Role of disulfide interchange enzyme in immunoglobulin synthesis

Sequence of protein disulphide isomerase and implications of its relationship to thioredoxin

The formation of disulphide bonds is essential to the structure and function of proteins. These bonds rapidly form either cotranslationally or immediately post-translationally in the lumen of the endoplasmic reticulum. Native disulphide pairing for such proteins has been achieved in vitro; however, the rates of reassembly are slow and the conditions non-physiological. To account for these observations, Anfinsen et al. proposed that a 'disulphide interchange protein' was the in vivo catalyst of disulphide bond rearrangement. Other groups discovered an activity with similar characteristics that catalysed the reductive cleavage of insulin and may be associated with insulin degradation, although this result has been disputed. The enzyme involved, protein disulphide isomerase (PDI; EC 5.3.4.1), may be the in vivo catalyst of disulphide bond formation. Here we describe the sequence of cloned rat liver PDI complementary DNA which predicts a protein with two distinct regions homologous with Escherichia coli thioredoxin, a known cofactor in oxidation-reduction reactions. Each of these regions contains the presumed active site sequence Trp-Cys-Gly-His-Cys-Lys, suggesting that PDI, similar in action to thioredoxin, catalyses disulphide bond interchange via an internal disulphide-sulphydryl interchange. The cDNA predicts a signal peptide consistent with the view that PDI is a luminal endoplasmic reticulum protein. PDI messenger RNA, although ubiquitous, is more highly concentrated in secretory cells.

Release of thioltransferase from rabbit polymorphonuclear leucocytes by immune complex in vitro and inhibition of the enzyme by chloramphenicol

Immune complex induced the release of thioltransferase from rabbit peritoneal exudates polymorphonuclear leucocytes in vitro. The release of thioltransferase occurs from viable cells and does not depend on a cytolysis. The catalytic activity of the released enzyme with S-sulfocysteine and glutathione as substrates had a distinct optimum pH at 7.6. On the contrary, opsonized zymosan was not effective as a stimulus for the liberation of thioltransferase from polymorphonuclear leucocytes. Thioltransferase liberated by the stimulation with immune complex was inhibited by chloramphenicol, but not by bacitracin. The inhibition was non-competitive (apparent Ki of 0.2 mM).

Characterization of an insulin degrading enzyme from cultured human lymphocytes

An insulin degrading enzyme from cultured human lymphocytes, IM-9 cells, has been purified and characterized. The biochemical, enzymatic and immunological characteristics of this enzyme were all found to be similar to the characteristics of insulin degrading enzymes previously isolated from rat and pig skeletal muscle. Furthermore, this insulin degrading enzyme was found to have no effect on the structure of the insulin receptor nor to be linked to the insulin receptor either on the plasma membrane of cells or when they are shed into the media. The present studies suggest that the IM-9 lymphocytes, which have been extensively used to study the human insulin receptor, may also be a good system for studying human insulin degrading enzymes.

Biosynthesis of von Willebrand protein by human endothelial cells: processing steps and their intracellular localization

Biosynthesis of von Willebrand protein by human umbilical vein endothelial cells involved distinct processing steps marked by the presence of several intermediate molecular species. Examination of endoglycosidase H sensitivity of these intracellular intermediates indicated that the processing steps occurred in at least two separate cellular compartments. In the pre-Golgi apparatus (most probably the endoplasmic reticulum), the high mannose carbohydrates were added onto the precursor monomer chains and the 260,000-mol-wt monomers dimerized by interchain disulfide bond formation. The other processing steps have been localized to the Golgi apparatus and later compartments (e.g., Weibel-Palade bodies). High mannose carbohydrate was converted to the complex type, leading to the appearance of a larger precursor subunit of 275,000 mol wt. The 275,000-mol-wt species was not formed if carbohydrate processing was inhibited by the ionophore monensin. From the large pool of dimers of precursor subunits, the high molecular weight multimers were built. These dimer molecules appeared to have free sulfhydryls which might have been involved in the interdimer disulfide bond formation. Simultaneously with multimerization, the precursor subunits were cleaved to the 220,000-mol-wt form. The cleavage of the pro-sequence was not likely to be an absolute requirement for von Willebrand protein multimerization or secretion, as the 275,000-mol-wt precursor subunit was present in secreted high molecular weight multimers of the protein.

Production and characterization of a monoclonal antibody to rat liver thiol: protein-disulfide oxidoreductase/glutathione-insulin transhydrogenase

Rat liver thiol:protein-disulfide oxidoreductase/glutathione-insulin transhydrogenase (glutathione:protein disulfide oxidoreductase, EC 1.8.4.2) was purified and found to give two bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis. A monoclonal antibody was produced against this enzyme preparation and found to remove all the insulin degrading activity of purified preparations of the enzyme. This monoclonal antibody was also found to react with the two different forms of the enzyme observed on gel electrophoresis. These results suggest that glutathione-insulin transhydrogenase can exist in more than one state.

Protein disulphide-isomerase of chick-embryo tendon

Protein disulphide-isomerase can be partially purified from the high-speed-supernatant fraction of extensively disrupted chick-embryo tendon tissue. The catalytic properties of the preparation resemble those of the enzyme from mammalian liver. Gel electrophoresis and isoelectric focusing show the enzyme to be very acidic, with pI 4.4 +/- 0.3. Gel filtration indicates an Mr for the active enzyme of 140 000. The enzyme can be partially purified by preparative gel filtration or isoelectric focusing, but its limited stability has prevented purification to homogeneity; active fractions from both gel filtration and isoelectric focusing show two major polypeptide components by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The major polypeptides present in partially purified preparations have Mr 45 000 and 55 000; the latter band co-distributes with the enzyme activity in fractionations by both gel filtration and isoelectric focusing. The subcellular location of the enzyme cannot be established from work on homogenates of whole tissue, which are extensively disrupted. In homogenates from isolated tendon cells, the enzyme is located in a vesicle fraction that is excluded from Sepharose 2B but is of low density and can only be sedimented at very high speeds. This fraction is identified as deriving from the endoplasmic reticulum on the grounds of marker-enzyme studies and electron microscopy.

The oocyte as a secretory cell

Xenopus laevis oocytes secrete a large variety of foreign secretory proteins after the microinjection of mRNA or DNA. Two classes of such proteins are discussed in detail. These are the chick oviduct proteins ovalbumin and lysozyme, and the mouse MOPC 21 immunoglobulin. The injection of mRNAs for mouse immunoglobulin heavy or light chain leads to the synthesis, segregation, but not secretion of the encoded proteins unless the two mRNAs are simultaneously or sequentially injected into the same oocytes. Chicken ovalbumin and lysozyme are synthesized and secreted from oocyte after the injection of either oviduct mRNA or cloned DNA (ovalbumin). The secreted lysozyme is exported considerably faster than ovalbumin; however, 40% of the lysozyme synthesized cannot be secreted and, after fractionation of oocytes on sucrose gradients, is found in a higher density position than ovalbumin. No competition at the level of secretion or translation was noted when different amounts of immunoglobulin and ovalbumin mRNAs were injected into oocytes. However, the co-injection of ovalbumin mRNA and mRNAs encoding anti-ovalbumin immunoglobins resulted in the formation of a complex of the two types of protein within the oocyte. In these circumstances, secretion of the immunoglobulin was severely reduced.

Ultrastructural studies of human lymphoid cells. mu and J chain expression as a function of B cell differentiation

J chain expression was examined as a function of the stage in differentiation along the B cell axis in humans. Intracellular distribution of J and mu chains in leukemic HLA-DR+ null and pre-B cells, and in normal B cells stimulated with pokeweed mitogen (PWM) was determined by immunoelectron microscopy and radioimmunoassay (RIA). J chain was detected in leukemic null and pre-B cells on free and membrane-bound ribosomes in the cytoplasm, or on perinuclear cisternae. Mu chain was found on free ribosomes and ribosomal clusters in leukemic pre-B cells but was absent in the leukemic null cells. In pre-B cell lines, mu chain was seen within rough endoplasmic reticulum (RER) and the Golgi apparatus whereas J chain was not detected in these organelles. However, both mu and J chain were detected in RER and the Golgi apparatus of immature and mature plasma cells induced by PWM stimulation of normal peripheral blood lymphocytes. Low levels of J chain were also detected by RIA in lysates of leukemic null and pre-B cells. Most of the intracellular J chain became detectable after reduction and alkylation of cell lysates, and free J chain was not found in the culture supernatants. The amount of intracellular and secreted immunoglobulin-bound J chain increased dramatically after PWM stimulation of peripheral blood lymphocytes. The majority of J chain-positive cells seen over an 8 d culture interval were lymphocytes and lymphoblasts, while mu chain was found primarily in plasma cells. These results suggest that J chain expression precedes mu chain synthesis during B cell differentiation and that a combination of the two chains for secretion is not initiated until the onset of plasma cells maturation.

Structural properties of homogeneous protein disulphide-isomerase from bovine liver purified by a rapid high-yielding procedure

Protein disulphide-isomerase from bovine liver was purified to homogeneity as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, two-dimensional electrophoresis and N-terminal amino acid analysis. The preparative procedure, a modification of that of Carmichael, Morin & Dixon [(1977) J. Biol. Chem. 252, 7163-7167], is much faster and higher-yielding than previous procedures, and the final purified material is of higher specific activity. The enzyme has Mr 57 000 as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, both in the presence and in the absence of thiol compounds. Gel-filtration studies on Sephadex G-200 indicate an Mr of 107 000, suggesting that the native enzyme is a homodimer with no interchain disulphide bonds. Ultracentrifugation studies give a sedimentation coefficient of 3.5S, implying that the enzyme sediments as the monomer. The isoelectric point, in the presence of 8 M-urea, is 4.2, and some microheterogeneity is detectable. The amino acid composition is comparable with previous analyses of this enzyme from bovine liver and of other preparations of thiol:protein disulphide oxidoreductases whose relation to protein disulphide-isomerase has been controversial. The enzyme contains a very high proportion of Glx + Asx residues (27%). The N-terminal residue is His. The pure enzyme has a very small carbohydrate content, determined as 0.5-1.0% by the phenol/H2SO4 assay. Unless specific steps are taken to remove it, the purified enzyme contains a small amount (5 mol/mol of enzyme) of Triton X-100 carried through the purification.

Kinetics and specificity of homogeneous protein disulphide-isomerase in protein disulphide isomerization and in thiol-protein-disulphide oxidoreduction

The protein disulphide-bond isomerization activity of highly active homogeneous protein disulphide-isomerase (measured by re-activation of 'scrambled' ribonuclease) is enhanced by EDTA and by phosphate buffers. As shown for previous less-active preparations, the enzyme has a narrow pH optimum around pH 7.8 and requires the presence of either a dithiol or a thiol. The dithiol dithiothreitol is effective at concentrations 100-fold lower than the monothiols reduced glutathione and cysteamine. The enzyme follows Michaelis-Menten kinetics with respect to these substrates; Km values are 4,620 and 380 microM respectively. The enzyme shows apparent inhibition by high concentrations of thiol or dithiol compounds (greater than 10 X Km), but the effect is mainly on the extent of reaction, not the initial rate. This is interpreted as indicating the formation of significant amounts of reduced ribonuclease in these more reducing conditions. The purified enzyme will also catalyse net reduction of insulin disulphide bonds by reduced glutathione (i.e. it has thiol:protein-disulphide oxidoreductase or glutathione:insulin transhydrogenase activity), but this requires considerably higher concentrations of enzyme and reduced glutathione than does the disulphide-isomerization activity. The Km for reduced glutathione in this reaction is an order of magnitude greater than that for the disulphide-isomerization activity, and the turnover number is considerably lower than that of other enzymes that can catalyse thiol-disulphide oxidoreduction. Conventional two-substrate steady-state analysis of the thiol:protein-disulphide oxidoreductase activity indicates that it follows a ternary-complex mechanism. The protein disulphide-isomerase and thiol:protein-disulphide oxidoreductase activities co-purify quantitatively through the final stages of purification, implying that a single protein species is responsible for both activities. It is concluded that previous preparations, from various sources, that have been referred to as protein disulphide-isomerase, disulphide-interchange enzyme, thiol:protein-disulphide oxidoreductase or glutathione:insulin transhydrogenase are identical or homologous proteins. The assay, nomenclature and physiological role of this enzyme are discussed.

Inter- and intramolecular disulfide bonding among lymphocyte plasma membrane proteins and glycoproteins

The disulfide bonding characteristics of the pig lymph node plasma membrane (PM) proteins and glycoproteins have been examined by 1- and 2-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Reaction of the purified PM vesicles with N-ethyl maleimide (NEM) prior to detergent solubilization was found to markedly reduce the extent of intermolecular disulfide bonding subsequently observed. Thus the blocking of free sulfhydryl groups with NEM prevented the detergent-induced disulfide bonding of numerous components, including PM-bound actin. The extent of intermolecular disulfide bonding among the NEM-pretreated PM glycoproteins purified by lentil lectin affinity chromatography was found to be relatively limited, with only 3% of the total glycoprotein present as intermolecular disulfide-bonded complexes. In contrast, the degree of intramolecular disulfide bonding revealed by a modified 1-dimensional SDS-PAGE technique was quite striking. Among those polypeptides demonstrating a clearly altered mobility upon reduction was the heavy chain of class I and beta-chain of class II major histocompatibility complex (MHC) antigens. The class II alpha-chain, however, was much less affected. These changes have been compared with those observed for proteins containing intramolecular disulfide-bonded domains of known size and number, and considered in the light of recent information on the structure of MHC antigens.

Identification of thiol:protein disulfide oxidoreductase activity in cultured human fibroblasts: dependence of enzyme activity on growth conditions

Thiol:protein disulfide oxidoreductase activity was assayed in extracts of cultured normal human skin fibroblasts. Enzyme activity in confluent fibroblasts was dependent on growth conditions. In serum-deprived fibroblasts grown in minimal medium enzyme activity was approximately 40% of that observed in fibroblasts maintained in medium supplemented with 10% fetal calf serum. In fibroblasts cultured in medium supplemented only with insulin, activity was 35% greater than that in fibroblasts cultured in unsupplemented defined medium. Antibodies raised against purified bovine liver thiol:protein disulfide oxidoreductase immunoprecipitated all of the activity present in fibroblast extracts. The thiol:protein disulfide oxidoreductase from human fibroblasts thus appears to share antigenic determinants with the bovine liver enzyme. The human fibroblast may serve as an in vitro model to study the regulation of the oxidoreductase.

Mutations affecting the structure and function of immunoglobulin M

Using a hybridoma cell line which secretes hapten-specific immunoglobulin M (IgM), we have isolated a variety of mutants which produce abnormal immunoglobulin. Immunoglobulin was tested for the size and composition of the component heavy and light chains and for variable and constant region related functional and serological activities. Some mutants secrete IgM which seems to be defective in hapten binding; others make IgM which appears not to activate complement. Many of the mutants secrete monomeric as opposed to pentameric IgM. In some cases, the defect apparently correlates with structural alterations in the mu heavy chain: partial deletion, polypeptide addition, and abnormal glycosylation have been observed. These mutant cell lines provide a means of identifying the structural basis of IgM function and of studying the biochemistry of IgM synthesis and processing.

Mutations affecting the structure and function of immunoglobulin M

Using a hybridoma cell line which secretes hapten-specific immunoglobulin M (IgM), we have isolated a variety of mutants which produce abnormal immunoglobulin. Immunoglobulin was tested for the size and composition of the component heavy and light chains and for variable and constant region related functional and serological activities. Some mutants secrete IgM which seems to be defective in hapten binding; others make IgM which appears not to activate complement. Many of the mutants secrete monomeric as opposed to pentameric IgM. In some cases, the defect apparently correlates with structural alterations in the mu heavy chain: partial deletion, polypeptide addition, and abnormal glycosylation have been observed. These mutant cell lines provide a means of identifying the structural basis of IgM function and of studying the biochemistry of IgM synthesis and processing.