Sulfhydryl Reagents Research Articles

Characterization of mENT1Δ11, a Novel Alternative Splice Variant of the Mouse Equilibrative Nucleoside Transporter 1

Mammalian cells require specific transport mechanisms for the cellular uptake and release of endogenous nucleosides such as adenosine, and nucleoside analogs used in chemotherapy. We have identified a novel splice variant of the mouse equilibrative nucleoside transporter, mENT1, that results from the exclusion of exon 11 during pre-RNA processing. This variant encodes a truncated protein (mENT1Delta11) missing the last three transmembrane domains of the full-length mENT1. The mENT1Delta11 transcript and protein were found to be differentially distributed among tissues relative to full-length mENT1. PK15-NTD (nucleoside transport deficient) cells were transfected with mENT1 or mENT1Delta11 and assessed for nucleoside transport function. No significant differences were observed between the mENT1 and mENT1Delta11 in terms of transport function or inhibitor binding affinity. PK15-mENT1Delta11 transfected cells bound the ENT1 probe [3H]nitrobenzylthioinosine (NBMPR) with high affinity and mediated the cellular accumulation of both [3H]2-chloroadenosine and [3H]uridine. The only significant differences between the mENT1 variants were that mENT1Delta11 could not be photolabeled with [3H]NBMPR and that mENT1Delta11 was insensitive to the transporter-modifying effects of N-ethylmaleimide. These data suggest that the last three transmembrane domains of mENT1 are not necessary for transport activity, but this region does contain the cysteines responsible for the sensitivity of mENT1 to sulfhydryl reagents, and the residues important for covalent modification of the protein with NBMPR. These results provide important guidelines for future mutagenesis studies aimed at elucidating the tertiary structure of the ENT1 protein and the domains involved in inhibitor binding and substrate translocation.

PARTIAL PURIFICATION AND CHARACTERIZATION OF TRANSGLUTAMINASE FROM THREADFIN BREAM (NEMIPTERUS SP.) LIVER

Transglutaminase (TGase) from the threadfin bream ([TB]Nemipterus sp.) liver was partially purified using ion exchange, size exclusion and affinity chromatography. Three protein bands with molecular weight (Mw) of 95, 63 and 43 kDa on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) were observed. Only one distinct fluorescent band appeared on the TGase activity staining of the native-PAGE. When the protein band was eluted and analyzed on the SDS-PAGE, it showed a single band with an Mw of 95 kDa. The enzyme required Ca2+up to 1 mM for full activation. TGase was also activated by 10 mM Sr2+. Dithiothreitol had no effect on activity. TGase activity was unaffected by NaCl up to 0.6 M and reduced to 75% at 1.2 M NaCl. Optimum pH and temperature was 8.5–9.0 and 50C, respectively. TGase activity was markedly inhibited by the sulfhydryl reagents. The enzyme catalyzed the cross-linking of the TB myosin heavy chains. PRACTICAL APPLICATIONS Threadfin bream (TB) liver transglutaminase (TGase) exhibited Ca2+-dependent characteristic similar to muscle TGase. The enzyme catalyzed the cross-linking of myosin, and therefore, could be applied to improve the textural properties of muscle proteins. The enzyme remained its high activity up to 1.2 M NaCl (≈7% w/v), indicating its potential application in foods containing a relatively high NaCl. The biochemical characteristics of the TB liver TGase are critical information leading to more understanding in the nature of the enzyme as well as determining the optimum conditions for food applications. Moreover, the presented purification scheme could be adopted to recover TGase from the TB liver, which is currently a solid waste of the surimi industry.

HIV-1 inactivation by 4-vinylpyridine is enhanced by dissociating Zn 2+ from nucleocapsid protein

Selective inactivation of critical cysteine residues in human immunodeficiency virus type one (HIV-1) was observed after treatment with 4-vinylpyridine (4-VP), with and without the membrane-permeable metal chelator N,N,N′ ,N′-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN). Chromatographic analysis showed that cysteines contained within nucleocapsid zinc fingers, in the context of whole virus or purified protein, were essentially unreactive, but became reactive when a chelator was included. Virus treated with 4-VP showed only a modest decrease in infectivity; after TPEN addition, nearly complete inactivation of HIV-1 occurred. Similarly, quantitation of viral DNA products from 4-VP-treated virus infections showed no significant effects on reverse transcription, but did show a 14-fold reduction in proviruses; when TPEN was added, a 10 5-fold decrease in late reverse transcription products was observed and no proviruses were detected. Since 4-VP effectiveness was greatly enhanced by TPEN, this strongly suggests that modification of nucleocapsid zinc fingers is necessary and sufficient for HIV-1 inactivation by sulfhydryl reagents.

Redox and heavy metal effects on the biochemical activities of an Arabidopsis polyadenylation factor subunit

The Arabidopsis CPSF30 ortholog (AtCPSF30) is an RNA-binding endonuclease that is part of the plant polyadenylation complex. Previous work (B. Addepalli, A.G. Hunt, Nucleic Acids Res. 35 (2007) 4453–4463) demonstrated that different zinc finger motifs in the protein were responsible for RNA-binding and nuclease activity, respectively. In this study, a more detailed functional map of AtCPSF30 is presented, a map that includes descriptions of novel biochemical activities. Elevated temperatures, the specific zinc chelator 1,10-phenanthroline, and the sulfhydryl reagent dithiothreitol all had differential inhibitory effects on the RNA-binding and nuclease activities. The endonuclease activity of AtCPSF30 was inhibited by relatively high (>100μM) concentrations of zinc, and this inhibition required a plant-specific N-terminal domain apart from the zinc finger core of the protein. ATP stimulated the nuclease activity in the presence of zinc, and this stimulation required a plant-specific C-terminal domain, again apart from the zinc finger core. These studies reveal a subtle and unexpected complexity to AtCPSF30, and raise the possibility that multiple avenues of regulation may impinge on this protein through different functional domains.

Studies on the Role of Subunit d for V‐ATPase Reversible Disassembly

V1V0‐ATPase proton pumps are multisubunit proteins consisting of integral and peripheral subunits regulated by reversible disassembly in response to the extracellular concentration of glucose; it is hypothesized that disassembly is an energy conserving strategy to preserve ATP due to limited glucose. In the absence of glucose, the peropheral sub‐complex (V1) is released into the cytoplasm and the membrane sub‐complex (Vo) remains integral. The disassembled enzyme is catalytically inactive but addition of glucose to glucose‐deprived cells yields reassembly of a functional holoenzyme. This project studies subunit d, a key subunit coupling ATP hydrolysis to proton transport. Subunit d remains bound to the membrane upon disassembly and interacts with different V1 and Vo subunits depending on the V‐ATPase's state of assembly. We used computational models of subunit d to introduce cysteine residues that will be photo‐chemically cross‐linked to neighboring V‐ATPase subunits using the sulfhydryl reagent MBP. This study will map subunit interactions involving subunit d in the assembled and disassembled states. It will generate new information regarding the roles of subunit d for V‐ATPase function and the mechanism of glucose‐dependent dissociation.Supported by NSF

Key Residues of Loop 3 in the Interaction with the Interface Residue at Position 14 in Triosephosphate Isomerase from Trypanosoma brucei

Cysteine 14 is an interface residue that is fundamental for the catalysis and stability of homodimeric triosephosphate isomerase from Trypanosoma brucei (TbTIM). Its side chain is surrounded by a deep pocket of 11 residues that are part of loop 3 of the adjacent monomer. Mutation of this residue to serine (producing single mutant C14S) yields a wild-type-like enzyme that is resistant to the action of sulfhydryl reagents methylmethane thiosulfonate (MMTS) and 5,5-dithiobis(2-nitrobenzoate) (DTNB). This mutant enzyme was a starting point for probing by cysteine scanning the role of four residues of loop 3 in the catalysis and stability of the enzyme. Considering that the conservative substitution of either serine or alanine with cysteine would minimally alter the structure and properties of the environment of the residue in position 14, we made double mutants C14S/A69C, C14S/S71C, C14S/A73C, and C14S/S79C. Three of these double mutants were similar in their kinetic parameters to wild-type TbTIM and the single mutant C14S, but double mutant C14S/A73C showed a greatly reduced k cat. All enzymes had similar CD spectra, but all mutants had thermal stabilities lower than that of wild-type TbTIM. Intrinsic fluorescence was also similar for all enzymes, but the double mutants bound up to 50 times more 1-anilino-8-naphthalene sulfonate (ANS) and were susceptible to digestion with subtilisin. The double mutants were also susceptible to inactivation by sulfhydryl reagents. Double mutant C14S/S79C exhibited the highest sensitivity to MMTS and DTNB, bound a significant amount of ANS, and had the highest sensitivity to subtilisin. Thus, the residues at positions 73 and 79 are critical for the catalysis and stability of TbTIM, respectively.

Inactivation of an invertebrate acetylcholinesterase by sulfhydryl reagents: A reconsideration of the implications for insecticide design

Previously we used site-directed mutagenesis, in vitro expression, and molecular modeling to investigate the inactivation of an invertebrate acetylcholinesterase, cholinesterase 2 from amphioxus, by the sulfhydryl reagents 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM). We created the mutants C310A, C466A, C310A/C466A and C310A/F312I to assess the roles of the two cysteines and a proposal that the increased rate of inactivation previously found in an F312I mutant was due to increased access of sulfhydryl reagents to Cys310. Our results indicated that both of the cysteines could be involved in inactivation by sulfhydryl reagents, but that the cysteine near the acyl pocket was more accessible. We speculated that the inactivation of aphid AChEs by sulfhydryl reagents was due to the presence of a cysteine homologous to Cys310 and proposed that this residue could be a target for a specific insecticide. Here we reconsider this proposal.

Aromatic residues at position 55 of rat α7 nicotinic acetylcholine receptors are critical for maintaining rapid desensitization

The rat alpha7 nicotinic acetylcholine receptor (nAChR) can undergo rapid onset of desensitization; however, the mechanisms of desensitization are largely unknown. The contribution of a tryptophan (W) residue at position 55 of the rat alpha7 nAChR subunit, which lies within the beta2 strand, was studied by mutating it to other hydrophobic and/or aromatic amino acids, followed by voltage-clamp experiments in Xenopus oocytes. When mutated to alanine, the alpha7-W55A nAChR desensitized more slowly, and recovered from desensitization more rapidly, than wildtype alpha7 nAChRs. The contribution of desensitization was validated by kinetic modelling. Mutating W55 to other aromatic residues (phenylalanine or tyrosine) had no significant effect on the kinetics of desensitization, whereas mutation to various hydrophobic residues (alanine, cysteine or valine) significantly decreased the rate of onset and increased the rate of recovery from desensitization. To gain insight into possible structural rearrangements during desensitization, we probed the accessibility of W55 by mutating W55 to cysteine (alpha7-W55C) and testing the ability of various sulfhydryl reagents to react with this cysteine. Several positively charged sulfhydryl reagents blocked ACh-induced responses for alpha7-W55C nAChRs, whereas a neutral sulfhydryl reagent potentiated responses; residue C55 was not accessible for modification in the desensitized state. These data suggest that W55 plays an important role in both the onset and recovery from desensitization in the rat alpha7 nAChR, and that aromatic residues at position 55 are critical for maintaining rapid desensitization. Furthermore, these data suggest that W55 may be a potential target for modulatory agents operating via hydrophobic interactions.

Functionally important conserved length of C-terminal regions of yeast and bovine ADP/ATP carriers, identified by deletion mutants studies, and water accessibility of the amino acids at the C-terminal region of the yeast carrier

Comparison of the amino acid sequence of yeast type 2 ADP/ATP carrier (yAAC2) with that of bovine type 1 AAC (bAAC1) revealed that the N- and C-terminus of yAAC2 are 15- and 6-amino acids longer, respectively, than those of bAAC1. In the present study, we focused on the difference in the C-terminal region between yAAC2 and bAAC1. Deletion of first six residues of C-terminus of yAAC did not markedly affect the function of yAAC2; however, further deletion of 1 amino acid (7th amino acid from the C-terminus) destroyed its function. On the contrary, deletion of the first amino acid residue of the C-terminus of bAAC1 caused failure of its functional expression in yeast mitochondria. Based on these results, we concluded that the 6-amino acid residue extension of the C-terminus of yAAC2 was not necessary for the function of this carrier and that the remainder of the C-terminal region of yAAC2, having a length conserved with that of bAAC1, is important for the transport function of AACs. We next prepared various single-Cys mutants in which each of 32 residues in the C-terminus of yAAC2 was replaced by a Cys residue. Since all mutants were successfully expressed in yeast mitochondria, we examined the reactivity of these cysteine residues with the membrane-impermeable sulfhydryl reagent eosin 5-maleimide (EMA). As a result, all cysteine residues that replaced the 9 continuous amino acids in Met310-Lys318 showed high reactivity with EMA regardless of the presence of carboxyatractyloside or bongkrekic acid; and so this region was concluded to be exposed to the water-accessible environment. Furthermore, based on the reactivities of cysteine residues that replaced amino acids in the sixth transmembrane segment, the probable structural features of the C-terminal region of this carrier in the presence of bongkrekic acid were discussed.

Conformational Changes in a Pore-lining Helix Coupled to Cystic Fibrosis Transmembrane Conductance Regulator Channel Gating

Cystic fibrosis transmembrane conductance regulator (CFTR), the protein dysfunctional in cystic fibrosis, is unique among ATP-binding cassette transporters in that it functions as an ion channel. In CFTR, ATP binding opens the channel, and its subsequent hydrolysis causes channel closure. We studied the conformational changes in the pore-lining sixth transmembrane segment upon ATP binding by measuring state-dependent changes in accessibility of substituted cysteines to methanethiosulfonate reagents. Modification rates of three residues (resides 331, 333, and 335) near the extracellular side were 10-1000-fold slower in the open state than in the closed state. Introduction of a charged residue by chemical modification at two of these positions (resides 331 and 333) affected CFTR single-channel gating. In contrast, modifications of pore-lining residues 334 and 338 were not state-dependent. Our results suggest that ATP binding induces a modest conformational change in the sixth transmembrane segment, and this conformational change is coupled to the gating mechanism that regulates ion conduction. These results may establish a structural basis of gating involving the dynamic rearrangement of transmembrane domains necessary for vectorial transport of substrates in ATP-binding cassette transporters.

Role of Ser-340 and Thr-341 in Transmembrane Domain IX of the Na+/Proline Transporter PutP of Escherichia coli in Ligand Binding and Transport

The Na+/solute symporter family comprises more than 400 members of pro- and eukaryotic origin. Using the Na+/proline transporter PutP of Escherichia coli as a model, the role of two conserved residues, Ser-340 and Thr-341, is investigated to obtain insights into the mechanism of transport catalyzed by members of this family. Substitution of these amino acids alters the transport kinetics of cells and proteoliposomes containing the PutP variants significantly. In particular, the apparent affinities for Na+ and Li+ are reduced by 2 orders of magnitude or more. Also proline binding is affected, albeit to a lesser extent than ion binding. Thereby, the presence of a hydroxyl group at position 341 is essential for high affinity ligand binding. Furthermore, Cys placed at position 340 or 341 reacts with sulfhydryl reagents of different polarity, indicating accessibility from the water phase. In addition, Cys cross-linking suggests proximity of the residues to other amino acids previously shown to be crucial for ligand binding. For these reasons it is suggested that Ser-340 and Thr-341 are located in a ligand translocation pathway. Furthermore, it is proposed that the side chain of Thr-341 directly participates in Na+ binding.

Studies on membrane-associated prostaglandin E synthase-2 with reference to production of 12L-hydroxy-5,8,10-heptadecatrienoic acid (HHT)

Membrane-associated prostaglandin (PG) E synthase (mPGE synthase)-2 catalyzes the conversion of PGH 2 primarily to PGE 2. The enzyme is activated by various sulfhydryl reagents including dithiothreitol, dihydrolipoic acid, and glutathione, and it is different from mPGE synthase-1 and cytosolic PGE synthase, both of which require specifically glutathione. Recently, other investigators reported that their preparation of mPGE synthase-2 containing heme converted PGH 2 to 12L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) rather than to PGE 2 [T. Yamada, F. Takusagawa, Biochemistry 46 (2007) 8414–8424]. As we examined presently, the heme-bound enzyme expressed and purified according to their method synthesized HHT from PGH 2, but also PGE 2 in a decreased amount. Whereas the PGE synthase activity was completely lost at 50 °C for 5 min, the HHT synthase activity remained even at 100 °C for 5 min. In contrast, when the heme-bound enzyme was purified in the presence of dithiothreitol, only PGE 2 was produced, but essentially no HHT was detected. Thus, native mPGE synthase-2 enzymatically catalyzes only the conversion of PGH 2 to PGE 2, but not to HHT, and heme is not involved in this reaction.

Acid Phosphatases from the Liver of Labeo rohita: Purification and Characterization

Low molecular weight acid phosphatase (LM-ACP) peak 2 (the isoenzyme corresponding to isoform 2, IF-2) from the liver of fish Rahu (Labeo rohita) was purified to homogeneity. 900 times purification resulted with specific activity of 35 U/mg of protein and recovery of 0.2%. The enzyme was found homogeneous on sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Molecular weight of 18 killo Daltons (kDa) was obtained. The peak 1 isoenzyme corresponding to isoform1 (IF-1) was partially purified about 160 times with specific activity of 7 U/mg of protein. Major protein band corresponding to 18 kDa was seen along with other protein faint bands. High molecular weight acid phosphatase (HM-ACP) was also partially purified. The molecular weight was estimated to be a 100 kDa by gel filtration on Sephadex G-100. LM-ACP isoenzymes and HM-ACP enzyme were studied for their substrate specificity, sensitivity to inhibitors or activators and other kinetic parameters. LM-ACP isoenzymes were not inhibited by tartrate and fluoride but were inhibited by sulfhydryl reagent whereas high molecular weight enzyme was strongly inhibited by fluoride and tartrate. Phosphate vanadate and molybdate inhibited both types of enzymes competitively, but their action was more pronounced in HM-ACP enzyme. LM-ACP was effectively activated by purine compounds whereas HM-ACP was not. LM-ACP showed strict substrate specificity while HM-ACP showed broad substrate specificity. The two types of acid phosphatases also differed in their rate of hydrolysis of alpha-naphthyl phosphate and beta-glyerophosphate.

Cadmium Trapping in an Epithelial Sodium Channel Pore Mutant

The putative selectivity filter of the epithelial sodium channel (ENaC) comprises a three-residue sequence G/SXS, but it remains uncertain whether the backbone atoms of this sequence or whether their side chains are lining the pore. It has been reported that the S589C mutation in the selectivity filter of alphaENaC renders the channel sensitive to block by externally applied Cd2+; this was interpreted as evidence for Cd2+ coordination with the thiol group of the side chain of alpha589C, pointing toward the pore lumen. Because the alphaS589C mutation alters the monovalent to divalent cation selectivity ratio of ENaC and because internally applied Cd2+ blocks wild-type ENaC with high affinity, we hypothesized that the inhibition of alphaS589C ENaC by Cd2+ results rather from the coordination of this cation with native cysteine residues located in the internal pore of ENaC. We show here that Cd2+ inhibits not only ENaC alphaS589C and alphaS589D but also alphaS589N mutants and that Ca2+ weakly interacts with the S589D mutant. The block of alphaS589C, -D, and -N mutants is characterized by a slow on-rate, is nearly irreversible, is voltage-dependent, and can be prevented by amiloride. The C546S mutation in the second transmembrane helix of gamma subunit in the background of the ENaC alphaS589C, -D, or -N mutants reduces the sensitivity to block by Cd2+ and renders the block rapidly reversible. We conclude therefore that the block by Cd2+ of the alphaS589C, -D, and -N mutants results from the trapping of Cd2+ ions in the internal pore of the channel and involves Cys-546 in the second transmembrane helix of the gammaENaC subunit.

Isolation and characterization of the DNA-binding protein (DBP) of the Autographa californica multiple nucleopolyhedrovirus

DNA-binding protein (DBP) of Autographa californica multiple nucleopolyhedrovirus (Ac MNPV) was expressed as an N-terminal His 6-tag fusion using a recombinant baculovirus and purified to near homogeneity. Purified DBP formed oligomers that were crosslinked by redox reagents resulting in predominantly protein dimers and tetramers. In gel retardation assays, DBP showed a high affinity for single-stranded oligonucleotides and was able to compete with another baculovirus SSB protein, LEF-3, for binding sites. DBP binding protected ssDNA against hydrolysis by a baculovirus alkaline nuclease AN/LEF-3 complex. Partial proteolysis by trypsin revealed a domain structure of DBP that is required for interaction with DNA and that can be disrupted by thermal treatment. Binding to ssDNA, but not to dsDNA, changed the pattern of proteolytic fragments of DBP indicating adjustments in protein structure upon interaction with ssDNA. DBP was capable of unwinding short DNA duplexes and also promoted the renaturation of long complementary strands of ssDNA into duplexes. The unwinding and renaturation activities of DBP, as well as the DNA binding activity, were sensitive to sulfhydryl reagents and were inhibited by oxidation of thiol groups with diamide or by alkylation with N-ethylmaleimide. A high affinity of DBP for ssDNA and its unwinding and renaturation activities confirmed identification of DBP as a member of the SSB/recombinase family. These activities and a tight association with subnuclear structures suggests that DBP is a component of the virogenic stroma that is involved in the processing of replicative intermediates.

Purification and characterization of a leucine aminopeptidase from the skeletal muscle of common carp ( Cyprinus carpio)

An aminopeptidase was purified from the skeletal muscle of common carp ( Cyprinus carpio) to homogeneity, with 1160-fold purification and a yield of 4.3%. The purification procedure consisted of ammonium sulfate fractionation and sequential chromatographic steps, including DEAE-Sephacel, Sephacryl S-200, hydroxyapatite, Phenyl Sepharose and Macro-Prep High Q columns. The molecular mass of the enzyme was estimated to be 105 kDa and 100 kDa by SDS-PAGE under reducing conditions and gel filtration chromatography, respectively, suggesting it to be a monomer. The enzymatic activity was optimal at 35 °C and pH 7.0. The metal-chelating agents EDTA, EGTA and 1,10-phenanthroline specifically inhibited the enzyme activity while inhibitors of other proteinases did not show much effect, indicating that it was a metalloproteinase. Furthermore, bestatin, a specific aminopeptidase inhibitor strongly inhibited its activity. Divalent cations Mn 2+, Mg 2+ and Ba 2+ slightly enhanced the enzymatic activity, while Co 2+, Cu 2+, Zn 2+, Ca 2+ and Fe 2+ inhibited the activity to different extents. In addition, a sulfhydryl reagent was necessary to maintain its activity. Substrate specificity study revealed that the purified enzyme preferentially hydrolyzed Leu-MCA, followed by Arg-MCA, Ala-MCA and Tyr-MCA and it was thus regarded as a leucine aminopeptidase (LAP, EC 3.4.11.1). The apparent K m and V max values of the enzyme were 4.6 μM and 9.6 μmol min −1 mg −1, respectively for substrate Leu-MCA. This is the first report concerning purification and characterization of LAP from freshwater fish.

Formation of stable platelet aggregates by lectin from Solanum tuberosum

The GlcNAc-specific lectin from Solanum tuberosum is shown to induce haptenic-sugar-resistant contacts in platelet aggregation but not to induce stable neutrophil and lymphocyte aggregation. The formation of such contacts in platelets was significantly hindered by the inhibitors of cAMP phosphodiesterase (papaverine) or arachidonic acid metabolism (indomethacin, aristolochic acid, or MK-886) and by a sulfhydryl reagent (N-ethylmaleimide). This lectin can be useful in studying the mechanisms of stable platelet aggregation, drug screening for antithrombotic activity, and developing the cell engineering techniques.

Photoexcitation of tryptophan groups induced reduction of disulfide bonds in hen egg white lysozyme

Prolonged exposure of hen egg white lysozyme to near-UV light results in an unusual fluctuating and red shift of fluorescence spectra. By chemical detection of free thiol groups with sulfhydryl reagent and spectroscopic measurement, it is found that the fluorescence changes are accompanied with the cleavage of disulfide bonds upon irradiation. However, tryptophan and the new generated free thiol groups are not stable within the protein and will further react during prolonged irradiation. The existence of photo-induced dimers is uncovered based on the fact that some intermediately formed thiol radicals in different lysozyme molecules can further form intermolecular S–S bonds. In addition, the decrease of UV absorption ability of lysozyme indicates that tryptophan will be reoxidated after the reduction of disulfide bonds. Then a conclusion is drawn that the disulfide bonds in lysozyme are only partially reduced and the reduction of Cys30-Cys115 may be preferential based on the structural analysis. Meanwhile, it is found that the tertiary structure of lysozyme is not very compact, and the secondary structure is also gradually disturbed during illumination.

Modulating the activity of avian pancreatic lipases by an alkyl chain reacting with an accessible sulfhydryl group

Both turkey (TPL) and chicken (CPL) pancreatic lipases possess only one exposed sulfhydryl residue (Cystein114). After preincubation with the lipase, the sulfhydryl reagent C 12-TNB was found to be a powerful inhibitor of TPL whereas it had no effect on the CPL activity. Based on the 3D structure modelling and the molecular dynamics, the bulky dodecyl chain might hamper the lid movement of the TPL leading to the lipase inhibition upon reaction with C 12-TNB. Meanwhile, the predicted position of the C 12 chain linked to Cystein114 of CPL could not block the lid opening mechanism which explains the absence of inhibition by C 12-TNB. Surprisingly, when added during the substrate hydrolysis, C 12-TNB activated the TPL but not the CPL that was slightly inhibited under these conditions. The 3D structure model generated for the open forms of C 12-TPL and C 12-CPL complexes showed that Cystein114 is still accessible and might react with C 12-TNB. Our models clearly explain the activation of TPL and the partial inhibition of CPL after the binding of the C 12 chain to the enzyme.

Functional Role of Extracellular Loop Cysteine Residues of the Epithelial Na+ Channel in Na+ Self-inhibition

The epithelial Na(+) channel (ENaC) is typically formed by three homologous subunits (alpha, beta, and gamma) that possess a characteristic large extracellular loop (ECL) containing 16 conserved cysteine (Cys) residues. We investigated the functional role of these Cys residues in Na(+) self-inhibition, an allosteric inhibition of ENaC activity by extracellular Na(+). All 16 Cys residues within alpha and gamma ECLs and selected beta ECL Cys residues were individually mutated to alanine or serine residues. The Na(+) self-inhibition response of wild type and mutant channels expressed in Xenopus oocytes was determined by whole cell voltage clamp. Individual mutation of eight alpha (Cys-1, -4, -5, -6, -7, -10, -13, or -16), one beta (Cys-7), and nine gamma (Cys-3, -4, -6, -7, -10, -11, -12, -13, or -16) residues significantly reduced the magnitude of Na(+) self-inhibition. Na(+) self-inhibition was eliminated by simultaneous mutations of either the last three alpha ECL Cys residues (Cys-14, -15, and -16) or Cys-7 within both alpha and gamma ECLs. By analyzing the Na(+) self-inhibition responses and the effects of a methanethiosulfonate reagent on channel currents in single and double Cys mutants, we identified five Cys pairs within the alphaECL (alphaCys-1/alphaCys-6, alphaCys-4/alphaCys-5, alphaCys-7/alphaCys-16, alphaCys-10/alphaCys-13, and alphaCys-11/alphaCys-12) and one pair within the gammaECL (gammaCys-7/gammaCys-16) that likely form intrasubunit disulfide bonds. We conclude that approximately half of the ECL Cys residues in the alpha and gamma ENaC subunits are required to establish the tertiary structure that ensures a proper Na(+) self-inhibition response, likely by formation of multiple intrasubunit disulfide bonds.