Bridging reagent for protein. I. The reaction of diisocyanates with lysine and enzyme proteins

Chemical and Enzymatic Methods for Post-Translational Protein-Protein Conjugation

Fusion proteins play an essential role in the biosciences but suffer from several key limitations, including the requirement for N-to-C terminal ligation, incompatibility of constituent domains, incorrect folding, and loss of biological activity. This perspective focuses on chemical and enzymatic approaches for the post-translational generation of well-defined protein-protein conjugates, which overcome some of the limitations faced by traditional fusion techniques. Methods discussed range from chemical modification of nucleophilic canonical amino acid residues to incorporation of unnatural amino acid residues and a range of enzymatic methods, including sortase-mediated ligation. Through summarizing the progress in this rapidly growing field, the key successes and challenges associated with using chemical and enzymatic approaches are highlighted and areas requiring further development are discussed.

Detection of Protein Changes in Serum of Workers following Inhalation Exposure to Toluene Diisocyanate Vapors

Serum samples of 10 workers undergoing occupational type inhalative challenge tests by toluene diisocyanate (TDI) were investigated by anion-exchange fast-protein-liquid-chromatography (FPLC) and polyacrylamide-gel electrophoresis (PAGE-SDS). Their serum chromatography profiles were compared to those of 20 unexposed individuals. The peak height of the first prealbumin peak in sera of workers after inhalative challenge tests was significantly different (p > 0, 01 Chi-square test) compared to that obtained before exposure and to that of unexposed subjects. In addition, qualitative changes of these peaks were also noted in sera of workers exposed to TDI. In the cases of exposed individuals, that peak was more diffuse with some shoulders and less symmetric in appearance. Similarly, PAGE-SDS of the serum proteins, followed by silver nitrate staining, revealed a different banding pattern after in vivo TDI exposure. One of the serum components at approximately 15 kD showed an increase of staining intensity after exposure (n = 10), compared to unexposed subjects or to patients before exposure. This serum fraction has not yet been identified. The results here demonstrate that it is possible to detect changes of serum proteins in TDI-exposed individuals within a relatively short analysis time. This could be useful for biological monitoring of exposure, since no method for such is yet available.

Macroporous copolymer matrix: 1. Effectiveness of different diisocyanate spacer arms to bind cyclodextrins

Cross-linked macroporous beaded polymer matrices, with pendant hydroxyl groups, were synthesized by the copolymerization of 2-hydroxyethyl methacrylate and ethylene glycol dimethacrylate using suspension polymerization methodology. Novel affinity chromatography matrices were synthesized using various diisocyanates as bifunctional reagents to couple the macroporous polymeric supports, of controlled particle size distribution, with alpha and beta-cyclodextrins. The optimal conditions to couple the hydroxyl groups of cyclodextrin (ligand) and the polymeric supports through urethane linkages were established iteratively using various diisocyanates. Efficacy of ligand binding on the matrix and nonspecific interactions of the synthesized affinity matrices were evaluated to establish the best support and spacer arm. 2,4-Tolylene diisocyanate was established as the best spacer arm on the basis of high ligand binding and low nonspecific interactions. The characteristics of the synthesized affinity matrices toward the adsorption of alpha and beta-cyclodextrin glycosyltransferase (CGTase) were investigated. The binding of beta-CGTase was the highest on affinity matrices with the polymeric methylene diisocyanate spacer. The optimal conditions to regenerate the matrices were also established.

2,4-toluenediisocyanate and hexamethylene-diisocyanate adducts with blood proteins: assessment of reactivity of amino acid residues in vitro

Diisocyanates, reactive compounds used in plastics industry and potent occupational allergens, readily bind to proteins both in vitro and in vivo, however, the pattern of adducts with individual amino acids has not been investigated systematically. In this study, potential of the proteinogenic amino acid residues for carbamoylation with 2,4-toluenediisocyanate (2,4-TDI) and hexamethylenediisocyanate (HDI) was evaluated. The diisocyanates were incubated in an in vitro system (buffer pH 7.4/dioxane 50:50) with: (a) a series of Nalpha-benzyloxycarbonyl amino acids (Z-amino acids) and N-acetylcysteine (Ac-Cys), model compounds for non-N-terminal amino acids of the protein chain; (b) dipeptides Val-Phe and Asp-Phe, model compounds for N-termini of globin and albumin, respectively. Reactivity of the compounds tested, evaluated from their depletion during incubation with the diisocyanates (measured by HPLC), was in the order: Ac-Cys = Asp-Phe > Val-Phe = Nalpha-Z-Lys >> Nalpha-Z-His for 2,4-TDI, and Ac-Cys > Asp-Phe > Val-Phe = Nalpha-Z-Lys > Nalpha-Z-His > N-Z-Tyr for HDI, however, the adducts with Ac-Cys were unstable. Reactions of other amino acid residues (e.g. Ser, Thr, Met, Trp, Arg, Asn, Gln) with 2,4-TDI and HDI were not observed. Thus, N-terminal amino acids and Lys residues are likely to produce most abundant adducts with diisocyanates in proteins. Further, three amino compounds with increasing pKa values (Val-Phe, Val and Nalpha-Z-Lys) were incubated with 2,4-TDI and N-acetyl-S-[4-(2-amino)tolylcarbamoyl]cysteine, a 2,4-TDI-derived thiocarbamate with carbamoylating activity, in media with 10% and no dioxane, respectively. Here, reactivity of the amino compounds was decreasing in the order: Val-Phe > Val > Nalpha-Z-Lys, which reflects the mechanism of the amine-isocyanate reaction. The experiments also demonstrate the effect of a solvent (organic phase content) on the yield of the carbamoylation reactions.

N-termini of EcoRI restriction endonuclease dimer are in close proximity on the protein surface

The N-terminal region of EcoRI endonuclease is essential for cleavage yet is invisible in the 2.5 A crystal structure of endonuclease-DNA complex [Kim, Y., Grable, J. C., Love, R., Greene, P. J., Rosenberg, J. M. (1990) Science 249, 1307-1309]. We used site-directed fluorescence spectroscopy and chemical cross-linking to locate the N-terminal region and assess its flexibility in the absence and presence of DNA substrate. The second amino acid in each subunit of the homodimer was replaced with cysteine and labeled with pyrene or reacted with bifunctional cross-linkers. The broad absorption spectra and characteristic excimer emission bands of pyrene-labeled muteins indicated stacking of the two pyrene rings in the homodimer. Proximity of N-terminal cysteines was confirmed by disulfide bond formation and chemical cross-linking. The dynamics of the N-terminal region were determined from time-resolved emission anisotropy measurements. The anisotropy decay had two components: a fast component with rotational correlation time of 0.3-3 ns representing probe internal motions and a slow component with 50-100 ns correlation time representing overall tumbling of the protein conjugate. We conclude that the N-termini are close together at the dimer interface with limited flexibility. Binding of Mg2+ cofactor or DNA substrate did not affect the location or flexibility of the N-terminal region as sensed by pyrene fluorescence and cross-linking, indicating that substrate binding is not accompanied by folding or unfolding of the N-terminus.

Carbendazim and n-butylisocyanate: metabolites responsible for benomyl double action on cytochrome P450 in HepG2 cells

Changes in the cytochrome P450 monooxygenase system were investigated in HepG2 cells treated for 24 h with 1.25, 2.5, 5, 10 and 20 microg/ml of carbendazim (MBC) and n-butylisocyanate (BIC), the principal benomyl metabolites. The results show that n-butylisocyanate leads to a decrease in both ethoxyresorufin deethylase (P4501A1) (EROD) and ethoxycoumarin deethylase (P4502B) (ECOD), whereas MBC has no effect on EROD and increases ECOD. The decrease in ECOD and EROD activities after BIC treatment can be attributed to the detrimental action of this substance. The MBC-induced increase in ethoxycoumarin can be considered an enzyme-specific inductive phenomenon. This hypothesis was confirmed by Western immunoblot analysis and treatment with actinomycin D 8 x 10(-4) microM: the first showed an increase in P4502B isoenzyme content and the second evidence of a partial block of the increase in ECOD activity induced by MBC. Given these results, MBC and BIC seem to be the metabolites responsible for the double opposite action of their parent compound benomyl. Data deriving from an equimolar mixture of the two metabolites suggest that benomyl activity on some cytochrome P450 isoenzymes is the result of a balance between the action of the single metabolites (Radice et al., 1996).

Biological monitoring of occupational exposure to toluene diisocyanate: measurement of toluenediamine in hydrolysed urine and plasma by gas chromatography-mass spectrometry

Exposure to toluene diisocyanate (TDI) was studied during 48 hours and biological samples from nine subjects were taken in a factory producing flexible polyurethane (PUR) foam. Five PUR workers, two white collar workers, and two volunteers were studied. The concentrations of TDI in air were determined by high performance liquid chromatography with the 9-(N-methylaminomethyl)-anthracene reagent. Urine and plasma samples were collected and the TDI related amines, 2,4-toluenediamine (2,4-TDA) and 2,6-toluenediamine (2,6-TDA), were determined (after hydrolysis) as pentafluoropropionic anhydride (PFPA) derivatives by capillary gas chromatography-mass spectrometry (GC-MS) with selected ion monitoring (SIM) in the negative chemical ionisation mode. The concentration of TDI in air was 1%-10% of the Swedish threshold limit value (TLV) of 40 micrograms/m3. The ratio between 2,4-TDI and 2,6-TDI varied in the air samples in the range of 60%:40%-5%:95%. Calibration plots for human urine spiked with 2,6-TDA and 2,4-TDA in the range of 0.2-12 micrograms/l were produced on eight different occasions during five weeks. The SDS of the calibration plot slopes (n = 8) were less than 4%. Urine and blood samples were taken on six occasions for eight of the studied subjects and on four occasions for one subject during a two day period. The five male PUR workers showed the highest average urinary elimination rate of TDA. Two PUR workers and the two white collar workers had an elimination rate of 20-70 ng on average for the sum of 2,6-TDA and 2,4-TDA per hour and three PUR workers had an average of 100-300 ng TDA per hour. The elimination rate curves for all the studied subjects had a linear relation with exposure to TDI. The concentrations of 2,4-TDA and 2,6-TDA in plasma for the PUR factory employees were virtually stable. No relation between the elimination rates of TDA in urine and plasma concentrations of TDA was found. The five PUR workers showed plasma concentrations of the sum of 2,4-TDA and 2,6-TDA in the range 1-8 ng per ml. The two white collar workers, present only on occasions in the factory, had 0.2- ng TDA per ml plasma. The two volunteers showed an increasing concentration of TDA in plasma with time. At the end of the study their plasma concentrations were 0.6 ng/ml and 0.2 ng/ml plasma. Three subjects had the same concentration of the two TDA isomers in plasma, two subjects had about double, and two subjects had 12 times higher concentrations of 2,6-TDA than 2,4-TDA. The presented study indicates that it is possible to monitor exposure to TDI by monitoring plasma concentrations of TDA.

Biological monitoring of isocyanates and related amines. IV. 2,4- and 2,6-toluenediamine in hydrolysed plasma and urine after test-chamber exposure of humans to 2,4- and 2,6-toluene diisocyanate

Two men were exposed to toluene diisocyanate (TDI) atmospheres at three different air concentrations (ca. 25, 50 and 70 micrograms/m3). The TDI atmospheres were generated by a gas-phase permeation method, and the exposures were performed in an 8-m3 stainless-steel test chamber. The effective exposure period was 4 h. The isomeric composition of the air in the test chamber was 30% 2,4-TDI and 70% 2,6-TDI. The concentration of TDI in air of the test chamber was determined by an HPLC method using the 9-(N-methyl-amino-methyl)-anthracene reagent and by a continuous-monitoring filter-tape instrument. Following the hydrolysis of plasma and urine, the related amines, 2,4-toluenediamine (2,4-TDA) and 2,6-toluenediamine (2,6-TDA), were determined as pentafluoropropionic anhydride (PFPA) derivatives by capillary gas chromatography using selected ion monitoring (SIM) in the electron-impact mode. In plasma, 2,4- and 2,6-TDA showed a rapid-phase elimination half-time of ca. 2-5 h, and that for the slow phase was greater than 6 days. A connection was observed between concentrations of 2,4- and 2,6-TDI in air and the levels of 2,4- and 2,6-TDA in plasma. The cumulated amount of 2,4-TDA excreted in the urine over 24 h was ca. 15%-19% of the estimated inhaled dose of 2,4-TDI, and that of 2,6-TDA was ca. 17%-23% of the inhaled dose of 2,6-TDI. A connection was found between the cumulated (24-h) urinary excretion of 2,4- and 2,6-TDA and the air concentration of 2,4- and 2,6-TDI in the test chamber.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrolysis of 14C-labelled proteins by rumen micro-organisms and by proteolytic enzymes prepared from rumen bacteria

Proteins were labelled with 14C in a limited reductive methylation using [14C]formaldehyde and sodium borohydride. The rate of hydrolysis of purified proteins was little (less than 10%) affected by methylation and the 14C-labelled digestion products were not incorporated into microbial protein during a 5 h incubation with rumen fluid in vitro. It was therefore concluded that proteins labelled with 14C in this way are valid substrates for study with rumen micro-organisms. The patterns of digestion of 14C-labelled fish meal, linseed meal and groundnut-protein meal by rumen micro-organisms in vitro were similar to those found in vivo. The rates of hydrolysis of a number of 14C-labelled proteins, including glycoprotein II and lectin from kidney beans (Phaseolus vulgaris), were determined with mixed rumen micro-organisms and with proteases extracted from rumen bacteria. Different soluble proteins were digested at quite different rates, with casein being most readily hydrolysed. Proteins modified by performic acid oxidation, by cross-linking using 1,6-di-iso-cyanatohexane or by diazotization were labelled with 14C. Performic acid treatment generally increased the susceptibility of proteins to digestion, so that the rates of hydrolysis of performic acid-treated proteins were more comparable than those of the unmodified proteins. Cross-linking resulted in a decreased rate of hydrolysis except with the insoluble proteins, hide powder azure and elastin congo red. Diazotization had little effect on the rate of hydrolysis of lactoglobulin and albumin, but inhibited casein hydrolysis and stimulated the breakdown of gamma-globulin.

Reversed immunosorbents: a simple method for specific antibody immobilization

A method is presented for permanently converting an antigen immunosorbent into a purified antibody immunosorbent which retains specific reactivity for antigen. Following immunological reaction of the antibodies in an antiserum with the antigen adsorbent, the specific immunoglobulin is chemically bound to the antigen by means of the divalent crosslinking agent. Various such agents, including glutaraldehyde, suberimidate, a diazide, and isocyanates, were employed. The conditions of preparation using the first two were optimized, and the characteristics of the resulting immobilized antibodies were investigated. Applications in radioimmunoassay, preparative chromatography, and affinity-constant studies discussed in view of the unique attributes of these reagents.