlaAT activities of 4 and 19nkatmg(-1) protein, respectively. However, we detected no in vitro tyrosine aminotransferase (TyrAT) activity and RNAi knockdown of the enzyme had no effect on phenotype, showing that TbCSL substrates might be channeled into redundant pathways. TbCSL is in vivo localized in the cytosol and functions as a C-S lyase or an aminotransferase in planta, but the purified enzyme converts at least two substrates specifically, and can thus be utilized for further in vitro applications.

Tyrosine aminotransferase contributes to benzylisoquinoline alkaloid biosynthesis in opium poppy

Tyrosine aminotransferase (TyrAT) catalyzes the transamination of L-Tyr and α-ketoglutarate, yielding 4-hydroxyphenylpyruvic acid and L-glutamate. The decarboxylation product of 4-hydroxyphenylpyruvic acid, 4-hydroxyphenylacetaldehyde, is a precursor to a large and diverse group of natural products known collectively as benzylisoquinoline alkaloids (BIAs). We have isolated and characterized a TyrAT cDNA from opium poppy (Papaver somniferum), which remains the only commercial source for several pharmaceutical BIAs, including codeine, morphine, and noscapine. TyrAT belongs to group I pyridoxal 5'-phosphate (PLP)-dependent enzymes wherein Schiff base formation occurs between PLP and a specific Lys residue. The amino acid sequence of TyrAT showed considerable homology to other putative plant TyrATs, although few of these have been functionally characterized. Purified, recombinant TyrAT displayed a molecular mass of approximately 46 kD and a substrate preference for L-Tyr and α-ketoglutarate, with apparent K(m) values of 1.82 and 0.35 mm, respectively. No specific requirement for PLP was detected in vitro. Liquid chromatography-tandem mass spectrometry confirmed the conversion of L-Tyr to 4-hydroxyphenylpyruvate. TyrAT gene transcripts were most abundant in roots and stems of mature opium poppy plants. Virus-induced gene silencing was used to evaluate the contribution of TyrAT to BIA metabolism in opium poppy. TyrAT transcript levels were reduced by at least 80% in silenced plants compared with controls and showed a moderate reduction in total alkaloid content. The modest correlation between transcript levels and BIA accumulation in opium poppy supports a role for TyrAT in the generation of alkaloid precursors, but it also suggests the occurrence of other sources for 4-hydroxyphenylacetaldehyde.

Sulfur metabolism in AIDS: cystamine as an anti-HIV agent

Numerous reports have documented disturbances of sulfur metabolism in AIDS patients. There is a generalized loss of sulfur from the body, measured as cysteine and glutathione. The enzyme, cystathionase, has been shown to be greatly decreased in the liver of AIDS patients. Cystathionase is known to catalyze beta elimination of cystine giving rise to sulfane sulphur, which has potent stimulatory properties for lymphocytes. When both cystine and cystathionase are deficient in AIDS, the lymphocytes would lack this important regulator, which might be replenished by giving cystamine. Cystamine is a small disulfide that gives rise to sulfane sulfur when it undergoes oxidation catalyzed by diamine oxidase (a ubiquitous enzyme in animals). Cystamine has been shown to cause marked suppression of HIV replication in cultured lymphocytes and macrophages; the inference is that the cystamine/diamine oxidase system may replace the cystine/cystathionase system as a source of sulfane sulfur. Sulfane sulfur could have two beneficial effects: (1) it could increase the vigor and resistance of the lymphocytes and (2) it could interfere with the HIV replication process. A clinical trial of cystamine in AIDS is indicated.

Cysteine S-conjugate beta-lyases

Cysteine S-conjugate beta-lyases are pyridoxal 5'-phosphate-containing enzymes that catalyze beta-elimination reactions with cysteine S-conjugates that possess an electron-withdrawing group attached at the sulfur. The end products of the beta-lyase reaction are pyruvate, ammonium and a sulfur-containing fragment. If the sulfur-containing fragment is reactive, the parent cysteine S-conjugate may be toxic, particularly to kidney mitochondria. Halogenated alkenes are examples of electrophiles that are bioactivated (toxified) by conversion to cysteine S-conjugates. These conjugates are converted by cysteine S-conjugate beta-lyases to thioacylating fragments. Several cysteine S-conjugates found in allium foods (garlic and onion) are beta-lyase substrates. This finding may account in part for the chemopreventive activity of allium products. This review (1) identifies enzymes that catalyze cysteine S-conjugate beta-lyase reactions, (2) suggests that toxicant channeling may contribute to halogenated cysteine S-conjugate-induced toxicity to mitochondria, and (3) proposes mechanisms that may contribute to the antiproliferative effects of sulfur-containing fragments eliminated from allium-derived cysteine S-conjugates.

Role of cysteine S-conjugate beta-lyase in the metabolism of cisplatin

Cisplatin is nephrotoxic, but the mechanism by which cisplatin kills renal proximal tubule cells is not well defined. Inhibition of gamma-glutamyl transpeptidase or pyridoxal 5'-phosphate (PLP)-dependent enzymes blocks the nephrotoxicity. Our hypothesis is that cisplatin is metabolized to a renal toxin through a platinum-glutathione conjugate to a reactive sulfur-containing compound. The final step in this bioactivation is the conversion of a platinum-cysteine S-conjugate to a reactive thiol by a PLP-dependent cysteine S-conjugate beta-lyase. LLC-PK1 cells, a proximal tubule cell line with low cysteine S-conjugate beta-lyase activity, are used to study cisplatin nephrotoxicity. We proposed that the beta-elimination reaction catalyzed by cysteine S-conjugate beta-lyase is the rate-limiting step in the metabolism of cisplatin to a toxin in these cells. In this study, LLC-PK1 cells were transfected with human glutamine transaminase K, which catalyzes the beta-elimination reaction. Cisplatin was significantly more toxic in confluent monolayers of cells with increased cysteine S-conjugate beta-lyase activity. In contrast, carboplatin, a non-nephrotoxic derivative of cisplatin, was 20-fold less toxic than cisplatin in confluent cells, and its toxicity was not altered by overexpression of cysteine S-conjugate beta-lyase. We propose that carboplatin is not nephrotoxic because it is not metabolized through this pathway. Dividing cells were more sensitive to both cisplatin and carboplatin toxicity. Overexpression of cysteine S-conjugate beta-lyase activity had no effect on the toxicity of either drug. These data demonstrate that cisplatin kills quiescent renal cells by a mechanism that is distinct from the mechanism by which it kills dividing cells and that the renal toxicity of cisplatin is dependent on cysteine S-conjugate beta-lyase activity.

A new member of plant CS-lyases. A cystine lyase from Arabidopsis thaliana

Cystine lyases catalyze the breakdown of l-cystine to thiocysteine, pyruvate, and ammonia. Until now there are no reports of the identification of a plant cystine lyase at a molecular level, and it is not clear what biological role this class of enzymes have in plants. A cystine lyase was isolated from Brassica oleracea (L.), and partial amino acid sequencing allowed the corresponding full-length cDNA (BOCL3) to be cloned. The deduced amino acid sequence of BOCL3 showed highest homology to the deduced amino acid sequences of several Arabidopsis thaliana genes annotated as tyrosine aminotransferase-like, including a coronatine, jasmonic acid, and salt stress-inducible gene, CORI3 (78.8% identity), and the unidentified rooty/superroot1 gene (44.8% identity). A full-length expressed sequence tag clone of CORI3 was obtained and recombinant CORI3 was synthesized in Escherichia coli. Isolated recombinant CORI3 catalyzed a cystine lyase reaction, but no aminotransferase reactions. The present study identifies, for the first time, a cystine lyase from plants at a molecular level and redefines the functional assignment of the only functionally identified member of a group of A. thaliana genes annotated as tyrosine aminotransferase-like.

Human mitochondrial and cytosolic branched-chain aminotransferases are cysteine S-conjugate beta-lyases, but turnover leads to inactivation

The mitochondrial and cytosolic branched-chain aminotransferases (BCAT(m) and BCAT(c)) are homodimers in the fold type IV class of pyridoxal 5'-phosphate-containing enzymes that also contains D-amino acid aminotransferase and 4-amino-4-deoxychorismate lyase (a beta-lyase). Recombinant human BCAT(m) and BCAT(c) were shown to have beta-lyase activity toward three toxic cysteine S-conjugates [S-(1,1,2,2-tetrafluoroethyl)-L-cysteine, S-(1,2-dichlorovinyl)-L-cysteine, and S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine] and toward beta-chloro-L-alanine. Human BCAT(m) is a much more effective beta-chloro-L-alanine beta-lyase than two aminotransferases (cytosolic and mitochondrial isozymes of aspartate aminotransferase) previously shown to possess this activity. BCAT(m), but not BCAT(c), also exhibits measurable beta-lyase activity toward a relatively bulky cysteine S-conjugate [benzothiazolyl-L-cysteine]. Benzothiazolyl-L-cysteine, however, inhibits the L-leucine-alpha-ketoglutarate transamination reaction catalyzed by both enzymes. Inhibition was more pronounced with BCAT(m). In the presence of beta-lyase substrates and alpha-ketoisocaproate (the alpha-keto acid analogue of leucine), no transamination could be detected. Therefore, with an amino acid containing a good leaving group in the beta position, beta-elimination is greatly preferred over transamination. Both BCAT isozymes are rapidly inactivated by the beta-lyase substrates. The ratio of turnover to inactivation per monomer in the presence of toxic halogenated cysteine S-conjugates is approximately 170-280 for BCAT(m) and approximately 40-50 for BCAT(c). Mitochondrial enzymes of energy metabolism are especially vulnerable to thioacylation and inactivation by the reactive fragment released from toxic, halogenated cysteine S-conjugates such as S-(1,1,2,2-tetrafluoroethyl)-L-cysteine. The present results suggest that BCAT isozymes may contribute to the mitochondrial toxicity of these compounds by providing thioacylating fragments, but inactivation of the BCAT isozymes might also block essential metabolic pathways.

Mitochondrial aspartate aminotransferase catalyses cysteine S-conjugate beta-lyase reactions

Rat liver mitochondrial aspartate aminotransferase (a homodimer) was shown to catalyse a beta-lyase reaction with three nephrotoxic halogenated cysteine S-conjugates [ S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(2-chloro-1,1,2-trifluoroethyl)-L-cysteine], and less effectively so with a non-toxic cysteine S-conjugate [benzothiazolyl-L-cysteine]. Transamination competes with the beta-lyase reaction, but is not favourable. The ratio of beta elimination to transamination in the presence of S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and 2-oxoglutarate is >100. Syncatalytic inactivation by the halogenated cysteine S-conjugates is also observed. The enzyme turns over approx. 2700 molecules of halogenated cysteine S-conjugate on average for every monomer inactivated. Kidney mitochondria are known to be especially sensitive to toxic halogenated cysteine S-conjugates. Evidence is presented that 15-20% of the cysteine S-conjugate beta-lyase activity towards S -(1,1,2,2-tetrafluoroethyl)-L-cysteine in crude kidney mitochondrial homogenates is due to mitochondrial aspartate aminotransferase. The possible involvement of mitochondrial aspartate aminotransferase in the toxicity of halogenated cysteine S-conjugates is also discussed.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

Co-purification of mitochondrial HSP70 and mature protein disulfide isomerase with a functional rat kidney high-M(r) cysteine S-conjugate beta-lyase

S-(1,1,2,2-Tetrafluoroethyl)-L-cysteine (TFEC, the cysteine S-conjugate of tetrafluoroethylene) is an example of a nephrotoxic, halogenated cysteine S-conjugate. Toxicity results in part from the cysteine S-conjugate beta-lyase(s)-catalyzed conversion of TFEC to a thioacylating fragment with the associated production of pyruvate and ammonia. In the present study, we have demonstrated that rat kidney homogenates contain at least three enzyme fractions that are capable of catalyzing a cysteine S-conjugate beta-lyase reaction with TFEC. One of these fractions contains a high-M(r) lyase. At least two proteins co-purify with this high-M(r) complex. N-Terminal analysis (15 cycles) revealed that the smaller species was mature protein disulfide isomerase (M(r) approximately 54,200) from which the 24 amino acid endoplasmic reticulum signal peptide had been removed. Internal amino acid sequencing (15 cycles) revealed that the larger species was mitochondrial HSP70 (mtHSP70; M(r) approximately 75,000). The present findings offer an explanation for the previous observation that mtHSP70 in kidney mitochondria is heavily thioacylated when rats are injected with TFEC (Bruschi et al., J Biol Chem 1993;268:23157-61).