Specificity of N-ethyl lysine of a monoclonal antibody to acetaldehyde-modified proteins prepared under reducing conditions

Aldehyde-modified proteins as mediators of early inflammation in atherosclerotic disease

Inflammation is widely accepted to play a major role in atherosclerosis and other cardiovascular diseases. However, the exact mechanism(s) by which inflammation exerts its pathogenic effect remains poorly understood. A number of oxidatively modified proteins have been associated with cardiovascular disease. Recently, attention has been given to the oxidative compound of malondialdehyde and acetaldehyde, two reactive aldehydes known to covalently bind and adduct macromolecules. These products have been shown to form stable malondialdehyde-acetaldehyde (MAA) adducts that are reactive and induce immune responses. These adducts have been found in inflamed and diseased cardiovascular tissue of patients. Antibodies to these adducted proteins are measurable in the serum of diseased patients. The isotypes involved in the immune response to MAA (i.e., IgM, IgG, and IgA) are predictive of atherosclerotic disease progression and cardiovascular events such as an acute myocardial infarction or coronary artery bypass grafting. Therefore, it is the purpose of this article to review the past and current knowledge of aldehyde-modified proteins and their role in cardiovascular disease.

Analysis of N(ε) -ethyllysine in human plasma proteins by gas chromatography-negative ion chemical ionization/mass spectrometry as a biomarker for exposure to acetaldehyde and alcohol

BACKGROUND

N(ε) -ethyllysine (NEL) is a major stable adduct formed by the reaction of acetaldehyde (AA) with lysine residues in proteins. However, its occurrence and levels in biological specimens and its relationship with AA/alcohol exposure-associated disorders have not been fully elucidated. In this study, we have developed a sensitive and specific method to quantitate NEL levels in human plasma proteins.

METHODS

The method consists of (1) purification of the protein fraction of interest by Sephadex G-15 to remove low molecular substances, (2) hydrolysis of proteins with Pronase E in the presence of stable isotope-labeled internal standards, (3) derivatization of amino acids with pentafluorobenzyl (PFB) bromide, and (4) quantification of the PFB derivatives of NEL and l-lysine using gas chromatography-negative ion chemical ionization/mass spectrometry in a selected ion monitoring mode.

RESULTS

Using the above method, the NEL levels in human plasma proteins obtained from 10 each of control subjects and alcoholic patients were measured. NEL was detected in all samples analyzed, the average level of NEL in the plasma proteins of alcoholic patients (1.17 ± 0.36 NEL/1,000 l-lysine) being significantly higher than that of control subjects (0.26 ± 0.07 NEL/1,000 l-lysine).

CONCLUSIONS

The method could be applied to molecular epidemiological studies to investigate possible associations between the NEL levels in human tissue proteins and human diseases associated with exposure to AA and alcohol.

Immunological response in alcoholic liver disease

The development of alcoholic liver disease (ALD) can be attributed to many factors that cause damage to the liver and alter its functions. Data collected over the last 30 years strongly suggests that an immune component may be involved in the onset of this disease. This is best evidenced by the detection of circulating autoantibodies, infiltration of immune cells in the liver, and the detection of hepatic aldehyde modified proteins in patients with ALD. Experimentally, there are numerous immune responses that occur when proteins are modified with the metabolites of ethanol. These products are formed in response to the high oxidative state of the liver during ethanol metabolism, causing the release of many inflammatory processes and potential of necrosis or apoptosis of liver cells. Should cellular proteins become modified with these reactive alcohol metabolites and be recognized by the immune system, then immune responses may be initiated. Therefore, it was the purpose of this article to shed some insight into how the immune system is involved in the development and/or progression of ALD.

PROTEIN ADDUCT SPECIES IN MUSCLE AND LIVER OF RATS FOLLOWING ACUTE ETHANOL ADMINISTRATION

AIMS

Previous immunohistochemical studies have shown that the post-translational formation of aldehyde-protein adducts may be an important process in the aetiology of alcohol-induced muscle disease. However, other studies have shown that in a variety of tissues, alcohol induces the formation of various other adduct species, including hybrid acetaldehyde-malondialdehyde-protein adducts and adducts with free radicals themselves, e.g. hydroxyethyl radical (HER)-protein adducts. Furthermore, acetaldehyde-protein adducts may be formed in reducing or non-reducing environments resulting in distinct molecular entities, each with unique features of stability and immunogenicity. Some in vitro studies have also suggested that unreduced adducts may be converted to reduced adducts in situ. Our objective was to test the hypothesis that in muscle a variety of different adduct species are formed after acute alcohol exposure and that unreduced adducts predominate.

METHODS

Rabbit polyclonal antibodies were raised against unreduced and reduced aldehydes and the HER-protein adducts. These were used to assay different adduct species in soleus (type I fibre-predominant) and plantaris (type II fibre-predominant) muscles and liver in four groups of rats administered acutely with either [A] saline (control); [B] cyanamide (an aldehyde dehydrogenase inhibitor); [C] ethanol; [D] cyanamide+ethanol.

RESULTS

Amounts of unreduced acetaldehyde and malondialdehyde adducts were increased in both muscles of alcohol-dosed rats. However there was no increase in the amounts of reduced acetaldehyde adducts, as detected by both the rabbit polyclonal antibody and the RT1.1 mouse monoclonal antibody. Furthermore, there was no detectable increase in malondialdehyde-acetaldehyde and HER-protein adducts. Similar results were obtained in the liver.

CONCLUSIONS

Adducts formed in skeletal muscle and liver of rats exposed acutely to ethanol are mainly unreduced acetaldehyde and malondialdehyde species.

Alcohol abuse, alcoholism, and damage to the immune system--a review

Chronic alcohol abuse exacts a major social and medical toll in the United States and other Western countries. One of the least appreciated medical complications of alcohol abuse is altered immune regulation leading to immunodeficiency and autoimmunity. The consequences of the immunodeficiency include increased susceptibility to bacterial pneumonia, tuberculosis, and other infectious diseases. In addition, the chronic alcoholic often has circulating autoantibodies, and recent investigations indicate that the most destructive complications of alcoholism, such as liver disease and liver failure, may have a component of autoimmunity. Current research on altered cytokine balance produced by alcohol is leading to new insights on the regulation of the immune system in the chronic alcoholic. There is also recent development of exciting new techniques designed to improve or restore immune function by manipulation of cytokine balance. Although much remains to be learned, both in the abnormalities produced by alcohol and in the techniques to reverse those abnormalities, current progress reflects a rapidly improving understanding of the basic immune disorders of the alcoholic.

Soluble proteins modified with acetaldehyde and malondialdehyde are immunogenic in the absence of adjuvant

Recent studies have shown that the alcohol metabolites malondialdehyde and acetaldehyde can combine to form a stable adduct (MAA) on proteins. This adduct has been detected in the livers of rats chronically consuming ethanol, and serum antibodies to MAA have been observed at significantly higher concentrations in ethanol-fed when compared with pair-fed or chow-fed control rats. More recently, preliminary studies have strongly suggested that the MAA adduct is capable of stimulating antibody responses to soluble proteins in the absence of adjuvants. The antibodies produced recognize either the MAA epitope or the carrier protein itself. Therefore, it was the purpose of this study to examine the potential immunogenicity of MAA-modified exogenous proteins in the absence of adjuvants. Balb/c mice were immunized in the presence or absence of adjuvant with different concentrations of unmodified or MAA-modified proteins. The antibody response to both the MAA epitope and unmodified protein epitopes were determined by ELISA. In the absence of adjuvant, significant antibody responses were induced to both the MAA epitope and nonmodified protein epitopes. Smaller immunizing doses of MAA-protein conjugate favored the production of antibodies to nonmodified proteins, whereas larger doses induced a strong anti-MAA response. In studies to begin determining a mechanism for the specificity of the response in the absence of adjuvants, peritoneal macrophages were found to bind and degrade MAA-adducted proteins through the use of a scavenger receptor. This indicated that MAA-adducted proteins may be specifically taken up and epitopes presented to the humoral immune system in the absence of adjuvants. Importantly, these are the first data showing that an alcohol-related metabolite can induce an antibody response in the absence of adjuvant and suggesting a mechanism by which antibody to the MAA adduct or its carrier (exogenous or endogenous) proteins may be generated in vivo.

Monoclonal and polyclonal antibodies recognizing acetaldehyde-protein adducts

Studies have investigated the hypothesis that metabolically derived acetaldehyde (AA) is capable of complexing with liver cell proteins to form AA-protein adducts that are capable of acting as antigens and inducing an immune response, as detected by the formation of unique antibodies. In an effort to better characterize and describe these adducts, mouse monoclonal and rabbit polyclonal antibodies specific for antigens prepared with AA under non-reducing (physiologic) and reducing (presence of sodium cyanoborohydride) conditions have been prepared. Two monoclonal antibodies were developed. The first antibody was RT1.1, which is specific to N-ethyl lysine (NEL); it is of the IgG2b isotype and recognizes all proteins modified with AA under reducing conditions. The other monoclonal antibody, NR-1, was of the IgG3 isotype; it recognizes proteins modified with AA under non-reducing conditions and cannot be inhibited by NEL. Affinity-purified and/or absorbed polyclonal antibodies were also produced to these epitopes. Using this panel of monoclonal and affinity-purified polyclonal antibodies, unique antigen-antibody binding occurred that: (1) detected only NEL; (2) reacted with the alpha-amino group on proteins prepared under reducing conditions; and (3) detected adducts on proteins prepared under non-reducing conditions. However, the only antibodies that recognized antigen(s) from alcohol-fed rat livers were those that were not specific to NEL or the alpha-amino group modified under reducing conditions. These data indicate that the relevant adduct in alcohol-fed rat livers is not NEL, and that it presumably is related to proteins modified with AA under non-reducing conditions.

Structural characterisation of acetaldehyde adducts formed by a synthetic peptide mimicking the N-terminus of the hemoglobin beta-chain under reducing and nonreducing conditions

This work was carried out to elucidate the structures resulting from acetaldehyde-induced modifications at the haemoglobin beta-chain N-terminal residues under different experimental conditions. A synthetic peptide (Val-His-Leu-Thr-Pro-Glu-Cys) of m/z 798, which represents the six N-terminal residues of the haemoglobin beta-chain N-terminus with an additional C-terminal cysteine, was used as a model compound. Peptide-acetaldehyde adducts were separated by reverse-phase HPLC. Fast-atom-bombardment MS and linked-scan (B/E) spectra were used to study adduct structures. Under nonreducing conditions at pH 7.4 (1:10 peptide/acetaldehyde molar ratio), two types of adducts of m/z 824 were formed, both with modifications at the N-terminal valine. These two adducts were shown to be a Schiff base, and a cyclic 2-methyl-imidazolidine-4-one. The 2-methyl-imidazolidine-4-one adduct was demonstrated to be formed via the Schiff base and to undergo dissociation gradually after 24 h. Reducing conditions at pH 7.4 (peptide /acetaldehyde molar ratio of 1:1 with 20 mM NaCNBH3) resulted in the formation of an adduct of m/z 826, which was shown to be an N-ethylated adduct of valine. A small amount of nonreduced adducts were also formed under these conditions. Reducing conditions at pH 9.0 (1:10 peptide/acetaldehyde molar ratio with 20 mM NaCNBH3) yielded two secondary, i.e. diethylated (m/z 854), products very rapidly. The cysteine residue of the peptide was not found to form an adduct with acetaldehyde under physiological pH.