Reductive alkylation of proteins with aromatic aldehydes and sodium cyanoborohydride

Tetrafunctional Epoxy Resin-Based Buoyancy Materials: Curing Kinetics and Properties

In order to synthesize a new kind of buoyancy material with high-strength, low-density and low-water-absorption and to study the curing reaction of tetraglycidylamine epoxy resin with an aromatic amine curing agent, the non-isothermal differential scanning calorimeter (DSC) method is used to calculate the curing kinetics parameters of N,N,N′,N′-tetraepoxypropyl-4,4′-diaminodiphenylmethane epoxy resin (AG-80) and the m-xylylenediamine (m-XDA) curing process. Further, buoyancy materials with different volume fractions of hollow glass microsphere (HGM) compounded with a AG-80 epoxy resin matrix were prepared and characterized. The curing kinetics calculation results show that, for the curing reaction of the AG-80/m-XDA system, the apparent activation energy increases with the conversion rates increasing and the reaction model is the Jander equation (three-dimensional diffusion, 3D, n = 1/2). The experimental results show that the density, compressive strength, saturated water absorption and water absorption rate of the composite with 55 v % HGM are 0.668 g·cm⁻³, 107.07 MPa, 0.17% and 0.025 h^−1/2, respectively. This kind of composite can probably be used as a deep-sea buoyancy material.

Terminomics Methodologies and the Completeness of Reductive Dimethylation: A Meta-Analysis of Publicly Available Datasets

Methods for analyzing the terminal sequences of proteins have been refined over the previous decade; however, few studies have evaluated the quality of the data that have been produced from those methodologies. While performing global N-terminal labelling on bacteria, we observed that the labelling was not complete and investigated whether this was a common occurrence. We assessed the completeness of labelling in a selection of existing, publicly available N-terminomics datasets and empirically determined that amine-based labelling chemistry does not achieve complete labelling and potentially has issues with labelling amine groups at sequence-specific residues. This finding led us to conduct a thorough review of the historical literature that showed that this is not an unexpected finding, with numerous publications reporting incomplete labelling. These findings have implications for the quantitation of N-terminal peptides and the biological interpretations of these data.

Extensive modification of protein amino groups by reductive addition of different sized substituents

The amino groups of ovomucoid, lysozyme and ovotransferrin have been extensively alkylated by reacting the proteins with various carbonyl reagents in the presence of sodim borohydride. The extent of modification ranged from 40 to 100%. Essentially monosubstitution was obtained with acetone, cyclopentanone, cyclohexanone and benzaldehyde, while 20--50% disubstitution was obtained with N-butanal and nearby 100% disubstitution was obtained with formaldehyde. Both the methylated and isopropylated derivatives of all three proteins were soluble and retained almost full biochemical activities, but introduction of the larger substituents caused precipitation with lysozyme and ovotransferrin.

Application of the S-pyridylethylation reaction to the elucidation of the structures and functions of proteins

Cysteine (Cys) and cystine residues in proteins are unstable under conditions used for acid hydrolysis of peptide bonds. To overcome this problem, we proposed the use of the S-pyridylethylation reaction to stabilize Cys residues as pyridylethyl-cysteine (PEC) protein derivatives. This suggestion was based on our observation that two synthetic derivatives formed by pyridylethylation of the SH group of Cys with either 2-vinylpyridine (2-VP) or 4-vinylpyridine (4-VP), designated as S-beta-(2-pyridylethyl)-L-cysteine (2-PEC) and S-beta-(4-pyridylethyl)-L-cysteine (4-PEC), were stable under acid conditions used to hydrolyze proteins. This was also the case for protein-bound PEC groups. Since their discovery over 30 years ago, pyridylethylation reactions have been widely modified and automated for the analysis of many structurally different proteins at levels as low as 20 picomoles, to determine the primary structures of proteins and to define the influence of SH groups and disulfide bonds on the structures and functional, enzymatic, medical, nutritional, pharmacological, and toxic properties of proteins isolated from plant, microbial, marine, animal, and human sources. Pyridylethylation has been accepted as the best method for the modification of Cys residues in proteins for subsequent analysis and sequence determination. The reaction has also been proposed to measure D-Cys, homocysteine, glutathione, tryptophan, dehydroalanine, and furanthiol food flavors. This integrated overview of the diverse literature on these reactions emphasizes general concepts. It is intended to serve as a resource and guide for further progress based on the reported application of pyridylethylation reactions to more than 150 proteins.

Effect of alkylation with different sized substituents on thermal stability of lysozyme

The amino groups of hen egg white lysozyme were reductively alkylated by the reaction with aliphatic aldehydes of various chain lengths and with two aldehydes of different steric hindrance at pH 7.5 and 4 degrees for 3 h. About four of the original six lysine residues were modified by the reaction with acetaldehyde, n-butylaldehyde or n-hexylaldehyde. About three lysine residues were 2,2-dimethylpropylated with trimethylacetaldehyde while a single residue was modified with benzaldehyde. The thermal stabilities of these alkylated lysozymes were investigated by differential scanning calorimetry (DSC) at different acidic pH values. Alkylation thermally destabilized the proteins, depending not only on the extent of modification but also on the size of the substituent. The alkylated derivatives were 8-19 kJ/mol less stable than native lysozyme at 25 degrees and pH 3.0. The temperature dependences of the activities of the alkylated lysozymes against ethylene glycol chitin indicated that the orders of the optimum temperatures and the maximum activities were exactly the same as the order of the thermal stabilities.

Effect of reductive alkylation on thermal stability of ribonuclease A and chymotrypsinogen A

In order to probe changes in the structural stability induced by the introduction of hydrophobic groups into proteins, the amino groups of ribonuclease A and chymotrypsinogen A were reductively alkylated by reaction with various aliphatic aldehydes, formaldehyde, acetaldehyde, n-butylaldehyde and n-hexylaldehyde, and their thermal stabilities were investigated by differential scanning calorimetry (DSC) at different acidic pH values. Ribonuclease A was thermally unstabilized by reductive alkylation, while chymotrypsinogen A was slightly stabilized, depending on both the size of the introduced alkyl groups and the extent of modification. These observations suggest that the effects induced by alkylation involve not only steric hindrance due to the entering bulky groups but also certain other factors such as the participation of the chemically introduced alkyl groups in hydrophobic interactions.

Formation, nutritional value, and safety of D-amino acids

The extent of racemization of L-amino acid residues to D-isomers in food proteins increases with pH, time, and temperature. The nutritional utilization of different D-amino acids vary widely, both in animals and humans. In addition, some D-amino acids may be deleterious. For example, although D-phenylalanine is nutritionally available as a source of L-phenylalanine, high concentrations of D-tyrosine inhibit the growth of mice. The antimetabolic effect of D-tyrosine can be minimized by increasing the L-phenylalanine content of the diet. Similarly, L-cysteine has a sparing effect on L-methionine when fed to mice; however, D-cysteine does not. The wide variation in the utilization of D-amino acids is exemplified by the fact that D-lysine is not utilized as a source of L-lysine, whereas the utilization of D-methionine as a source of the L-isomer for growth is dose-dependent, reaching 76% of the value obtained with L-methionine. Both D-serine and the mixture of L-L and L-D isomers of lysinoalanine induce histological changes in the rat kidneys. D-tyrosine, D-serine, and lysinoalanine are produced in significant amounts under the influence of even short periods of alkaline treatment. Unresolved is whether the biological effects of D-amino acids vary, depending on whether they are consumed in the free state or as part of a food protein. Possible, metabolic interaction, antagonism, or synergism among D-amino acids in vivo also merits further study. The described results with mice complement related studies with other species and contribute to the understanding of nutritional and toxicological consequences of ingesting D-amino acids. Such an understanding will make it possible to devise food processing conditions to minimize or prevent the formation of undesirable D-amino acids in food proteins and to prepare better and safer foods.

Protein-polymer grafts, IV. A. Modification of amino acids: IIai. Modification of free lysine with reductive arylation

Inability to increase the yield of reaction between 2,4 dihydroxybenzaldehyde and gelatin beyond 55 and 60% has led to an extensive investigation of reductive alkylation with free lysine. Even with free lysine, the extent of reaction was about 60%. Since this could be attributed to the electron donation by the phenolic hydroxyls, reductive alkylation was performed between o-, m-, and p-nitrobenzaldehydes and free lysine; o- and m-nitrobenzaldehyde were ineffective in increasing the yield while with p-nitrobenzaldehyde a yield of 72% was achieved. The unreacted 28% of the lysines are susceptible to epichlorohydrin. These results suggest that the slow, reversible first step in reductive alkylation, the formation of the Schiff's base, is responsible for the low yield.

Drug-protein conjugates--XVI. Studies of sorbinil metabolism: formation of 2-hydroxysorbinil and unstable protein conjugates

The metabolism of sorbinil [+)6-fluoro-spiro (chroman-4, 4'-imidazolidine)-2',5' dione), an aldose reductase inhibitor associated with immunological adverse reactions, was studied in vivo and in vitro with particular reference to the formation of protein conjugates of 2-hydroxysorbinil and their further metabolism. [8-3H]Sorbinil was rapidly and extensively metabolized in the rat. 2-Hydroxysorbinil (2HSB) and a phenolic primary alcohol (2,4-imidazolidinedione 5-(2-hydroxyethyl)-5-(5-fluoro-2-hydroxyphenyl); IHFH) were its principal urinary metabolites; over 0-24 hr, they represented 17.0 +/- 0.7% (mean +/- SD, N = 4) and 7.1 +/- 0.7% of the dose, respectively. [3H]2HSB isolated from urine and re-administered was converted to IHFH. Chronic dosing with sorbinil (150 mg/kg x 5) induced 2-hydroxylation of the drug, the 0-24 hr urinary excretion of 2HSB increasing from 17.0 +/- 0.7% to 24.7 +/- 3.4% of the dose (P less than 0.05 by Students' paired t-test). The biotransformation of 2HSB to IHFH was rationalized in terms of an open-chain aldehyde intermediate. Since aldehydes form both stable and unstable protein adducts, 2HSB was potentially a pro-reactive metabolite and initiator of the hypersensitivity reaction associated with sorbinil. However, [3H]2HSB was neither metabolized by human liver microsomes nor underwent irreversible binding to the microsomal protein. Nevertheless, the mild reductant sodium cyanoborohydride, although without effect on microsomal binding of [3H]2HSB, enhanced binding to human serum albumin. Formation of unstable Schiff base adducts was indicated.

Preparation and characterization of N epsilon-(2,3-dihydroxypropyl)-L-lysine and its phenylthiohydantoin derivative

Convenient methods for preparative synthesis of N epsilon-(2,3-dihydroxypropyl)-L-lysine and its phenylthiohydantoin derivative are described. The former compound was characterized by elemental analysis, melting point, and ion-exchange chromatography and the latter by elemental analysis, melting point, UV-spectrum, HPLC and thin-layer chromatography. This study was performed for investigations of lysine residues in proteins.

Chemical modification of proteins: comments and perspectives

The use of chemical modification of proteins has increased exponentially during the past two decades. Today the many different uses of chemical modification include determination of relative reactivities of side chain groups, the quantitation of individual amino acids, development of affinity reagents, mechanism-based reagents for pharmaceutical uses, cross-linking reagents, special techniques for bioprostheses, blocking reagents for peptide synthesis, and reagents for specific cleavages of peptide bonds. Chemical modification should continue to be a primary tool in protein chemistry. It can supply information or products difficult or impossible to attain by the newer powerful technique of in vitro mutagenesis as well as serve as a supplementary procedure for the latter.

Novel materials from protein-polymer grafts

Proteins are the most underrated and under-used polymers: their impressive properties include infusibility, great mechanical strength and inherent adhesive capability due to a highly flexible backbone and many functional side chains. The notion of moisture sensitivity of proteins is not universally true. Barnacle cement (which can adhere to Teflon) and mussel and clam byssus, all of which are 99% protein, set in the presence of water and resist enzymatic as well as chemical degradation at ambient temperature. This observation suggests that proteins that are capable of tight three-dimensional cross-linking can overcome sensitivity to moisture and enzymatic attack. It should then be possible to achieve similar resistance by appropriate chemical manipulation of proteins, leading to cross-linking. We have achieved such a result with an ordinary protein, commercially available gelatin, which was chemically modified and then epoxidized. When cured such a material binds to metals and plastics. Any protein that has modifiable amino acids can be used for this purpose.

Radioiodination of proteins by reductive alkylation

The use of the aliphatic aldehyde, para-hydroxyphenylacetaldehyde as the reactive moiety in the radioiodination of proteins by reductive alkylation is described. The para-hydroxyphenyl group is radiolabeled with 125I, reacted through its aliphatic aldehyde group with primary amino groups on proteins to form a reversible Schiff base linkage which can then be stabilized with the mild reducing agent NaCNBH3. The introduction of the methylene group between the benzene ring and the aldehyde group increases its reactivity with protein amino groups permitting efficient labeling at low aldehyde concentrations. Using this method, radioiodinated proteins with high specific activity can be produced. The reductive alkylation procedure is advantageous in that the labeling conditions are mild, the reaction is specific for lysyl residues, and the modification of the epsilon-ammonium group of lysine results in ionizable secondary amino groups avoiding major changes in protein charge.

Pyridine borane as a reducing agent for proteins

Pyridine borane has been reported as a superior reagent over a wide pH range, 5-9, for the reductive methylation of amino groups of proteins with formaldehyde [J. C. Cabacungan , A. I. Ahmed , and R. E. Feeney (1982) Anal. Biochem. 124, 272-278]. It has also been reported to reduce tryptophan to dihydrotryptophan and to inactivate lysozyme in trifluoroacetic acid [M. Kurata , Y. Kikugawa , T. Kuwae , I. Koyama , and T. Takagi (1980) Chem. Pharm . Bull 28, 2274-2275]. In the present study the specificity of pyridine borane for the two different modifications under different reaction conditions has been demonstrated, and extended to the application to the synthesis of protein containing reductively attached carbohydrates. In the acid reduction, pyridine borane selectively reduced all six tryptophans in lysozyme to dihydrotryptophan while all other amino acids remained intact. On similar treatment no cleavage of the carbohydrate moiety from chicken ovomucoid, and no losses of activity of ovomucoid or ribonuclease, two proteins devoid of tryptophan, were observed. Nearly complete methylation of the lysines of lysozyme, chicken ovomucoid, and ribonuclease was achieved with formaldehyde at pH 7.0 after 2 h at room temperature, with the retention of full activity of the protein without any destruction of tryptophan. The same chemistry was applied to covalently attach glucose and lactose to bovine serum albumin. Parameters, including pH, temperature, and methanol, that affect the reactions were investigated. Incremental additions of pyridine borane during the course of the reactions increased the rate of modification. The covalent attachment of sugar to the epsilon-amino group of lysine was demonstrated by the synthesis of N-alpha- acetylglucitollysine and comparison with acid hydrolysates of the bovine serum albumin-sugar derivatives.

llC labelling of a protein: concanavalin A

A method is described for the 11C labelling of a phytoagglutinin, concanavalin A, involving reductive methylation by formaldehyde and sodium cyanoborohydride. The reaction mixture is then chromatographed by gel permeation. The quantities (20-80 mCi) and specific activities (20-170 mCi/mg) obtained are such that a practical application of the method is possible. No evidence was found, however, of any preferential uptake of concanavalin A by Krebs II ascite cells in Swiss mice.

Comparative studies on radiolabeling of lysozyme by iodination and reductive methylation

Before attempting to radiolabel proteins it is essential that conditions be found for optimal reaction by use of cold reagents. Iodination by the chloramine-T method was not suitable for radiolabeling of lysozyme as it resulted in reduced solubility, large conformational changes, loss of enzymic activity and a decrease in immunochemical reactivity. On the other hand, reductive methylation of lysozyme by formaldehyde and sodium cyanoborohydride was considered suitable for radiolabeling of lysozyme. The extent of reaction with the free amino groups was dependent on the concentration of lysozyme and the molar ratios of the reactants (lysozyme, NaCNBH3 and HCHO). The molecular weight, net charge and enzymic activity of the lysozyme derivatives were similar to the native molecule. The immunochemical reactivity was reduced by 6-13% when more than 6 amino groups were modified. Reductively methylated rabbit IgG showed unaltered molecular weight, net charge and very little conformational changes compared to native IgG. Partial reaction, by reductive methylation using [14C]HCHO, lysozyme with specific activity of 11.1 X 10(6) cpm/mg protein and pig anti lysozyme antibody with specific activity of 2.95 X 10(5) cpm/mg protein were prepared.

Alternative reducing agents for reductive methylation of amino groups in proteins

Dimethylamine borane and trimethylamine borane were compared with sodium cyanoborohydride as reducing agents for the reductive methylation of amino groups in proteins. Dimethylamine borane performed nearly as well as cyanoborohydride; either reducing agent (15 mM) with 20 mM formaldehyde gave extensive methylation of turkey ovomucoid (5 mg/ml) in 2 h at 22 degrees. Trimethylamine borane gave equivalent levels of methylation only with a higher concentration of formaldehyde, suggesting that it is an even milder reducing agent than the other two under these conditions. With all three reducing agents, the pH optimum for methylation covered a range of pH 6-9. Methylations should be performed at the lowest possible pH to avoid side reactions of formaldehyde with the protein. Possible effects of these reducing agents on the disulfide bonds of proteins were studied. No reduction of disulfides of turkey ovomucoid was observed using each of the three reducing agents under conditions for methylation.

Complete inactivation and labeling of methionyl-tRNA synthetase by periodate-treated initiator tRNA in the presence of sodium cyanohydridoborate

Methionyl-tRNA synthetase from Escherichia coli can react with periodate-treated tRNA to form a Schiff's base through the epsilon-amino group of a lysine within the enzymic active center and the 2',3'-aldehyde groups created at the 3'-terminal ribose of tRNA. At alkaline pH, the Schiff's base equilibrium can be continuously and specifically displaced by reduction in situ with sodium cyanohydridoborate, which on the other hand leaves intact the reacting aldehyde groups of oxidized tRNA. The effects of temperature, pH and of reducing agent concentration on the rate and extent of reduction of the Schiff's base are analysed. Conditions are described (37 degrees C, pH 8.0, in the presence of 1 mM cyanohydridoborate) which allowed rapid and complete conversion of the monomeric trypsin-modified methionyl-tRNA synthetase into its 1:1 covalent complex with tRNAfMet.

Factors affecting cyanoborohydride reduction of aromatic Schiff's bases in proteins

Reductive alkylation of bovine serum albumin (BSA) and wool by a aromatic aldehydes and sodium cyanoborohydride has been investigated. The aldehydes used were chosen to allow convenient quantitative measurement of binding by ultraviolet spectroscopy. Alkylation of BSA occurred primarily at two highly reactive sites. Variation of time, PH, reactant concentration, and addition of urea had little effect on the extent of alkylation of BSA. However, more extensive alkylation was achieved in buffered aqueous dimethyl sulfoxide. The unusual reactivity of two epsilon-amino groups on the BSA molecule is attributed to closely placed lysine residues in the primary sequence rather than to favorable placement of unrelated, distant reactive centers. Similarly, only a few of the potentially available epsilon-amino groups of wool were observed to react.