Formation of protein adducts

Alcohol-related liver diseases are complex, and ethanol has been shown to interact with a large number of molecular targets. Ethanol can interfere with hepatic lipid metabolism in a number of ways and is known to induce both inflammation and necrosis in the liver. Ethanol increases the formation of superoxide by Kupffer cells thus implicating oxidative stress in ethanol-induced liver disease. Similarly prooxidants (reactive oxygen species) are produced in the hepatocytes by partial reactions in the action of CYP2E1, an ethanol-induced CYP isoform. The formation of protein adducts in the microtubules by acetaldehyde, the metabolic product formed from ethanol by alcohol dehydrogenase, plays a role in the impairment of VLDL secretion associated with ethanol. 

Formation of protein adducts with reactive aldehydic products may provide a general basis for observed pathogenesis. 

Ichihashi, K., Osawa, T., Toyokuni, S., and Uchida, K. Endogenous formation of protein adducts with carcinogenic aldehydes Implication for oxidative stress. J. Biol. Chem. 276 2001 23903-23913. 

Hydroxynonenal is one of the major aldehyde products of lipid peroxidation and has been reported to be the most toxic aldehyde formed (Bene-detti et al. 1979, 1980, Benedetti and Comporti 1987). Li et al. (1996) found a formation of protein adducts in alveolar macrophages from C3H/HeJ and C57BL/6J mice in a dose-dependent manner. Alveolar macrophages from both strains had extensive apoptosis at 100 [iM 4-hydroxyonenal. 

Nucleic acids in the DNA contain a high number of nucleophilic sites that can be attacked by electrophilic intermediates (metabolites) of chemical compounds. DNA adducts formed may cause alterations in the expression of a critical gene in the cell and thus lead to cell death. For example, modification of p53 tumor suppressor gene may inactivate the functions of the p53 protein and render cells sensitive to malignant transformation. Also, formation of RNA adducts may inhibit key cellular events because RNA is essential for protein synthesis. 

There are many instances for azurin and plastocyanin where limiting kinetic behaviour is observed, and attributed to the formation of an adduct between the protein and the inorganic complex followed by electron transfer. Values of the association constants and of the electron-transfer rate constants may then be calculated. This situation has not been observed in the case of stellacyanin, which differs from azurin and plastocyanin in that it has an overall positive charge at pH 7 (of +7 in the case of the reduced protein). The electron-transfer rate constants are often associated with fairly large negative values for the entropy of activation (in the range -84 to -210 J K1 mol-1), which are not expected for electron transfer within a compact assembly. 

R. Bucala, J. Fishman, and A. Cerami, Formation of covalent adducts between cortisol and 16a-hydroxyestrone and protein Possible role in the pathogenesis of cortisol toxicity and systemic lupus erythematosus, Proc. Natl. Acad. Sci. USA 79 3320 (1982). 

Formation of hybrid adducts with acetaldehyde and malondialdehyde (MAA adducts) has been shown to act in a synergistic manner and may be involved in the stabilisation of protein adducts in vivo. Malondialdehyde (HOCH=CH-CHO) is a highly reactive dialdehyde originating from the non-enzymatic lipid peroxidation of a variety of unsaturated fatty acids. 

Within cells, sulfur mustard forms adducts with DNA, primarily those described in Toxicity of sulfur mustard, above. Adducts can also be formed with nucleophilic sites of amino acids and proteins. Contrary to DNA adducts, there is no specific mechanism to reverse protein adduct formation. For this reason, there is a strong forensic interest in the detection of protein adducts of sulfur mustard as these may provide evidence of sulfur mustard exposure for prolonged periods after the incident. 

Detailed metabolism studies have been reported for the RCAs, CS and CR, and to a lesser extent, capsaicin, but sensitive analytical methods for the metabolites have yet to be developed. The formation of covalent adducts with proteins has been little studied, although observations have suggested that CS and CN react with proteins. In the case of CS and capsaicin, major metabolites are derived from an initial hydrolysis with loss of some of the carbon skeleton, and it needs to be established if background levels of these metabolites occur in non-exposed individuals. 

---