Rearrangement Of Disulfide Bonds Research Articles

The Role of Low pH and Disulfide Shuffling in the Entry and Fusion of Semliki Forest Virus and Sindbis Virus

Semliki Forest virus (SFV), an enveloped alphavirus, infects cells via a membrane fusion reaction that is induced by the low pH in endocytic vesicles. The role of low pH in the entry of the alphavirus Sindbis virus (SIN) is unclear, and an alternative fusion mechanism involving receptor-induced disulfide bond rearrangements at neutral pH has been proposed. The entry properties of SFV and SIN were here compared in parallel using treatment with the weak base NH4Cl or the vacuolar ATPase inhibitors bafilomycin A-1 or concanamycin to neutralize endosome pH. Three membrane impermeant thiol modifying reagents, 5,5′-dithio-bis(2-nitrobenzoic acid) (DTNB),p-chloromercuriphenylsulfonic acid (pCMBS), and monobromotrimethylammoniobimane (Thiolyte MQ), were used to inhibit thiol–disulfide exchange reactions. Primary infection by both SFV and SIN was inhibited by neutralization of endosome pH using NH4Cl, bafilomycin, or concanamycin. The concentration of NH4Cl or bafilomycin required for inhibition correlated with the pH dependence of membrane fusion for SFV, SIN, and a pH-shift mutant of SFV. SFV and SIN infection were partially inhibited by the thiol blocker DTNB, but not by pCMBS or Thiolyte MQ. Our data suggest that acidic endosomal pH induces the fusion activity of both SFV and SIN during virus infection.

Modification of functional properties by rearrangement of disulfide bonds and change of ratio of protein fractions

The growing use of different protein isolates (and concentrates) in food production in many cases requires modification of functional properties of isolates in order to assure the optimal physical (nutritional) properties of the endproduct. A number of possible ways of modification is known including chemical, physical procedures and also use of recombinant gene technology. Such technologies are in many cases expensive and are connected with food safety problems. In search for relatively cheap and safety process we studied the possibilities of applying rearrangement of disulfide bonds and change of ratio of protein fractions. The models used in experiments were: gluten-, soy-, pea-, and wheat germ proteins. It was found that gluten polypeptide mixtures of same composition reoxydized under different conditions were transformed to protein masses of quite different functional properties. The same procedure resulted in only slight changes in the case of reduction and reoxydation of legume- and wheat germ proteins. The study of some protein mixtures showed that the functional properties did not change linearly with the composition of the mixture. In many cases maxima of some parameters were observed.

The multi-domain structure of protein disulfide isomerase is essential for high catalytic efficiency

Protein disulfide isomerase (PDI) catalyzes protein folding linked to disulfide bond formation in secreted proteins. It consists of four major domains, denoted a , b , b′ and a′ . The a and a′ domains each contain an active site motif, -CGHC-, which is directly involved in thiol-disulfide exchange reactions during catalysis. The roles of the b and b′ domains and the functional necessity for the multi-domain structure of PDI are unknown. We now demonstrate that full catalytic activity requires the involvement of multiple PDI domains and that the b′ domain has a particularly important role in catalysis. Reconstruction of the PDI molecule from the isolated a and a′ domains results in a progressive increase in catalytic efficiency as further domains are added. These effects are especially significant in the catalysis of disulfide bond rearrangements in folded substrates, for which all the domains of the protein are required for maximum catalytic efficiency. It is likely that all of the domains of PDI participate in substrate binding interactions and that PDI has evolved its multidomain structure as an adaptation that allows it to catalyze transformations involving difficult conformational changes.

Contributions of substrate binding to the catalytic activity of DsbC.

DsbA and DsbC are involved in protein disulfide bond formation in the periplasm of Gram-negative bacteria. The two proteins are thought to fulfill different functions in vivo, DsbA as a catalyst of disulfide bond formation and DsbC as a catalyst of disulfide bond rearrangement. To explore the basis of this catalytic complementarity, the reaction mechanism of DsbC has been examined using unstructured model peptides that contain only one or two cysteine residues as substrates. The reactions between the various forms of the peptide and DsbC occur at rates up to 10(6)-fold faster than those that involve glutathione and DsbC, and they were constrained to occur at only one sulfur atom of disulfide bonds involving the peptide. Mixed disulfide complexes of DsbC and the peptide were 10(4)-fold more stable than the corresponding mixed disulfides with glutathione. These observations suggest that noncovalent binding interactions occur between the peptide and DsbC, which contribute to the very rapid kinetics of substrate utilization. The interactions between DsbC and the peptide appear to be more substantial than those between DsbA and the same peptide. The differences in the reaction of the peptide at the active sites of DsbA and DsbC provide insight into why DsbC is the better catalyst of disulfide bond rearrangement and how the active site chemistry of these structurally related proteins has been adapted to fulfill complementary functions.

Crystallization of DsbC, the disulfide bond isomerase of Escherichia coli.

DsbC is a 2 x 23 kDa soluble dimeric protein molecule involved in protein disulfide bond formation in the E. coli periplasm, primarily catalyzing disulfide bond rearrangements. Crystals of both the native and selenomethione protein suitable for structure determination were obtained using the hanging-drop vapour-diffusion method. The best crystals were obtained using 18-22% (v/v) polyethylene glycol 550 monomethyl ether in 100 mM Tris-HCl (pH 8.9). Seeding methods were used to produce large crystals diffracting to 2 A resolution, and the detergent n-octyl-beta-glucoside was used to improve crystal quality. Significant variation in cell dimensions and crystal order was observed. Cell dimensions obtained for frozen crystals were in the range a = 58.8 (0.3), b = 78.9 (0.5), c = 95.2 (5.0) A. The lattice is orthorhombic and systematic absences indicate that the space group is P2(1)2(1)2(1).

Molecular chaperones and protein folding in plants.

Protein folding in vivo is mediated by an array of proteins that act either as 'foldases' or 'molecular chaperones'. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.

Covalent Association of Protein Disulfide Isomerase with Recombinant Human Interleukin 2in Vitro

Protein disulfide isomerase has broad specificity in the catalysis of the formation and rearrangement of native disulfide bonds in proteins. This enzyme has two independent thioredoxin-like active sites (-CGHC-) and a peptide binding site. However, the mechanisms involving the catalytic processes are not clearly understood. It was reported that the enzyme associates with scrambled pancreatic ribonuclease Ain vitro,and with misfolded human lysozymein vivo.In the present study, recombinant human interleukin 2 has been chosen to probe the reaction intermediate in the reaction with the enzyme. We have identified and characterized a covalent associate formedin vitroby SDS-PAGE and Western blot analysis. This associate has a molecular weight of 71–72 kDa, the approximate sum of the molecular weights of the enzyme and the substrate. Western blot analysis confirmed that it formed via an intermolecular disulfide bond. Upon treatment with 2-mercaptoethanol, this bond was cleaved.

Catalysis of oxidative protein folding by mutants of protein disulfide isomerase with a single active-site cysteine.

Protein disulfide isomerase (PDI), a very abundant protein in the endoplasmic reticulum, facilitates the formation and rearrangement of disulfide bonds using two nonequivalent redox active-sites, located in two different thioredoxin homology domains [Lyles, M. M., & Gilbert, H. F. (1994) J. Biol. Chem. 269, 30946-30952]. Each dithiol/disulfide active-site contains the thioredoxin consensus sequence CXXC. Four mutants of protein disulfide isomerase were constructed that have only a single active-site cysteine. Kinetic analysis of these mutants show that the first (more N-terminal) cysteine in either active site is essential for catalysis of oxidation and rearrangement during the refolding of reduced bovine pancreatic ribonuclease A (RNase). Mutant active sites with the sequence SGHC show no detectable activity for disulfide formation or rearrangement, even at concentrations of 25 microM. The second (more C-terminal) cysteine is not essential for catalysis of RNase disulfide rearrangements, but it is essential for catalysis of RNase oxidation, even in the presence of a glutathione redox buffer. Mutant active sites with the sequence CGHS show 12%-50% of the kcat activity of wild-type active sites during the rearrangement phase of RNase refolding but < 5% activity during the oxidation phase. In addition, mutants with the sequence CGHS accumulate significant levels of a covalent PDI-RNase complex during steady-state turnover while the wild-type enzyme and mutants with the sequence SGHC do not. Since both active-site cysteines are essential for catalysis of disulfide formation, the dominant mechanism for RNase oxidation may involve direct oxidation by the active-site PDI disulfide. Although it is not essential for catalysis of RNase rearrangements, the more C-terminal cysteine does contribute 2-8-fold to the rearrangement activity. A mechanism for substrate rearrangement is suggested in which the second active-site cysteine provides PDI with a way to "escape" from covalent intermediates that do not rearrange in a timely fashion. The second active-site cysteine may normally serve the wild-type enzyme as an internal clock that limits the time allowed for intramolecular substrate rearrangements.

Catalysis of disulfide isomerization in thrombospondin 1 by protein disulfide isomerase.

Thrombospondin 1 is a multidomain glycoprotein from platelets and most cells that participates in diverse biological processes. The structure and some functional properties of thrombospondin 1 are regulated by disulfide interchange in the Ca(2+)-binding repeats and C-globular domain. The recent identification of the enzyme, protein disulfide isomerase, on the platelet surface suggested that protein disulfide isomerase may catalyze disulfide isomerization in platelet thrombospondin 1. Protein disulfide isomerase was found to form disulfide-linked complexes with thrombospondin 1, which is consistent with protein disulfide isomerase-mediated rearrangement of disulfide bonds in thrombospondin 1. To quantitate disulfide interchange in thrombospondin 1, perturbation of the enzyme inhibitory properties of platelet thrombospondin 1 were measured, specifically changes in the apparent dissociation constant for inhibition of neutrophil cathepsin G by thrombospondin 1. The inhibition constant increased > or = 10-14-fold following incubation of either Ca(2+)-replete or Ca(2+)-depleted thrombospondin 1 with protein disulfide isomerase and reduced glutathione. The rate of protein disulfide isomerase-catalyzed disulfide interchange in thrombospondin 1 increased linearly with protein disulfide isomerase concentration and the K(m) for reduced glutathione was 0.4 +/- 0.2 mM. Disulfide isomerization in both platelet and fibroblast thrombospondin 1 was probed by measuring perturbation in epitopes for two anti-thrombospondin 1 monoclonal antibodies. Antibody D4.6 binds to the C-terminal Ca(2+)-binding domains which are involved in disulfide interchange, whereas antibody HB8432 binds toward the N-terminus of the thrombospondin 1 subunit. In accordance with the location of these epitopes, incubation of platelet thrombospondin 1 or fibroblast thrombospondin 1 with protein disulfide isomerase and reduced glutathione resulted in 2-fold enhancement of binding of D4.6, whereas binding of HB8432 did not significantly change. In summary, protein disulfide isomerase catalyzes disulfide interchange in thrombospondin 1 which alters binding of neutrophil cathepsin G and antibody D4.6 to thrombospondin 1.

Nucleotide sequence of a wheat cDNA encoding protein disulfide isomerase.

PDI (EC 5.3.4.1) is an ER-resident protein involved in the rearrangement of disulfide bonds of secretory proteins. This protein also appears as one of the two subunits of another ER resident protein, prolyl 4-hydroxylase (EC 1.14.11.2), and may be involved in various cellular processes (Noiva and Lennarz, 1992). PDI appears to be very abundant in developing wheat grains (Roden et al., 1982) and was also demonstrated to catalyze intrachain disulfide bond formation in a wheat y-gliadin storage protein (Bulleid and Freedman, 1988). To analyze the expression of PDI in developing grains and to study further its role in the folding and assembly of wheat storage proteins, we have cloned a full-length wheat PDI cDNA by screening a root-tip cDNA library using maize PDI as a DNA probe (Table I). Sequence analysis of the wheat cDNA revealed an open reading frame encoding a protein of 515 amino acids, which also exhibited 96 and 63% amino acid sequence identity with full-length PDI cDNAs from barley (accession No. L33250) and alfalfa (Shorrosh and Dixon, 1991), respectively. The deduced amino acid sequence of the wheat PDI also contained an N-terminal signal peptide of 25 amino acids, two conserved thioredoxin-like active sites, a potential N-linked glycosylation site, as well as a C-terminal KDEL sequence generally present in ER-resident proteins.

Effects of DsbA on the disulfide folding of bovine pancreatic trypsin inhibitor and alpha-lactalbumin.

DsbA is a protein found in the periplasm of Escherichia coli that is required for the formation of disulfide bonds in secreted proteins. It contains only two cysteine residues, which can form reversibly a very unstable disulfide bond that has been proposed to be the oxidant that introduces disulfide bonds into secreted proteins. The present study investigates the effect of DsbA on the well-characterized disulfide-coupled refolding processes of BPTI and of alpha-lactalbumin. Disulfide-bonded DsbA in stoichiometric amounts proved to be a very potent donor of disulfide bonds to reduced BPTI but showed little catalytic activity at neutral pH in the presence of a glutathione redox buffer. In contrast to the related eukaryotic enzyme protein disulfide isomerase, DsbA did not substantially catalyze the usual intramolecular disulfide bond rearrangements of quasi-native folding intermediates of BPTI. Neither did DsbA catalyze the intramolecular rearrangements observed in the three disulfide-bonded "molten globule" form of alpha-lactalbumin at neutral pH. Thiol-disulfide exchange is normally very slow at acidic pH but occurs rapidly with DsbA; consequently, DsbA catalyzed the disulfide folding of BPTI under acidic conditions. It was then possible to detect some increase in the rates of disulfide rearrangements, but only with stoichiometric amounts of DsbA and on the hour time scale. These results suggest that the primary role of DsbA in the bacterial periplasm is to introduce disulfide bonds into newly secreted proteins.

Catalysis of Protein Folding by Agarose-Immobilized Protein Disulfide Isomerase

Protein disulfide isomerase (PDI) catalyzes the formation and rearrangement of disulfide bonds during protein folding. PDI coupled to cyanogen bromide-activated agarose retains its catalytic activity, and a column of this material increases both the rate and the yield for folding disulfide-containing proteins. For reduced, denatured ribonuclease, the overall yield of fully active ribonuclease isolated from the PDI column in one pass was 85-98% of the applied protein. Under the same conditions in the absence of PDI, ribonuclease regained only 16% of its native activity. The oxidative folding of reduced denatured lysozyme is complicated by aggregation so that in the absence of PDI optimal yields of only ≤ 25% are obtained at lysozyme concentrations of 1.6 mg/ml. When reduced, denatured lysozyme (1.6 mg/ml) is passed over a PDI column in 1-2 M urea in the presence of a glutathione redox buffer, the specific activity of the recovered lysozyme is identical to that of the native enzyme and the total recovery of the applied protein is 50-65%.

Formation and rearrangement of disulfide bonds during maturation of the Sindbis virus E1 glycoprotein.

The rigidly ordered icosahedral lattice of the Sindbis virus envelope is composed of a host-derived membrane bilayer in which the viral glycoproteins E1 and E2 reside. E1-E1 interactions stabilized by intramolecular disulfide bridges play a significant role in maintaining the envelope's structural integrity (R. P. Anthony and D. T. Brown, J. Virol. 65:1187-1194, 1991; R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have examined the acquisition of disulfide bridges within E1 during its maturation. Prior to exit from the endoplasmic reticulum, E1 folds via at least three intermediates, differing in the number and/or arrangement of their disulfides, into a single, compact form. This E1 species remains stable with respect to its disulfides until late in the secretory pathway, when E1 attains a metastable conformation. At this point, when appropriately triggered, intramolecular thiol-disulfide exchange reactions within E1 can occur, resulting in the generation of alternative E1 species. This metastable nature of mature E1 may have important implications for the mechanism of virus disassembly during the initial stages of the infection process (B. Abell and D. T. Brown, J. Virol. 67:5496-5501, 1993).

Protein engineering strategies in examining protein folding intermediates: Current opinion in Structural Biology 1993, 3:594–600

Mutagenesis has proven to be a powerful tool for examining the structures and stabilities of intermediates which appear during the folding of globular proteins. Combining site-directed mutagenesis with other, more classical, methods of protein chemistry, such as chemical labeling and cleavage, has advanced the investigation of these transient, marginally stable species. The high cooperativity of the folding reaction has been reduced by mutagenesis to produce a series of reactions involving stable, highly populated, partially folded forms. New insights into inherently slow processes, such as proline isomerization reactions and disulfide bond rearrangements, have also resulted from protein engineering experiments.

Pathway of disulfide-coupled unfolding and refolding of bovine alpha-lactalbumin.

alpha-Lactalbumin's four disulfide bonds have been used to probe the nature of its native, molten globule, and unfolded states. The Ca2+ concentration could be used to vary the stability of the native state because it binds a single calcium ion with high affinity. In the Ca(2+)-bound native state, the Cys6-120 and Cys28-111 disulfides were reduced sequentially, reflecting their relative accessibilities. The native disulfides do not stabilize the native conformation as much as in other proteins, nor was the unfolding process as cooperative. After two disulfides were reduced, only part of the protein molecule appeared to remain folded. In the absence of Ca2+, the native state was much less stable, and a molten globule-type conformation tended to be adopted. Formation of disulfide bonds in the reduced protein was initially noncooperative and nonspecific. The molten globule state increased the rate at which disulfides were formed, by tending to keep close in space cysteine residues that are distant in the sequence. A large number of nonnative disulfide pairings were preferred, and the disulfides were not stabilized to any great extent. The alpha-lactalbumin molten globule seems to be stabilized by nonspecific interactions and without a well-defined topology. The native protein was regenerated only when Ca2+ bound to and stabilized the three-disulfide intermediate lacking the Cys6-120 disulfide, which then rapidly formed the fourth disulfide bond. The rate-limiting intramolecular steps were rearrangement of disulfide bonds to native combinations in species with two and three disulfide bonds.

Kinetic role of nonnative species in the folding of bovine pancreatic trypsin inhibitor.

We have shown previously that during the oxidative folding of bovine pancreatic trypsin inhibitor only intermediates with native disulfide bonds are well populated. Nevertheless, these studies also confirmed the earlier conclusion [Creighton, T. E. (1977) J. Mol. Biol. 113, 275-293] that the rate-limiting transition in the kinetically preferred route for folding involves intramolecular disulfide bond rearrangements. Consequently, intermediates with nonnative disulfide bonds must form transiently during folding. Two specific nonnative species, denoted [30-51; 5-14] and [30-51; 5-38], in which numbers indicate residues participating in a disulfide bond, can be detected at low levels in kinetic folding experiments with bovine pancreatic trypsin inhibitor. By working with purified reversibly trapped intermediates, the role of these two nonnative species has been examined directly. These species are found to be in relatively rapid exchange with each other and with an initially formed native two-disulfide intermediate [30-51; 14-38]. Thus, the low abundance of the two nonnative species detected in kinetic folding experiments reflects primarily their low thermodynamic stability as compared to this native intermediate. To a small extent, these nonnative species form the productive native intermediate [30-51; 5-55], which is the immediate precursor to the native protein. However, an equal amount of [5-55; 14-38], a nonproductive dead-end intermediate, is also produced. Thus, the nonnative species detected during the folding of bovine pancreatic trypsin inhibitor are not committed to forming the productive native intermediate, nor do they serve to direct folding specifically toward a productive route.

Presentation of insulin and insulin a chain peptides to mouse T cells: Involvement of cysteine residues

The requirements for insulin presentation and recognition by A b α A b β- and A b α A k β-restricted mouse T cells were studied using a variety of derivatives of the insulin A chain. It was found that A chain peptides with irreversibly blocked Cys residues are non-stimulatory for the T cells. This suggests that at least one of the Cys residues is essential for recognition. On the other hand, all A chain peptides containing Cys residues modified in a way reversible by reaction with thiols are stimulatory yet differ in antigenic potency. All these A chain derivatives including a 14 amino acid fragment require uptake by antigen presenting cells (APC) for efficient presentation. Differences in stimulatory potency between the A chain peptides derived from the same insulin appear to be mainly due to the efficiency of uptake and/or processing by APC. Based on these findings we propose that processing in the case of insulin and its A chain derivatives involves the reductive deblocking of Cys residues or the rearrangement of disulfide bonds apart from a possible proteolytic cleavage. The same may apply to other proteins if Cys residues in the presented peptides are important for the interaction with Ia or the T cell receptor.

Conformational change of the coronavirus peplomer glycoprotein at pH 8.0 and 37 degrees C correlates with virus aggregation and virus-induced cell fusion

We have obtained biochemical and electron microscopic evidence of conformational changes at pH 8.0 and 37 degrees C in the coronavirus spike glycoprotein E2 (S). The importance of these changes is reflected in the loss of virus infectivity, the aggregation of virions, and increased virus-induced cell fusion at the same pH. Coronavirus (MHV-A59) infectivity is exquisitely sensitive to pH. The virus was quite stable at pH 6.0 and 37 degrees C (half-life, approximately 24 h) but was rapidly and irreversibly inactivated by brief treatment at pH 8.0 and 37 degrees C (half-life, approximately 30 min). Virions treated at pH 8.0 and 37 degrees C formed clumps and large aggregates. With virions treated at pH 8.0 and 37 degrees C, the amino-terminal peptide E2N (or S1) was released from virions and the remaining peptide, E2C (S2), was aggregated. Viral spikes isolated from detergent-treated virions also aggregated at pH 8.0 and 37 degrees C. Loss of virus infectivity and E2 (S) aggregation at pH 8.0 and 37 degrees C were markedly enhanced in the presence of dithiothreitol. On the basis of the effects of dithiothreitol on the reactions of the peplomer, we propose that release of E2N (S1) and aggregation of E2C (S2) may be triggered by rearrangement of intramolecular disulfide bonds. The aggregation of virions and the isolated E2 (S) glycoprotein at pH 8.0 and 37 degrees C or following treatment with guanidine and urea at pH 6.0 and 37 degrees C indicate that an irreversible conformational change has been induced in the peplomer glycoprotein by these conditions. It is interesting that coronavirus-induced cell fusion also occurred under mildly alkaline conditions and at 37 degrees C. Some enveloped viruses, including influenza viruses and alphaviruses, show conformational changes of spike glycoproteins at a low pH, which correlates with fusion and penetration of those viruses in acidified endocytic vesicles. For coronavirus MHV-A59, comparable conformational change of the spike glycoprotein E2 (S) and cell fusion occurred at a mildly alkaline condition, suggesting that coronavirus infection-penetration, like that of paramyxoviruses and lentiviruses, may occur at the plasma membrane, rather than within endocytic vesicles.

Catalysis of thiol/disulfide exchange: single-turnover reduction of protein disulfide-isomerase by glutathione and catalysis of peptide disulfide reduction.

Protein disulfide-isomerase, a protein localized to the lumen of the endoplasmic reticulum of eukaryotic cells, catalyzes the posttranslational formation and rearrangement of protein disulfide bonds. As isolated from bovine liver, the enzyme contains 0.8 free sulfhydryl group per mole of protein monomer and 3.1 disulfide bonds. Single-turnover experiments in which the disulfide bonds of the native enzyme are reduced by glutathione reveal three distinct reduction steps corresponding to the sequential reduction of the three disulfide bonds. The fastest disulfide to be reduced undergoes a change in the rate-determining step with increasing GSH concentration from a step which is second-order with respect to GSH concentration to a step which is first-order in GSH concentration. The disulfide which is reduced at an intermediate rate displays kinetics that are first-order in GSH concentration, and the slowest disulfide to be reduced exhibits kinetics which are second-order in GSH concentration. The enzyme catalyzes the steady-state reduction of a disulfide-containing hexapeptide (CYIQNC) by GSH. Initial velocity kinetic experiments are consistent with a sequential addition of the substrates to the enzyme. Saturation behavior is not observed at high levels of both substrates (Km for GSH much greater than 14 mM, Km for CYIQNC much greater than 1 mM). Only one of the three disulfides appears to be kinetically competent in the steady-state reduction of CYIQNC by GSH. The second-order thiol/disulfide exchange reactions catalyzed by the enzyme are 400-6000-fold faster than the corresponding uncatalyzed reactions.

Studies on the hyaluronate binding properties of newly synthesized proteoglycans purified from articular chondrocyte cultures

Primary cultures of rabbit articular chondrocytes have been maintained for 10 days and labeled with [ 35S]sulfate, [ 3H]leucine, and [ 35S]cysteine in pulsechase protocols to study the structure and hyaluronate binding properties of newly synthesized proteoglycan monomers. Radiolabeled monomers were purified from medium and cell-layer fractions by dissociative CsCl gradient centrifugation with bovine carrier monomer, and analyzed for hyaluronate binding affinity on Sepharose CL-2B in 0.5 m Na acetate, 0.1% Triton X-100, pH 6.8. Detergent was necessary to prevent self-association of newly synthesized monomers during chromatography. Monomers secreted during a 30-min pulse labeling with [ 35S]sulfate had a low affinity relative to carrier. Those molecules released into the medium during the first 12 h of chase (about 40% of the total) remained in the low affinity form whereas those retained by the cell layer rapidly acquired high affinity. In cultures where more than 90% of the preformed cell-layer proteoglycan was removed by hyaluronidase digestion before radiolabeling the newly synthesized low affinity monomers also rapidly acquired high affinity if retained in the cell layer. Cultures labeled with amino acid precursors were used to establish the purity of monomer preparations and to isolate core proteins for study. Leucine- or cysteine-labeled core proteins derived from either low or high affinity monomer preparations migrated as a single major species on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with electrophoretic mobility very similar to that of core protein derived from extracted proteoglycan monomer. Purified low affinity monomers were converted to the high affinity form by treatment at pH 8.6; however, this change was prevented by guanidinium-HCl at concentrations above 0.8 m. Conversion to high affinity was also achieved by incubation of monomers in aggregate with hyaluronic acid (HA) at pH 6.8 followed by dissociative reisolation of monomer. At both pH 6.8 and 8.6 the conversion process was slow, requiring up to 48 h for the maximum increase in affinity. It is suggested that the slow increase in HA binding affinity seen during extracellular processing of proteoglycans in cartilage and chondrocyte cultures is the result of an irreversible structural change in the HA binding domain following the binding of monomer to hyaluronate. The available evidence suggests that this change involves the formation or rearrangement of disulfide bonds.