Sulfhydryl chemistry of Salmonella typhimurium phosphoribosylpyrophosphate synthetase: identification of two classes of cysteinyl residues

Sulfhydryl-specific reagents were used to study the reactivities and function of the four cysteinyl residues per subunit present in Salmonella typhimurium 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP) synthetase. In the presence of high concentrations of denaturants all four cysteinyl residues reacted with sulfhydryl-specific reagents. In the absence or in the presence of low levels of denaturing agents, two classes of cysteinyl residues were identified. A single sulfhydryl reacted rapidly with iodoacetamide and 5,5'-dithiobis(nitrobenzoic acid) (DTNB) without significant loss of enzymatic activity. This single sulfhydryl was identified as Cys-229 by reaction with iodo[1-14C]acetamide, followed by isolation and sequence analysis of a single radiolabeled peptide. The three remaining sulfhydryls reacted to various extents depending on the conditions and sulfhydryl-specific reagents employed. At low Pi concentrations, these residues reacted fully with DTNB, leading to an 80 to 90% loss of enzymatic activity. ATP and high levels of Pi prevented this reaction. These results, along with studies comparing the S. typhimurium PRPP synthetase sequence with the sequences of PRPP synthetases from other species, suggest that the cysteinyl residues in the Salmonella enzyme are not catalytically essential. That one or more of the three less reactive residues may lie in or near the active site is not excluded.     

Evidence for a reactive cysteine at the nucleotide binding site of spinach ribulose-5-phosphate kinase

Ribulose-5-phosphate kinase from spinach was rapidly inactivated by N-bromoacetylethanolamine phosphate in a bimolecular fashion with a k2 of 2.0 M-1 S-1 at 2 degrees C and pH 8.0. Ribulose 5-phosphate had little effect on the rate of inactivation, whereas complete protection was afforded by ADP or ATP. The extent of incorporation as determined with 14C-labeled reagent was about 1 molar equivalent per subunit in the presence of ATP with full retention of enzymatic activity, and about 2 molar equivalents per subunit in the completely inactivated enzyme. Amino acid analyses of enzyme derivatized with 14C-labeled reagent reveal that all of the covalently incorporated reagent was associated with cysteinyl residues. Hence two sulfhydryls are reactive, but the inactivation correlates with alkylation of one cysteinyl residue at or near the enzyme's nucleotide binding site. The kinase was also extremely sensitive to the sulfhydryl reagents 5,5'-dithiobis(2-nitrobenzoic acid) and N-ethyl-maleimide. The reactive sulfhydryl groups are likely those generated by reduction of a disulfide during activation.     

Characterization of the cysteinyl-containing peptides of the gamma subunit of coupling factor 1

The cysteinyl peptides of the gamma subunit of chloroplast coupling factor 1 (CF1) have been analyzed by high performance liquid chromatography. Analysis of the reduced enzyme alkylated with 4-vinylpyridine showed that the gamma subunit contains four cysteinyl residues. Two of these residues are involved in a disulfide linkage in CF1 either in solution or bound to washed thylakoid membranes. Two free sulfhydryls, one that is readily attacked by alkylating reagents and another that is less reactive, were also detected. Each of these four cysteinyl residues is present in a separate tryptic peptide derived from the gamma subunit. These results show that 4-vinylpyridine is an excellent reagent for the analysis of cysteinyl-containing peptides and support our analyses of the roles of cysteinyl residues in the gamma subunit in ATP synthesis and hydrolysis.     

Identification of two cysteine residues forming a pair of vicinal thiols in glucosamine-6-phosphate deaminase from Escherichia coli and a study of their functional role by site-directed mutagenesis.

The nucleotide sequence of the nagB gene in Escherichia coli, encoding glucosamine-6-phosphate deaminase, located four cysteinyl residues at positions 118, 219, 228, and 239. Chemical modification studies performed with the purified enzyme had shown that the sulfhydryl groups of two of these residues form a vicinal pair in the enzyme and are easily modified by thiol reagents. The allosteric transition to the more active conformer (R), produced by the binding of homotropic (D-glucosamine 6-phosphate or 2-deoxy-2-amino-D-glucitol 6-phosphate) or heterotropic (N-acetyl-D-glucosamine 6-phosphate) ligands, completely protected these thiols against chemical modification. Selective cyanylation of the vicinal thiols with 2-nitro-5-(thiocyanato)benzoate, followed by alkaline hydrolysis to produce chain cleavage at the modified cysteines, gave a pattern of polypeptides which allowed us to identify Cys118 and Cys239 as the residues forming the thiol pair. Subsequently, three mutated forms of the gene were constructed by oligonucleotide-directed mutagenesis, in which one or both of the cysteine codons were changed to serine. The mutant proteins were overexpressed and purified, and their kinetics were studied. The dithiol formed by Cys118 and Cys239 was necessary for maximum catalytic activity. The single replacements and the double mutation affected catalytic efficiency in a similar way, which was also identical to the effect of the chemical block of the thiol pair. However, only one of these cysteinyl residues, Cys239, had a significant role in the allosteric transition, and its substitution for serine reduced the allosteric interaction energy, due to a lower value of KT.     

Essential cysteines in 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli. Analysis by chemical modification and site-directed mutagenesis of the phenylalanine-sensitive isozyme.

The phenylalanine-sensitive isozyme of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli was inactivated by the sulfhydryl modifying reagents 5,5-dithiobis-(2-nitrobenzoate), bromopyruvate, and N-ethylmaleimide and protected from inactivation by the presence of its metal activator, Mn2+, and substrate, phosphoenolpyruvate. Inactivation by 5,5-dithiobis-(2-nitrobenzoate) was correlated with modification of two of the seven cysteine sulfhydryls of the enzyme monomer. The kinetics of 5,5-dithiobis-(2-nitrobenzoate) modification were altered significantly and distinctively by both substrates (phosphoenolpyruvate and erythrose 4-phosphate), by Mn2+, and by L-phenylalanine, suggesting that ligand binding has significant effects on the conformation of the enzyme. Site-directed mutagenesis was used to create multiple substitutions at the two invariant cysteine residues of the polypeptide, Cys-61 and Cys-328. Analysis of purified mutant enzymes indicated that Cys-61 is essential for catalytic activity and for metal binding. Cys-328 was found to be nonessential for catalytic activity, although mutations at this position had significant negative effects on Vmax, KmMn, and KmPEP.     

Identification of an essential cysteinyl residue for the structure of glutamine synthetase α from Phaseolus vulgaris

We have studied the possible role, in a plant glutamine synthetase (GS), of the different cysteinyl residues present in this enzyme. For this purpose we carried out the site-directed mutagenesis of the cDNA for α-GS polypeptide from Phaseolus vulgaris in the positions corresponding to Cys-92, Cys-159, and Cys-179, followed by heterologous expression in E. coli and enzymatic characterisation of WT and mutant proteins. The results show that neither Cys-92 nor Cys-179 residues were essential for enzyme activity, but the replacement of Cys-159 by alanine or serine strongly affects the quaternary structure and function of the GS enzyme molecule, resulting in a complete loss of enzymatic activity. Other studies using sulfhydryl specific reagents such as pHMB (p-hydroxymercuribenzoate) or DTNB (5,5′-dithiobis-2-nitrobenzoate) confirmed that the profound inhibition produced is associated with an important alteration of the quaternary structure of GS, and suggest that Cys-159 might be the residue responsible for the enzyme inhibition. All these results suggest that the Cys-159 residue is essential for the enzyme structure. The results are also consistent with previous reports based on classical biochemistry studies indicating the presence of essential cysteinyl residues for the enzyme activity of higher plant GS.     

The sulfhydryls of firefly luciferase are not essential for activity

Firefly luciferase, containing an average of seven free sulfhydryls per two 50 000-dalton polypeptides, was modified by various sulfhydryl reagents. The differential reactivities of the sulfhydryls in luciferase protected by substrates allow one to define three categories of these groups: Class SH-III contains three sulfhydryls that are not involved in enzymatic activity. Class SH-II contains two sulfhydryls whose modification by different reagents causes varying effects on activity ranging from 0 to 60% inactivation. These sulfhydryls are not essential but may be important structurally or sterically. Class SH-I contains two sulfhydryls that are protected by substrates, either dehydroluciferyl adenylate or dehydroluciferin alone, and are located at or near the active site. The SH-I sulfhydryls are vicinal in the enzyme as demonstrated by their ability to form a disulfide bond. They have also been shown to exist on a single polypeptide chain. Modification of the SH-I groups by most reagents results in complete loss of enzymatic activity; reaction with methyl methanethiosulfonate produces an enzyme that emits only red light whereas native luciferase emits yellow-green light. Evidence is presented that the modified enzyme, while catalytically active, has a distorted active site. It is concluded that these two SH-I sulfhydryls are not essential for activity.     

Avian 3-hydroxy-3-methylglutaryl-CoA lyase: sensitivity of enzyme activity to thiol/disulfide exchange and identification of proximal reactive cysteines.

Catalysis by purified avian 3-hydroxy-3-methylglutaryl-CoA lyase is critically dependent on the reduction state of the enzyme, with less than 1% of optimal activity being observed with the air-oxidized enzyme. The enzyme is irreversibly inactivated by sulfhydryl-directed reagents with the rate of this inactivation being highly dependent upon the redox state of a critical cysteine. Methylation of reduced avian lyase with 1 mM 4-methylnitrobenzene sulfonate results in rapid inactivation of the enzyme with a k(inact) of 0.178 min-1. The oxidized enzyme is inactivated at a sixfold slower rate (k(inact) = 0.028 min-1). Inactivation of the enzyme with the reactive substrate analog 2-butynoyl-CoA shows a similar dependence upon the enzyme's redox state, with a sevenfold difference in k(inact) observed with oxidized vs. reduced forms of the enzyme. Chemical cross-linking of the reduced enzyme with stoichiometric amounts of the bifunctional reagents 1,3-dibromo-2-propanone (DBP) or N,N'-ortho-phenylene-dimaleimide (PDM) coincides with rapid inactivation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of enzyme treated with bifunctional reagent reveals a band of twice the molecular weight of the lyase monomer, indicating that an intersubunit cross-link has been formed. Differential labeling of native and cross-linked protein with [1-14C]iodoacetate has identified as the primary cross-linking target a cysteine within the sequence VSQAACR, which maps at the carboxy-terminus of the cDNA-deduced sequence of the avian enzyme (Mitchell, G.A., et al., 1991, Am. J. Hum. Genet. 49, 101). In contrast, bacterial HMG-CoA lyase, which contains no corresponding cysteine, is not cross-linked by comparable treatment with bifunctional reagent. These results provide evidence for a potential regulatory mechanism for the eukaryotic enzyme via thiol/disulfide exchange and identify a cysteinyl residue with the reactivity and juxtaposition required for participation in disulfide formation.     

Spinach leaf ribulose-5-phosphate kinase: examination of sulfhydryls by chemical modification and spin-labeling

Methodology has been developed for complete or selective modification of the cysteinyl sulfhydryls of ribulose-5-phosphate (Ru5P) kinase. Using native enzyme, iodoacetate modifies four sulfhydryls with varying levels of completeness. The most reactive sulfhydryl in the native enzyme can be selectively titrated with iodoacetate; complete loss of activity occurs. Composition and N-terminal analyses of the peptide bearing this essential sulfhydryl indicate that the alkylated residue (Cys-16) is identical to the site modified by other modification reagents (M. A. Porter and F. C. Hartman (1986) Biochemistry 25, 7314-7318). In the presence of ATP, a nonessential sulfhydryl of the native enzyme is carboxymethylated. The peptide bearing this modified cysteine has been isolated and its composition and N-terminal sequence determined. Enzyme that is carboxymethylated in the presence of ATP retains activity and can be oxidatively inactivated in a reversible fashion. This suggests that the cysteine targeted by iodoacetate in the presence of ATP is not a residue that participates in regulation of enzyme activity. Using a spin-labeled analog of iodoacetate, both essential and nonessential cysteines have been selectively modified. ESR measurements suggest that the environment of these cysteines is not highly constrained. Modest effects on spin-label mobility are observed upon occupancy of Ru5P or ATP sites on the modified enzyme. These effects are dependent on the presence of divalent cations, suggesting that a binary enzyme-cation complex must form prior to productive enzyme-substrate interactions.     

Studies on the reactivity of the essential cysteine of thymidylate synthetase

The reaction of one of the four cysteinyl residues of thymidylate synthetase from methotrexate-resistant Lactobacillus casei with a variety of sulfhydryl reagents results in complete inhibition of the enzyme. Kinetic studies indicate that the rates of reactivity of the reagents tested are N-ethylmaleimide > iodoacetamide > N-(iodoacetylaminoethyl)-S-naphthylamine-1-sulfonic acid > iodoacetic acid. The enzyme is also inactivated by 5-Hg-deoxyuridylate, a compound which reacts stoichiometrically with a single cysteine. Unlike the other reagents, the inhibition produced by this compound can be completely reversed by added thiols. The same cysteine appears to react with all of the sulfhydryl reagents, as shown by competition experiments and by protection against inactivation by deoxyuridylate. Even at a 100-fold excess of the alkylating agents, only one of the four cysteines in the native enzyme was reactive, attesting to the uniqueness of this residue. Carboxypeptidase A inactivation of the enzyme does not affect either the binding of deoxyuridylate to the enzyme or the reactivity of N-ethylmaleimide with the “catalytic” cysteine. Under denaturing conditions, all four cysteinyl residues react with N-ethylmaleimide or iodoacetate, as shown by identifying the reaction products by amino acid analysis. The covalent ternary complex [(+)5,10-methylenetetrahydrofolate-5-fluorodeoxyuridylate-thymidylate synthetase] (molar ratio = 2:2:1) revealed only two cysteinyl residues capable of reacting with N-ethylmaleimide or iodoacetate upon denaturation. From these data, it appears that one cysteine is involved in the binding of deoxyuridylate and that two of the enzyme's four cysteines are responsible for binding 5-fluorodeoxyuridylate in the ternary complex.     

Role of strategic cysteine residues in oxidative damage to the yeast plasma membrane H<Superscript>+</Superscript>-ATPase caused by Fe- and Cu-containing fenton reagents

Damage caused to Saccharomyces cerevisiae SY4 plasma membrane H(+)-ATPase by Fe- and Cu-Fenton reagents was determined in secretory vesicles containing enzyme in which Cys residues were replaced singly or in pairs by Ala. Cys-221 situated in a beta-sheet domain between M2 and M3 segments, phosphorylation domain-located Cys-409 and Cys-532 situated at the ATP-binding site play a role in the inactivation. In the presence of all three residues the enzyme exhibited a certain basic inactivation, which did not change when Cys-532 was replaced with Ala. In mutants having intact Cys-532 but lacking one or both other cysteines, replacement of Cys-221 with Ala led to lower inactivation, suggesting that Cys-221 may serve as a target for metal-catalyzed oxidation and intact Cys-532 promotes this target role of Cys-221. In contrast, the absence of Cys-409 caused higher inactivation by Fe-Fenton. Cys-532 thus seems to serve as a target for Fe-Fenton, intact Cys-409 causing a conformational change that makes Cys-532 less accessible to oxidation. The mutant lacking both Cys-221 and Cys-409 is more sensitive to Fe-Fenton than to Cu-Fenton and the absence of both Cys residues thus seems to expose presumable extra Fe-binding sites. These data and those on protection by ATP, ADP, 1,4-dithiothreitol and deferrioxamine B point to complex interactions between individual parts of the enzyme molecule that determine its sensitivity towards Fenton reagents. ATPase fragmentation caused by the two reagents differed in that the Fe-Fenton reagent produced in Western blot "smears" whereas the Cu-Fenton reagent produced defined fragments.     

Zinc binding and its trapping by allosteric transition in glucosamine-6-phosphate deaminase from Escherichia coli

Glucosamine-6-phosphate isomerase deaminase from Escherichia coli, a typical allosteric enzyme, becomes less cooperative and 50% inhibited when treated with zinc. This metal cation behaving as a tight-bound and slow partial inhibitor. Modification of a pair of vicinal reactive thiols with some sulfhydryl reagents mimics this effect. On the other hand, sulfhydryl reactivity disappears in the presence of saturating concentrations of Zn2+, which does not modify the kinetics of S-methylated enzyme, a finding that indicates that vicinal thiols are an essential part of the zinc-binding site. Allosteric activation of the deaminase causes trapping of the metal, which cannot be released by dialysis against a buffer containing EDTA. Cadmium and nickel(II) cations also produce a similar effect.     

Primary structure and analysis of the location of the regulatory disulfide bond of pea chloroplast NADP-malate dehydrogenase

Purified pea chloroplast NADP-malate dehydrogenase (S)-malate: NADP⁺ oxidoreductase, EC 1.1.1.82) was digested with trypsin and the resulting peptides were separated by HPLC and sequenced. Together with the information from earlier work (Fickenscher, K. et al. (1987) Eur. J. Biochem. 168, 653–658) the total sequence is now known to an extent of 78%. Comparison with the sequence of the corn NADP-malate dehydrogenase deduced from its cDNA (Metzler, M.C. et al. (1989) Plant Mol. Biol. 12, 713–722) showed 84% agreement; however, the 11 N-terminal residues exhibit only 27% similarity. The N- and C-terminal extrapeptides of the pea NADP-malate dehydrogenase when aligned with non-regulatory NAD-malate dehydrogenases from bacteria or mammals consist of 30 and 17 amino acids, respectively. Since all cysteine-containing peptides were sequenced, the number of eight cysteines per subunit of the pea enzyme was established. The native, oxidized enzyme ss characterized by an extremely slow reactivity of two thiols. Titration of the thiols of the denatured, oxidized enzyme both with DTNB and with pCMB resulted in six thiols not involved in disulfide formation. Therefore, one disulfide bridge must be present per 38.9 kDa subunit. Analysis of disulfide bonds by urea gel electrophoresis confirmed this finding. Using digestion products of NADP-malate dehydrogenase with aminopeptidase K, the location of the single disulfide bridge was established to be on the N-terminal arm (Cys-12 and Cys-17) of the polypeptide chain.     

Recombinant rat liver guanidinoacetate methyltransferase: reactivity and function of sulfhydryl groups

Rat liver guanidinoacetate methyltransferase, produced in Escherichia coli by recombinant DNA technique, possesses five cysteine residues per molecule. No disulfide bond is present. Analysis of the chymotryptic peptides derived from the iodo[14C]acetate-modified enzyme shows that Cys-90, Cys-15, Cys-219, and Cys-207 are alkylated by the reagent in order of decreasing reactivity. Incubation of the enzyme with excess 5,5'-dithiobis(2-nitrobenzoate) (DTNB) in the absence and presence of cystamine [2,2'-dithiobis(ethylamine)] causes the appearance of 4 and 5 mol of 2-nitro-5-mercaptobenzoate/mol of enzyme, respectively. Reaction of the methyltransferase with an equimolar amount of DTNB results in an almost quantitative disulfide cross-linking of Cys-15 and Cys-90 with loss of a large portion of the activity. The methyltransferase is completely inactivated by iodoacetate following nonlinear kinetics. Comparison of the extent of inactivation with that of modification of cysteine residues and the experiment with the enzyme whose Cys-15 and Cys-90 are cross-linked suggest that alkylation of Cys-15 and Cys-90 results in a partially active enzyme and that carboxymethylation of Cys-219 completely eliminates enzyme activity. The inactivation of guanidinoacetate methyltransferase by iodoacetate or DTNB is not protected by substrates. Furthermore, disulfide cross-linking of Cys-15 and Cys-90 or carboxymethylation of Cys-219 does not impair the enzyme's capacity to bind S-adenosylmethionine. Thus, these cysteine residues appear to occur outside the active-site region, but their integrity is crucial for the expression of enzyme activity.     

The two fast-reacting thiols of 3-phosphoglycerate kinase are structurally juxtaposed. Chemical modification with bifunctional reagents

The two fast-reacting thiol groups of pig muscle 3-phosphoglycerate kinase can be simultaneously blocked by one mole equivalent of bifunctional reagent: either mercuric chloride (HgCl2) or 1,4-bis(bromomercuri)butane. The reactions are accompanied by an enzyme activity loss of about 50-70% and 60-80% with mercuric chloride and 1,4-bis(bromomercuri)butane respectively. Removal of either of the reagents with excess cysteine leads to the recovery of at least 70-90% of the original enzymic activity. Gel chromatographic analysis revealed no change in the molecular mass of the enzyme modified with mercuric chloride, while an increase of about 30% of the apparent molecular mass was observed after the reaction with 1,4-bis(bromomercuri)butane. Since no dimer formation could be detected by independent crosslinking, the increase of the apparent molecular mass is probably due to modification causing protein conformational change. The results strongly suggest that the fast-reacting thiols are intramolecularly connected by either of the above bifunctional reagents. In the light of the known structural data on the enzyme, it may follow that the two fast-reacting thiols belong to the two sequentially neighbouring cysteinyl residues.     

Substrate-induced changes in sulfhydryl reactivity of bacterial D-amino acid transaminase

D-Amino acid transaminase from Bacillus sphaericus strain ATCC 14577 is a dimer with eight cysteinyl residues per molecule (T.S. Soper, W.M. Jones, and J.M. Manning (1979) J. Biol. Chem. 254, 10,901-10,905). The reaction of the cysteinyl residues with a variety of sulfhydryl reagents has been explored to gain insight into the physical environments around these cysteinyl residues in the absence or the presence of substrates. The native enzyme, in the pyridoxal-P conformation, appears to be a symmetrical dimer, whose SH groups react in pairs with anionic reagents such as 5,5'-dithiobis(2-nitrobenzoic acid) or the halo acids. Two SH groups react with either reagent without altering enzymatic activity. Two additional SH groups react with DTNB with loss of catalytic activity. Positively charged reagents such as beta-bromoethylamine are much more effective in inactivating the pyridoxal-P conformation of the enzyme with almost five of the eight SH groups reacting and this results in a significant loss in catalytic activity. The neutral reagent dithiodipyridine is able to detect some asymmetry in the pyridoxal-P conformation. Upon addition of a D-amino acid substrate, the enzyme is transformed into the pyridoxamine-P conformation. This conformation is much more reactive with anionic reagents and much less reactive with cationic reagents, suggesting that there is a significant change in the net charge around one of the SH groups in the pyridoxamine-P conformation. Also, titration with DTNB indicates that the enzyme is a much more asymmetric dimmer in the pyridoxamine-P conformation than in the pyridoxal-P conformation. Thus, upon binding of a D-amino acid substrate, D-amino acid transaminase is transformed into the pyridoxamine-P conformation. This results in a significant change in the environment of four of the sulfhydryl groups of the enzyme. We conclude that the enzyme is transformed from a symmetrical dimer into an asymmetrical dimer and that the net charge of one of the pairs of cysteinyl groups is changed from a net negative charge into a net positive charge. These results suggest that there is a significant conformational change that occurs during the transition from the pyridoxal-P into the pyridoxamine-P form of this transaminase.     

Site-specific modification or rabbit muscle aldolase with fluorescent probes

The site-specific modification of rabbit muscle aldolase A by labeling of thiol residues of Cys-289 with 5-(2-((iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid and Cys-239 with 5-iodoacetamidofluorescein or 4-dimethylamino-phenylazophenyl-4'-maleimide has been described. The method is based on the differences in kinetics of the chemical modification of aldolase thiols with the above reagents either in the presence or in the absence of a competitive inhibitor. The spectral properties of the doubly labeled aldolase derivatives were compared with those of the singly labeled enzyme. The doubly labeled aldolase derivatives exhibited full catalytic activity.     

Reactive oxygen species generated by thiol-modifying phenylarsine oxide stimulate the expression of protein L-isoaspartyl methyltransferase

Expression of the repair enzyme protein l-isoaspartyl methyltransferase (PIMT) has been reported to play important roles in brain. However, little is known about the regulation of PIMT expression following protein damage by oxidation in brain. Phenylarsine oxide (PAO) is an arsenical compound that alters proteins by forming disulfide bond with vicinal cysteinyl residues. Here we report that PIMT was rapidly up-regulated by PAO in U-87 human astroglioma cells. We also confirmed that PIMT up-regulation by PAO was mediated by the reaction with vicinal cysteines. Furthermore, we showed that PIMT induction by PAO was dependent on formation of reactive oxygen species (ROS). Crucially, both ROS formation and PIMT induction by PAO were inhibited by antioxidant N-acetyl-l-cysteine and NADPH oxidase inhibitor diphenyleneiodonium chloride. Importantly, down-regulation of PIMT by siRNA strikingly enhanced PAO-induced ROS. Together, these results highlight that PIMT expression is regulated by ROS and could primarily act as an antioxidant enzyme.     

Oxidation of cysteines activates cGMP-dependent protein kinase.

The functional significance of the oxidation/reduction state of sulfhydryl groups of cGMP-dependent protein kinase (cGMP kinase) was studied at 30 degrees C using different metal ions as oxidizing agents. Mn2+, Zn2+, Fe2+, Ni2+, and Co2+ failed to activate cGMP kinase, whereas Cu2+, Cu+, Fe3+, Hg2+, and Ag+ activated cGMP kinase by oxidation with an activity ratio (-cGMP/+cGMP) of about 0.7. The activation was not caused by degradation of the enzyme to a cGMP-independent constitutively active form. Reduction of the Cu(2+)-activated and gel-filtered enzyme with dithiothreitol lowered the activity ratio in the absence of cGMP to 0.17. Oxidation did not change the kinetic and binding parameters of cGMP kinase significantly but reduced the number of titratable sulfhydryl groups from 9.5 +/- 0.7 to 6.0 +/- 0.4 cysteines/75-kDa subunit. The free cysteinyl residues of the native and Cu(2+)-oxidized cGMP kinase were labeled with 4-dimethylaminoazobenzene-4'-iodoacetamide or N-(7-dimethylamino-4-methyl-3-coumarinyl)maleimide. Tryptic peptides of the labeled proteins were isolated and sequenced. The cysteinyl residues oxidized by Cu2+ were identified as disulfide bonds between Cys-117 and Cys-195 and Cys-312 and Cys-518, respectively. Cu2+ activation of cGMP kinase was prevented by mild carboxymethylation of the reduced enzyme with iodoacetamide, which apparently modified these four cysteinyl groups. The results show that cGMP kinase is activated by the formation of at least one intrachain disulfide bridge.