Impeded electron transfer from a pathogenic FMN domain mutant of methionine synthase reductase and its responsiveness to flavin supplementation.

Methionine synthase reductase (MSR) is a diflavin oxidoreductase that transfers electrons from NADPH to oxidized cobalamin and plays a vital role in repairing inactive cobalamin-dependent methionine synthase. MSR deficiency is a recessive genetic disorder affecting folate and methionine metabolism and is characterized by elevated levels of plasma homocysteine. In this study, we have examined the molecular basis of MSR dysfunction associated with a patient mutation, A129T, which is housed in the FMN binding domain and is adjacent to a cluster of conserved acidic residues found in diflavin oxidoreductases. We show that the substitution of alanine with threonine destabilizes FMN binding without affecting the NADPH coenzyme specificity or affinity, indicating that the mutation's effects may be confined to the FMN module. The A129T MSR mutant transfers electrons to ferricyanide as efficiently as wild type MSR but the rate of cytochrome c, 2,6-dichloroindophenol, and menadione reduction is decreased 10-15 fold. The mutant is depleted in FMN and reactivates methionine synthase with 8% of the efficiency of wild type MSR. Reconstitution of A129T MSR with FMN partially restores its ability to reduce cytochrome c and to reactivate methionine synthase. Hydrogen-deuterium exchange mass spectrometric studies localize changes in backbone amide exchange rates to peptides in the FMN-binding domain. Together, our results reveal that the primary biochemical penalty associated with the A129T MSR mutant is its lower FMN content, provide insights into the distinct roles of the FAD and FMN centers in human MSR for delivering electrons to various electron acceptors, and suggest that patients harboring the A129T mutation may be responsive to riboflavin therapy.

Analysis of methionine/selenomethionine oxidation and methionine sulfoxide reductase function using methionine-rich proteins and antibodies against their oxidized forms.

Methionine (Met) residues are present in most proteins. However, this sulfur-containing amino acid is highly susceptible to oxidation. In cells, the resulting Met sulfoxides are reduced back to Met by stereospecific reductases MsrA and MsrB. Reversible Met oxidation occurs even in the absence of stress, is elevated during aging and disease, but is notoriously difficult to monitor. In this work, we computationally identified natural Met-rich proteins (MRPs) and characterized three such proteins containing 21-33% Met residues. Oxidation of multiple Met residues in MRPs with H(2)O(2) and reduction of Met sulfoxides with MsrA/MsrB dramatically influenced the mobility of these proteins on polyacrylamide gels and could be monitored by simple SDS-PAGE. We further prepared antibodies enriched for reduced and Met sulfoxide forms of these proteins and used them to monitor Met oxidation and reduction by immunoblot assays. We describe applications of these reagents for the analysis of MsrA and MsrB functions, as well as the development of the assay for high-throughput analysis of their activities. We also show that all Met sulfoxide residues in an MRP can be reduced by MsrA and MsrB. Furthermore, we prepared a selenomethionine form of an MRP and found that selenomethionine selenoxide residues can be efficiently reduced nonenzymatically by glutathione and other thiol compounds. Selenomethionine selenoxide residues were not recognized by antibodies specific for the Met sulfoxide form of an MRP. These findings, reagents, assays, and approaches should facilitate research and applications in the area of Met sulfoxide reduction, oxidative stress, and aging.

Low molecular weight protein tyrosine phosphatase (LMW-PTP) and its possible physiological functions of redox signaling in the eye lens.

Low molecular weight protein tyrosine phosphatase (LMW-PTP) was cloned from human lens epithelial B3 cells (HLE B3) and the recombinant enzyme was purified to homogeneity. The pure enzyme reacted positively with anti-LMW-PTP antibody, displayed tyrosine-specific phosphatase activity and was extremely sensitive to H(2)O(2). The inactivated LMW-PTP could be regenerated by thioltransferase (TTase)/GSH system as demonstrated by both activity assay and by mass spectrometry (MS). The MS study also showed that an intramolecular disulfide bond was formed between C13 and C18 at the active site, and was reduced by the TTase/GSH system. The putative role of LMW-PTP in regulating platelet derived growth factor (PDGF)-stimulated cell signaling was demonstrated in wild type mouse lens epithelial cells (LEC) in which LMW-PTP was transiently inactivated, corroborated with the transient phosphorylation of Tyr857 at the active site of PDGF receptor and the downstream signaling components of Akt and ERK1/2. In contrast, LMW-PTP activity in PDGF-stimulated LEC from TTase(-/-) mice was progressively lost, concomitant with the high basal and sustained high phosphorylation levels at Tyr857, Akt and ERK1/2. We conclude that the reversible LMW-PTP activity regulated by ROS-mediated oxidation and TTase/GSH reduction is the likely mechanism of redox signaling in lens epithelial cells.

Equilibrium and kinetic folding of rabbit muscle triosephosphate isomerase by hydrogen exchange mass spectrometry.

Unfolding and refolding of rabbit muscle triosephosphate isomerase (TIM), a model for (betaalpha)8-barrel proteins, has been studied by amide hydrogen exchange/mass spectrometry. Unfolding was studied by destabilizing the protein in guanidine hydrochloride (GdHCl) or urea, pulse-labeling with 2H2O and analyzing the intact protein by HPLC electrospray ionization mass spectrometry. Bimodal isotope patterns were found in the mass spectra of the labeled protein, indicating two-state unfolding behavior. Refolding experiments were performed by diluting solutions of TIM unfolded in GdHCl or urea and pulse-labeling with 2H2O at different times. Mass spectra of the intact protein labeled after one to two minutes had three envelopes of isotope peaks, indicating population of an intermediate. Kinetic modeling indicates that the stability of the folding intermediate in water is only 1.5 kcal/mol. Failure to detect the intermediate in the unfolding experiments was attributed to its low stability and the high concentrations of denaturant required for unfolding experiments. The folding status of each segment of the polypeptide backbone was determined from the deuterium levels found in peptic fragments of the labeled protein. Analysis of these spectra showed that the C-terminal half folds to form the intermediate, which then forms native TIM with folding of the N-terminal half. These results show that TIM folding fits the (4+4) model for folding of (betaalpha)8-barrel proteins. Results of a double-jump experiment indicate that proline isomerization does not contribute to the rate-limiting step in the folding of TIM.

Androgen-stimulated UDP-glucose dehydrogenase expression limits prostate androgen availability without impacting hyaluronan levels.

UDP-glucose dehydrogenase (UGDH) oxidizes UDP-glucose to UDP-glucuronate, an essential precursor for production of hyaluronan (HA), proteoglycans, and xenobiotic glucuronides. High levels of HA turnover in prostate cancer are correlated with aggressive progression. UGDH expression is high in the normal prostate, although HA accumulation is virtually undetectable. Thus, its normal role in the prostate may be to provide precursors for glucuronosyltransferase enzymes, which inactivate and solubilize androgens by glucuronidation. In this report, we quantified androgen dependence of UGDH, glucuronosyltransferase, and HA synthase expression. Androgen-dependent and androgen-independent human prostate cancer cell lines were used to test the effects of UGDH manipulation on tumor cell growth, HA production, and androgen glucuronidation. Dihydrotestosterone (DHT) increased UGDH expression approximately 2.5-fold in androgen-dependent cells. However, up-regulation of UGDH did not affect HA synthase expression or enhance HA production. Mass spectrometric analysis showed that DHT was converted to a glucuronide, DHT-G, at a 6-fold higher level in androgen-dependent cells relative to androgen-independent cells. The increased solubilization and elimination of DHT corresponded to slower cellular growth kinetics, which could be reversed in androgen-dependent cells by treatment with a UDP-glucuronate scavenger. Collectively, these results suggest that dysregulated expression of UGDH could promote the development of androgen-independent tumor cell growth by increasing available levels of intracellular androgen.

Characterization of a novel dithiocarbamate glutathione reductase inhibitor and its use as a tool to modulate intracellular glutathione.

Thiol redox state (TRS) is an important parameter to reflect intracellular oxidative stress and is associated with various normal and abnormal biochemical processes. Agents that can be used to increase intracellular TRS will be valuable tools in TRS-related research. Glutathione reductase (GR) is a critical enzyme in the homeostasis of TRS. The enzyme catalyzes the reduction of GSSG to GSH to maintain a high GSH:GSSG ratio. Inhibition of the enzyme can be used to increase TRS. Despite the reports of various GR inhibitors, N,N-bis(2-chloroethyl)-N-nitrosourea, an anticancer drug with IC(50) = 647 microm against yeast GR, remains the most commonly used GR inhibitor in the literature. However, the toxicity caused by nonspecific interactions, as well as inhibition of DNA synthesis, complicates the use of N,N-bis(2-chloroethyl)-N-nitrosourea as a GR inhibitor. We report 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylthiocarbonylamino)phenylthiocarbamoylsulfanyl]propionic acid (2-AAPA) as a novel irreversible GR inhibitor. 2-AAPA was prepared by one-step synthesis from commercially available reagents. The K(i) and k(inact) of 2-AAPA against yeast GR were determined to be 56 microm and 0.1 min(-1), respectively. At the concentration that produced >80% yeast GR inhibition, 2-AAPA showed no inhibition against glutamylcysteine synthetase, glutathione synthetase, catalase, and superoxide dismutase, but minimal inhibition against glutathione S-transferase and glutathione peroxidase. In CV-1 cells, 2-AAPA (0.1 mm) produced 97% GR inhibition, 25% GSH reduction, and a 5-fold increase in GSSG in 20 min. The compound can be a useful tool in TRS-related research.

Formation of a stabilized cysteine sulfinic acid is critical for the mitochondrial function of the parkinsonism protein DJ-1.

The formation of cysteine-sulfinic acid has recently become appreciated as a modification that links protein function to cellular oxidative status. Human DJ-1, a protein associated with inherited parkinsonism, readily forms cysteine-sulfinic acid at a conserved cysteine residue (Cys106 in human DJ-1). Mutation of Cys106 causes the protein to lose its normal protective function in cell culture and model organisms. However, it is unknown whether the loss of DJ-1 protective function in these mutants is due to the absence of Cys106 oxidation or the absence of the cysteine residue itself. To address this question, we designed a series of substitutions at a proximal glutamic acid residue (Glu18) in human DJ-1 that alter the oxidative propensity of Cys106 through changes in hydrogen bonding. We show that two mutations, E18N and E18Q, allow Cys106 to be oxidized to Cys106-sulfinic acid under mild conditions. In contrast, the E18D mutation stabilizes a cysteine-sulfenic acid that is readily reduced to the thiol in solution and in vivo. We show that E18N and E18Q can both partially substitute for wild-type DJ-1 using mitochondrial fission and cell viability assays. In contrast, the oxidatively impaired E18D mutant behaves as an inactive C106A mutant and fails to protect cells. We therefore conclude that formation of Cys106-sulfinic acid is a key modification that regulates the protective function of DJ-1.

Transcriptional activation of dehalorespiration. Identification of redox-active cysteines regulating dimerization and DNA binding.

Desulfitobacterium dehalogenans can use chlorinated aromatics including polychlorinated biphenyls as electron acceptors in a process called dehalorespiration. Expression of the cpr gene cluster involved in this process is regulated by CprK, which is a member of the CRP/FNR (cAMP-binding protein/fumarate nitrate reduction regulatory protein) family of helix-turn-helix transcriptional regulators. High affinity interaction of the chlorinated aromatic compound with the effector domain of CprK triggers binding of CprK to an upstream target DNA sequence, which leads to transcriptional activation of the cpr gene cluster. When incubated with oxygen or diamide, CprK undergoes inactivation; subsequent treatment with dithiothreitol restores activity. Using mass spectrometry, this study identifies two classes of redox-active thiol groups that form disulfide bonds upon oxidation. Under oxidative conditions, Cys105, which is conserved in FNR and most other CprK homologs, forms an intramolecular disulfide bond with Cys111, whereas an intermolecular disulfide bond is formed between Cys11 and Cys200. SDS-PAGE and site-directed mutagenesis experiments indicate that the Cys11/Cys200 disulfide bond links two CprK subunits in an inactive dimer. Isothermal calorimetry and intrinsic fluorescence quenching studies show that oxidation does not change the affinity of CprK for the effector. Therefore, reversible redox inactivation is manifested at the level of DNA binding. Our studies reveal a strategy for limiting expression of a redox-sensitive pathway by using a thiol-based redox switch in the transcription factor.

A bidomain nonribosomal peptide synthetase encoded by FUM14 catalyzes the formation of tricarballylic esters in the biosynthesis of fumonisins.

Fumonisins are a group of polyketide-derived mycotoxins produced by Fusarium verticillioides, a filamentous fungus infecting corn and contaminating food and feeds. Fumonisins contain two tricarballylic esters that are critical for toxicity. Here, we present genetic and biochemical data for the esterification mechanism. FUM14 in F. verticillioides has been deleted by homologous recombination, and the resultant mutant lost the ability to produce fumonisins. Two new metabolites, HFB(3) and HFB(4), which are biosynthetic precursors of fumonisins lacking the tricarballylic esters, were detected in the mutant. The results suggest that FUM14 is required for the esterification of fumonisins. FUM14 was predicted to encode a nonribosomal peptide synthetase (NRPS) containing two domains, peptidyl carrier protein and condensation domain. Both the intact Fum14p and the condensation domain have been expressed in Escherichia coli and purified for activity assays. Fum14p was able to convert HFB(3) and HFB(4) to the tricarballylic esters-containing fumonisins, FB(3) and FB(4), respectively, when incubated with tricarballylic thioester of N-acetylcysteamine. In addition, the condensation domain was able to convert HFB(1) to FB(1). These data provide direct evidence for the role of Fum14p in the esterification of fumonisins. More interestingly, the results are the first example of an NRPS condensation domain catalyzing a C-O bond (ester) formation, instead of the typical C-N bond (amide) formation in nonribosomal peptides. The understanding of the esterification mechanism provides useful knowledge for mycotoxin reduction and elimination. The study also provides new insight into the reactions catalyzed by NRPS.