A novel heterodimeric antimicrobial peptide from the tree-frog Phyllomedusa distincta

We present here the purification and the analysis of the structural and functional properties of distinctin, a 5.4 kDa heterodimeric peptide with antimicrobial activity from the tree-frog Phyllomedusa distincta. This peptide was isolated from the crude extract of skin granular glands by different chromatographic steps. Its minimal inhibitory concentration was determined against pathogenic Escherichia coli, Staphylococcus aureus, Enterococcus faecalis and Pseudomonas aeruginosa strains. Amino acid sequencing and mass spectrometric investigations demonstrated that distinctin is constituted of two different polypeptide chains connected by an intermolecular disulphide bridge. Circular dichroism and Fourier-transformed infrared spectroscopy studies showed that this molecule adopts, in water, a structure containing a significant percentage of anti-parallel beta-sheet. A conformational variation was observed under experimental conditions mimicking a membrane-like environment. Database searches did not show sequence similarities with any known antimicrobial peptides. In the light of these results, we can consider distinctin as the first example of a new class of antimicrobial heterodimeric peptides from frog skin.     

Isolation and identification of three bactericidal domains in the bovine alpha-lactalbumin molecule

Proteolytic digestion of alpha-lactalbumin by pepsin, trypsin and chymotrypsin yielded three polypeptide fragments with bactericidal properties. Two fragments were obtained from the tryptic digestion. One was a pentapeptide with the sequence EQLTK (residues 1-5) and the other, GYGGVSLPEWVCTTF ALCSEK (residues (17-31)S-S(109-114)), was composed of two polypeptide chains held together by a disulfide bridge. Fragmentation of alpha-lactalbumin by chymotrypsin yielded CKDDQNPH ISCDKF (residues (61-68)S-S(75-80)), also a polypeptide composed of two polypeptide chains held together by a disulfide bridge. The three polypeptides were synthesized and found to exert antimicrobial activities. The polypeptides were mostly active against Gram-positive bacteria. Gram-negative bacteria were only poorly susceptible to the bactericidal action of the polypeptides. GYGGVSLPEWVCTTF ALCSEK was most, EQLTK least bactericidal. Replacement of leucine (23) with isoleucine, having a similar chemical structure but higher hydrophobicity, in the sequence GYGGVSLPEWVCTTF ALCSEK significantly reduced the bactericidal capacity of the polypeptide. Digestion of alpha-lactalbumin by pepsin yielded several polypeptide fragments without antibacterial activity. alpha-Lactalbumin in contrast to its polypeptide fragments was not bactericidal against all the bacterial strains tested. Our results suggest a possible antimicrobial function of alpha-lactalbumin after its partial digestion by endopeptidases.     

Chain assembly intermediate in the biosynthesis of type III procollagen in chick embryo blood vessels

A general mechanism for the assembly of procollagens is proposed from a biosynthetic study of procollagen III. This was shown to proceed by a stepwise process punctuated by disulfide bond formation and an assembly intermediate was recovered. The biosynthesis of type III procollagen in excised chick embryo blood vessels was studied by radioactive labeling for 30 min. Velocity sedimentation under denaturing conditions and purified antibodies specific against bovine amino propeptide III were used to identify and characterize monomeric pro alpha 1 III chains and a type III procollagen intermediate which is interchain disulfide-linked only at the carboxyl end but not at the amino end. The monomeric chains presumably have intrachain disulfide bonds within the propeptides. The monomeric pro alpha 1 III chains were also found when alpha, alpha'-dipyridyl was present during incubation. Pulse-chase experiments show that the monomeric chains and the intermediate are biosynthetic precursors of type III procollagen. Furthermore, it is shown that monomeric pro alpha 1 chains are not triple helical when extracted under nondenaturing conditions. The results indicate that the assembly of pro alpha 1 III chains into type III procollagen starts with the association of the folded carboxyl propeptides and is followed by formation of disulfide bonds between carboxyl propeptides, folding of the triple helix, and formation of disulfide bonds between amino propeptides. All procollagens may follow a similar assembly sequence.     

Structure and conformation of the disulfide bond in dimeric lung surfactant peptides SP-B1-25 and SP-B8-25

Raman spectroscopy was used to determine the conformation of the disulfide linkage between cysteine residues in the homodimeric construct of the N-terminal alpha helical domain of surfactant protein B (dSP-B(1-25)). The conformation of the disulfide bond between cysteine residues in position 8 of the homodimer of dSP-B(1-25) was compared with that of a truncated homodimer (dSP-B(8-25)) of the peptide having a disulfide linkage at the same position in the alpha helix. Temperature-dependent Raman spectra of the S-S stretching region centered at approximately 500 cm(-1) indicated a stable, although highly strained disulfide conformation with a chi(CS-SC) dihedral angle of +/-10 degrees for the dSP-B(1-25) dimer. In contrast, the truncated dimer dSP-B(8-25) exhibited a series of disulfide conformations with the chi(CS-SC) dihedral angle taking on values of either +/-30 degrees or 85+/-20 degrees . For conformations with chi(CS-SC) close to the +/-90 degrees value, the Raman spectra of the 8-25 truncated dimers exhibited chi(SS-CC) dihedral angles of 90/180 degrees and 20-30 degrees . In the presence of a lipid mixture, both constructs showed a nu(S-S) band at approximately 488 cm(-1), corresponding to a chi(CS-SC) dihedral angle of +/-10 degrees . Polarized infrared spectroscopy was also used to determine the orientation of the helix and beta-sheet portion of both synthetic peptides. These calculations indicated that the helix was oriented primarily in the plane of the surface, at an angle of approximately 60-70 degrees to the surface normal, while the beta structure had approximately 40 degrees tilt. This orientation direction did not change in the presence of a lipid mixture or with temperature. These observations suggest that: (i) the conformational flexibility of the disulfide linkage is dependent on the amino acid residues that flank the cysteine disulfide bond, and (ii) in both constructs, the presence of a lipid matrix locks the disulfide bond into a preferred conformation.     

Monomeric and dimeric GDF-5 show equal type I receptor binding and oligomerization capability and have the same biological activity

Growth and differentiation factor 5 (GDF-5) is a homodimeric protein stabilized by a single disulfide bridge between cysteine 465 in the respective monomers, as well as by three intramolecular cysteine bridges within each subunit. A mature recombinant human GDF-5 variant with cysteine 465 replaced by alanine (rhGDF-5 C465A) was expressed in E. coli, purified to homogeneity, and chemically renatured. Biochemical analysis showed that this procedure eliminated the sole interchain disulfide bond. Surprisingly, the monomeric variant of rhGDF-5 is as potent in vitro as the dimeric form. This could be confirmed by alkaline phosphatase assays and Smad reporter gene activation. Furthermore, dimeric and monomeric rhGDF-5 show comparable binding to their specific type I receptor, BRIb. Studies on living cells showed that both the dimeric and monomeric rhGDF-5 induce homomeric BRIb and heteromeric BRIb/BRII oligomers. Our results suggest that rhGDF-5 C465A has the same biological activity as rhGDF-5 with respect to binding to, oligomerization of and signaling through the BMP receptor type Ib.     

Replacement of disulfides by amide bonds in the relaxin-like factor (RLF/INSL3) reveals a role for the A11-B10 link in transmembrane signaling

The relaxin-like factor (RLF) also named insulin-like 3 (INSL3) consists of two polypeptide chains linked by two interchain and one intrachain disulfide bond. RLF binds to its receptor (LGR8 also named RXFP2) through the B chain and initiates transmembrane communication by activating the adenylate cyclase through the N-terminal region of both chains. Cystine A11-B10 occupies a unique position on the molecular surface just outside the binding region and between the two signaling ports. We have synthesized an RLF analogue in which the disulfide A11-B10 was replaced by a peptide bond and found that cAMP production ceased while receptor binding was not affected. In contrast, replacing the disulfide A24-B22 by a peptide bond reduced potency proportional to the binding affinity and lowered efficacy to 65%, while replacing disulfide A10-A15 by a peptide bond reduced binding affinity to 32% and lowered potency to 7% but maintained 100% efficacy. The exceptional properties of the derivative bearing an A11-B10 isopeptide cross-link suggests that the disulfide has a special role in signal transduction. We propose that disulfide A11-B10 serves as an insulator between the two ports, whereas the amide functionality disturbs the signal transmission complex likely due to changes in polarity. The clear separation between receptor binding and signal activation sites within this small protein permits one to study how the relaxin-like factor initiates the signal on the receptor that induces intracellular cAMP production.     

The road to the first,fully active and more stable human insulin variant with an additional disulfide bond

Insulin, a small peptide hormone, is crucial in maintaining blood glucose homeostasis. The stability and activity of the protein is directed by an intricate system involving disulfide bonds to stabilize the active monomeric species and by their non‐covalent oligomerization. All known insulin variants in vertebrates consist of two peptide chains and have six cysteine residues, which form three disulfide bonds, two of them link the two chains and a third is an intra‐chain bond in the A‐chain. This classical insulin fold appears to have been conserved over half a billion years of evolution. We addressed the question whether a human insulin variant with four disulfide bonds could exist and be fully functional. In this review, we give an overview of the road to engineering four‐disulfide bonded insulin analogs. During our journey, we discovered several active four disulfide bonded insulin analogs with markedly improved stability and gained insights into the instability of analogs with seven cysteine residues, importance of dimerization for stability, insulin fibril formation process, and the conformation of insulin binding to its receptor. Our results also open the way for new strategies in the development of insulin biopharmaceuticals. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Cyclic Tritrpticin Analogs with Distinct Biological Activities

Tritrpticin is a Trp-, Arg-, and Pro-rich cathelicidin peptide with promising antimicrobial activity. Cyclic analogs of tritrpticin were designed using two different approaches: circularization of the backbone by a head-to-tail peptide bond (TritrpCyc) or disulfide bridging between two Cys residues introduced at the termini of the peptide (TritrpDisu). Compared to the parent peptide, TritrpCyc has greatly improved therapeutic potential, showing stronger bactericidal activities and diminished hemolytic activity. Unexpectedly, the opposite effect was observed for TritrpDisu, which has lost its antimicrobial activity and is very hemolytic. In a membrane mimetic environment, NMR spectra show that TritrpDisu adopts an amphipathic turn-turn structure similar to linear tritrpticin. The structure of membrane-bound TritrpCyc has some similarity to that of TritrpDisu; however, the lipid interactions were not sufficient to restrain the structure of the former peptide in a single well-defined conformation. To help explain the distinct biological properties of the analogs, experiments investigating alternative antimicrobial targets were pursued: the membrane bilayer, lipopolysaccharides, and DNA. Although the hemolytic activity of TritrpDisu can be explained by the peptide’s ability to induce higher leakage from the model mammalian membranes, TritrpCyc and TritrpDisu show no significant differences in these functional assays. Overall, our studies show that TritrpCyc holds great promise as a candidate for further development toward antimicrobial therapy.     

A novel concatenated dimer of recombinant bovine somatotropin

A novel protein concatenated dimer structure was generated during the folding/oxidation of inclusion bodies of recombinant bovine somatotropin synthesized inEscherichia coli. The structure of this dimeric molecule was determined by peptide mapping with trypsin, and limited proteolysis by thrombin. Peptide mapping demonstrated that the two disulfide pairs in bovine somatotropin dimer were identical to those in monomer. Limited proteolysis with thrombin resulted in the cleavage of only a single peptide bond between arginine-132 and alanine-133 in bovine somatotropin dimer. This single peptide bond cleavage was sufficient to convert this dimer to a monomeric molecular weight species as analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and HPLC. Since the single cleaved peptide bond is present in the large disulfide loop of bovine somatotropin, these data demonstrate that the dimeric molecule exists as a novel concatenated structure through the interlocking of the disulfide loops of this protein.     

A redox-sensitive loop regulates plasminogen activator inhibitor type 2 (PAI-2) polymerization

Plasminogen activator inhibitor type 2 (PAI-2) is the only wild-type serpin that polymerizes spontaneously under physiological conditions. We show that PAI-2 loses its ability to polymerize following reduction of thiol groups, suggesting that an intramolecular disulfide bond is essential for the polymerization. A novel disulfide bond was identified between C79 (in the CD-loop) and C161 (at the bottom of helix F). Substitution mutants in which this disulfide bond was broken did not polymerize. Reactive center loop peptide insertion experiments and binding of bis-ANS to hydrophobic cavities indicate that the C79-C161 disulfide bond stabilizes PAI-2 in a polymerogenic conformation with an open A-beta-sheet. Elimination of this disulfide bond causes A-beta-sheet closure and abrogates the polymerization. The finding that cytosolic PAI-2 is mostly monomeric, whereas PAI-2 in the secretory pathway is prone to polymerize, suggests that the redox status of the cell could regulate PAI-2 polymerization. Taken together, our data suggest that the CD-loop functions as a redox-sensitive switch that converts PAI-2 between an active stable monomeric and a polymerogenic conformation, which is prone to form inactive polymers.     

Probing membrane permeabilization by the antimicrobial peptide distinctin in mercury-supported biomimetic membranes

The mechanism of membrane permeabilization by the antimicrobial peptide distinctin was investigated by using two different mercury-supported biomimetic membranes, namely a lipid self-assembled monolayer and a lipid bilayer tethered to the mercury surface through a hydrophilic spacer (tethered bilayer lipid membrane: tBLM). Incorporation of distinctin into a lipid monolayer from its aqueous solution yields rapidly ion channels selective toward inorganic cations, such as Tl(+) and Cd(2+). Conversely, its incorporation in a tBLM allows the formation of ion channels permeable to potassium ions only at non-physiological transmembrane potentials, more negative than -340mV. These channels, once formed, are unstable at less negative transmembrane potentials. The kinetics of their formation is consistent with the disruption of distinctin clusters adsorbed on top of the lipid bilayer, incorporation of the resulting monomers and their aggregation into hydrophilic pores by a mechanism of nucleation and growth. Comparing the behavior of distinctin in tBLMs with that in conventional black lipid membranes strongly suggests that distinctin channel formation in lipid bilayer requires the partitioning of distinctin molecules between the two sides of the lipid bilayer. We can tentatively hypothesize that an ion channel is formed when one distinctin cluster on one side of the lipid bilayer matches another one on the opposite side.     

Hierarchical formation of disulfide bonds in the immunoglobulin Fc fragment is assisted by protein-disulfide isomerase

Antibodies provide an excellent system to study the folding and assembly of all beta-sheet proteins and to elucidate the hierarchy of intra/inter chain disulfide bonds formation during the folding process of multimeric and multidomain proteins. Here, the folding process of the Fc fragment of the heavy chain of the antibody MAK33 was investigated. The Fc fragment consists of the C(H)3 and C(H)2 domains of the immunoglobulin heavy chain, both containing a single S-S bond. The folding process was investigated both in the absence and presence of the folding catalyst protein-disulfide isomerase (PDI), monitoring the evolution of intermediates by electrospray mass spectrometry. Moreover, the disulfide bonds present at different times in the folding mixture were identified by mass mapping to determine the hierarchy of disulfide bond formation. The analysis of the uncatalyzed folding showed that the species containing one intramolecular disulfide predominated throughout the entire process, whereas the fully oxidized Fc fragment never accumulated in significant amounts. This result suggests the presence of a kinetic trap during the Fc folding, preventing the one-disulfide-containing species (1S2H) to reach the fully oxidized protein (2S). The assignment of disulfide bonds revealed that 1S2H is a homogeneous species characterized by the presence of a single disulfide bond (Cys-130-Cys-188) belonging to the C(H)3 domain. When the folding experiments were carried out in the presence of PDI, the completely oxidized species accumulated and predominated at later stages of the process. This species contained the two native S-S bonds of the Fc protein. Our results indicate that the two domains of the Fc fragment fold independently, with a precise hierarchy of disulfide formation in which the disulfide bond, especially, of the C(H)2 domain requires catalysis by PDI.     

Epigenetic profiling of the antitumor natural product psammaplin A and its analogues

A collection of analogues of the dimeric natural product psammaplin A that differ in the substitution on the (halo)tyrosine aryl ring, the oxime and the diamine connection has been synthesized. The effects on cell cycle, induction of differentiation and apoptosis of the natural-product inspired series were measured on the human leukaemia U937 cell line. Epigenetic profiling included induction of p21(WAF1), effects on global H3 histone and tubulin acetylation levels as well as in vitro enzymatic assays using HDAC1, DNMT1, DNMT3A, SIRT1 and a peptide domain with p300/CBP HAT activity. Whereas the derivatives of psammaplin A with modifications in the length of the connecting chain, the oxime bond and the disulfide unit showed lower potency, the analogues with changes on the bromotyrosine ring exhibited activities comparable to those of the parent compound in the inhibition of HDAC1 and in the induction of apoptosis. The lack of HDAC1 activity of analogues modified on the disulfide bond suggests that its cleavage must occur in cells to produce the monomeric Zn(2+)-chelating thiol. This assumption is consistent with the molecular modelling of the complex of psammaplin A thiol with h-HDAC8. Only a weak inhibition of DNMT1, DNMT3A and residual activities with SIRT1 and a p300/CBP HAT peptide were measured for these compounds.     

Structural studies and model membrane interactions of two peptides derived from bovine lactoferricin.

The powerful antimicrobial properties of bovine lactoferricin (LfcinB) make it attractive for the development of new antimicrobial agents. An 11-residue linear peptide portion of LfcinB has been reported to have similar antimicrobial activity to lactoferricin itself, but with lower hemolytic activity. The membrane-binding and membrane-perturbing properties of this peptide were studied together with an amidated synthetic version with an added disulfide bond, which was designed to confer increased stability and possibly activity. The antimicrobial and cytotoxic properties of the peptides were measured against Staphylococcus aureus and Escherichia coli and by hemolysis assays. The peptides were also tested in an anti-cancer assay against neuroblastoma cell lines. Vesicle disruption caused by these LfcinB derivatives was studied using the fluorescent reporter molecule calcein. The extent of burial of the two Trp residues in membrane mimetic environments were quantitated by fluorescence. Finally, the solution NMR structures of the peptides bound to SDS micelles were determined to provide insight into their membrane bound state. The cyclic peptide was found to have greater antimicrobial potency than its linear counterpart. Consistent with this property, the two Trp residues of the modified peptide were suggested to be embedded deeper into the membrane. Although both peptides adopt an amphipathic structure without any regular alpha-helical or beta-sheet conformation, the 3D-structures revealed a clearer partitioning of the cationic and hydrophobic faces for the cyclic peptide.     

Role of cysteine residues and disulfide bonds in the activity of a legume root nodule-specific, cysteine-rich peptide

The root nodules of certain legumes including Medicago truncatula produce >300 different nodule-specific cysteine-rich (NCR) peptides. Medicago NCR antimicrobial peptides (AMPs) mediate the differentiation of the bacterium, Sinorhizobium meliloti into a nitrogen-fixing bacteroid within the legume root nodules. In vitro, NCR AMPs such as NCR247 induced bacteroid features and exhibited antimicrobial activity against S. meliloti. The bacterial BacA protein is critical to prevent S. meliloti from being hypersensitive toward NCR AMPs. NCR AMPs are cationic and have conserved cysteine residues, which form disulfide (S-S) bridges. However, the natural configuration of NCR AMP S-S bridges and the role of these in the activity of the peptide are unknown. In this study, we found that either cysteine replacements or S-S bond modifications influenced the activity of NCR247 against S. meliloti. Specifically, either substitution of cysteines for serines, changing the S-S bridges from cysteines 1-2, 3-4 to 1-3, 2-4 or oxidation of NCR247 lowered its activity against S. meliloti. We also determined that BacA specifically protected S. meliloti against oxidized NCR247. Due to the large number of different NCRs synthesized by legume root nodules and the importance of bacterial BacA proteins for prolonged host infections, these findings have important implications for analyzing the function of these novel peptides and the protective role of BacA in the bacterial response toward these peptides.     

Real-time Monitoring of Intermediates Reveals the Reaction Pathway in the Thiol-Disulfide Exchange between Disulfide Bond Formation Protein A (DsbA) and B (DsbB) on a Membrane-immobilized Quartz Crystal Microbalance (QCM) System

Disulfide bond formation protein B (DsbB_S-S,S-S) is an inner membrane protein in Escherichia coli that has two disulfide bonds (S-S, S-S) that play a role in oxidization of a pair of cysteine residues (SH, SH) in disulfide bond formation protein A (DsbA_SH,SH). The oxidized DsbA_S-S, with one disulfide bond (S-S), can oxidize proteins with SH groups for maturation of a folding preprotein. Here, we have described the transient kinetics of the oxidation reaction between DsbA_SH,SH and DsbB_S-S,S-S. We immobilized DsbB_S-S,S-S embedded in lipid bilayers on the surface of a 27-MHz quartz crystal microbalance (QCM) device to detect both formation and degradation of the reaction intermediate (DsbA-DsbB), formed via intermolecular disulfide bonds, as a mass change in real time. The obtained kinetic parameters (intermediate formation, reverse, and oxidation rate constants (k_f, k_r, and k_cat, respectively) indicated that the two pairs of cysteine residues in DsbB_S-S,S-S were more important for the stability of the DsbA-DsbB intermediate than ubiquinone, an electron acceptor for DsbB_S-S,S-S. Our data suggested that the reaction pathway of almost all DsbA_SH,SH oxidation processes would proceed through this stable intermediate, avoiding the requirement for ubiquinone.     

Synthesis of the bis-cystinyl-fragment 225-232/225'-232' of the human IgGl hinge region

In human IgGl the two heavy chains are crosslinked in the central portion of the molecule by two disulfide bridges forming a double chain bis-cystinyl cyclic peptide in parallel alignment. For our synthetic studies we have chosen the sequence portion 225-232/225'-232', i.e. [H-Thr-C1ys-Pro-Pro-C1ys-Pro-Ala-Pro-OH]2. By the use of a combination of the S-tert.-butylthio and the S-acetamidomethyl groups selective cysteine pairings in two successive steps produced the hinge hexadecapeptide in parallel and antiparallel alignment as homogeneous and well characterized compounds. Thiol disulfide interchange experiments on the antiparallel dimer led to over 90% conversion to the parallel isomer. Similarly random air-oxidation was found to generate again mainly the parallel dimer, thus strongly suggesting that this sequence portion contains sufficient structural information for a correct assembly of the two heavy chains of immunoglobulins without decisive contribution of a protein disulfide isomerase.     

Analysis of the Link between the Redox State and Enzymatic Activity of the HtrA (DegP) Protein from Escherichia coli

Bacterial HtrAs are proteases engaged in extracytoplasmic activities during stressful conditions and pathogenesis. A model prokaryotic HtrA (HtrA/DegP from Escherichia coli) requires activation to cleave its substrates efficiently. In the inactive state of the enzyme, one of the regulatory loops, termed LA, forms inhibitory contacts in the area of the active center. Reduction of the disulfide bond located in the middle of LA stimulates HtrA activity in vivo suggesting that this S-S bond may play a regulatory role, although the mechanism of this stimulation is not known. Here, we show that HtrA lacking an S-S bridge cleaved a model peptide substrate more efficiently and exhibited a higher affinity for a protein substrate. An LA loop lacking the disulfide was more exposed to the solvent; hence, at least some of the interactions involving this loop must have been disturbed. The protein without S-S bonds demonstrated lower thermal stability and was more easily converted to a dodecameric active oligomeric form. Thus, the lack of the disulfide within LA affected the stability and the overall structure of the HtrA molecule. In this study, we have also demonstrated that in vitro human thioredoxin 1 is able to reduce HtrA; thus, reduction of HtrA can be performed enzymatically.     

Structural Characterization of the Recombinant P40 Heavy Chain Subunit Monomer and Homodimer of Murine IL-12

Interleukin-12 (IL-12) is a heterodimeric cytokine that consists of two structurally unrelated subunits, P35 and P40. However, when expressed alone in Chinese hamster ovary (CHO) cells, murine P40 showed two species of different molecular weights under nonreducing conditions, a monomeric form of 45 kDa and a homodimer of >97 kDa. Under reducing conditions the two forms migrated as an identical array of species of 40-45 kDa. The monomer was separated from the homodimer under nonreducing conditions by heparin affinity chromatography and the disulfide bond structures of both species were determined by peptide mapping, Edman sequencing, and mass spectrometry. The peptide maps of the two species were identical except for a single peak that changed retention time. Sequencing showed that this peak contained two peptides of identical sequences in both maps. Mass spectrometric analysis of the peak from the >97-kDa species revealed an ion of double the expected mass, thus indicating that the peptide pair had dimerized. Mass analysis of the peak from the 40- to 45-kDa species showed that the peptide pair contained a mass difference that corresponded to that of an extra cysteine and which disappeared upon reduction. Amino acid analysis confirmed that the monomeric form of rmP40 is modified by a reducible cysteine. Structural analysis of the remainder of the cysteine-containing peaks showed that both species of rmP40 contained the same set of intramolecular disulfide bonds. The murine P40 homodimer arises from formation of a single intermolecular disulfide bond at Cys¹⁷⁵. In the monomeric P40, however, this cysteine is capped by an additional cysteine. Purified rmP40 monomer and homodimer inhibited the IL-12-dependent induction of interferon-γ, but neither appeared capable of inducing IL-12-like biological activity.     

Reductive chain separation of botulinum A toxin--a prerequisite to its inhibitory action on exocytosis in chromaffin cells

Cleavage of the disulfide bond linking the heavy and the light chains of tetanus toxin is necessary for its inhibitory action on exocytotic release of catecholamines from permeabilized chromaffin cells [(1989) FEBS Lett. 242, 245-248; (1989) J. Neurochem., in press]. The related botulinum A toxin also consists of a heavy and a light chain linked by a disulfide bond. The actions of both neurotoxins on exocytosis were presently compared using streptolysin O-permeabilized bovine adrenal chromaffin cells. Botulinum A toxin inhibited Ca2+-stimulated catecholamine release from these cells. Addition of dithiothreitol lowered the effective doses to values below 5 nM. Under the same conditions, the effective doses of tetanus toxin were decreased by a factor of five. This indicates that the interchain S-S bond of botulinum A toxin must also be split before the neurotoxin can exert its effect on exocytosis.