Macromolecules, covalent binding

Covalent binding of chemicals to biological macromolecules can also cause toxicity. During biotransformation and metabolic activation, chemical compounds can be changed to free radicals, which have an unpaired... 

The first is cell injury (cytotoxicity), which can be severe enough to result in cell death. There are many mechanisms by which xenobiotics injure cells. The one considered here is covalent binding to cell macromol-ecules of reactive species of xenobiotics produced by metabolism. These macromolecular targets include DNA, RNA, and protein. If the macromolecule to which the reactive xenobiotic binds is essential for short-term cell survival, eg, a protein or enzyme involved in some critical cellular function such as oxidative phosphorylation or regulation of the permeability of the plasma membrane, then severe effects on cellular function could become evident quite rapidly. 

Both enzymatic and nonenzymatic biotransformation of acrylonitrile occurs. Acrylonitrile is capable of covalently binding to proteins and other macromolecules such as lipids or nucleic acids, or acrylonitrile can also be directly conjugated to glutathione and excreted in urine as cyanoethylmercapturic acid. 

A study by Young et al. (1977) showed that retention and excretion of acrylonitrile are not directly proportional to dose. The data suggest a saturation process, perhaps due to covalent binding to tissue macromolecules. Seventy-two hours after administration of single oral doses of either 0.1 or 10 mg/kg, the proportion of the dose retained in the carcass was 37% at the low dose (0.1 mg/kg) and 27% at the high dose (10 mg/kg). 

Covalent binding of chemical carcinogens to cellular macromolecules, DNA, RNA and protein, is wel1-accepted to be the first step in the tumor initiation process ( 1, 2). Most carcinogens, including polycyclic aromatic hydrocarbons (PAH), require metabolic activation to produce the ultimate electrophilic species which react with cellular macromolecules. Understanding the mechanisms of activation and the enzymes which catalyze them is critical to elucidating the tumor initiation process. 

A rare but serious event that can result from irreversible CYP inhibition is the development of a hypersensitivity reaction. The bioactivation of a drug and the formation of a covalent adduct between the activated substrate and the enzyme can lead to hapten formation and eventually to an idiosyncratic autoimmune response (usually in the form of autoimmune hepatitis) [14]. The hapten formation is the first key step toward the autoimmune response. The CYP macromolecule is made immunogenic ( foreign ) by the covalent binding of the electrophilic metabolites, and the immune reaction follows with the production of autoantibodies against the target molecule (not necessarily alkylated). 

In situ growth via covalent binding of a hybridizing component to a nanocarbon can be achieved in the case of polymers, dendrons and various other macromolecules which are synthesized in a stepwise manner. The in situ synthesis of such macromolecules potentially increases binding site density while steric effects of the nanocarbon can lead to increased variation in average polymer chain length (polydispersity) [101 103]. 

Ilett KF, Reid WD, Sipes IG, et al. 1973. Chloroform toxicity in mice Correlation of renal and hepatic necrosis with covalent binding of metabolites to tissue macromolecules. Exp Mol Pathol 19 215-229. 

The mechanism of benzene-induced toxicity appears to involve the concerted action of several benzene metabolites. Benzene is metabolized, primarily in the liver, to a variety of hydroxylated and opened-ring products that are transported to the bone marrow, where secondary metabolism occurs. Metabolites may induce toxicity both by covalent binding to cellular macromolecules and by inducing oxidative damage. Metabolites may also inhibit stromal cells, which are necessary to support growth of differentiating and maturing marrow cells. ... 

It is tempting, given the above, to conclude that any degree of covalent binding of drug species to macromolecules will lead to a toxic response. However, this is clearly not the case as there are plenty of examples where covalent binding has had no toxicological consequence. 

From a purely pragmatic perspective, it is clear that reactive metabolites are linked with toxicity and that a circumstantial link can be made to idiosyncratic toxicides. Consequently, even though the mechanism of this toxicity is not fully understood, since assays are available to measure the potential for bioactivation in an ideal world one would not carry this liability forward. Conversely, it is not an ideal world, all drug molecules have challenges and the definition of therapeutic index (i.e., the ratio between the toxic exposure and the therapeutic exposure) is critical. Covalent binding of reactive metabolites to macromolecules is a crude measure and not a full predictor of toxicity and it is well known that toxicity can be ameliorated by a lower dose. Furthermore, the so-called definitive assays require radiolabeled drug material which is expensive and generally slow to produce. 

Bentley, R, Waechter, E, Bieri, E, Staubli, W. Muecke, W. (1986a) Species differences in the covalent binding of p-chloro-o-toluidine to DNA. Arch. Toxicol., Suppl. 9, 163-166 Bendey, R, Bieri, E, Muecke, W., Waechter, E. Staubli, W. (1986b) Species differences in the toxicity of p-chloro-o-toluidine to rats and mice. Covalent binding to hepatic macromolecules and hepatic non-parenchymal cell DNA and an investigation of effects upon the incorporation of [ Hjthymidine into capillary endothelial cells. Chem.-biol. Interact., 57, 27-40... 

Ibtal body accumulation reflects both total intake and the rate of elimination. Factors important in the rate of elimination include pharmacokinetics, lipid solubility, metabolism of the parent compound, profile of metabolites formed, rate of formation of reactive intermediates, degree of enzyme induction, amount of relevant covalent binding with subceUular macromolecules, and the rate of removal from the cell... 

Covalent Binding The irreversible interaction of xenobiotics or their metabolites with macromolecules such as lipids, proteins, nucleic acids. 

Some chemicals that strongly bind to biological receptors (ligands) can produce toxicity directly. However, most toxic chemicals are not intrinsically reactive and must be metabolized to reactive intermediates that often covalently bind to macromolecules (DNA, proteins, etc.), and, if present at a sufficient level, lead to toxicity. Metabolism generally serves to make lipophilic compounds more hydrophilic in order to facilitate excretion through the liver and into the bile for excretion into the feces or through the kidneys and into the urine. This process generally... 

Oxidative stress and covalent binding to macromolecules. Oxidation to the epoxide occurs via a tetrahedral intermediate, which can form either an epoxide or a phenol directly (see the scheme below). The epoxide can covalently bind nucleophiles, such as DNA or proteins, to open up the epoxide to a phenol and make toxic covalent adducts. The phenols can be further oxidized to bisphenols, which can in turn form quinones. Quinones can cause serious oxidative damage to cells through radical pathways, or can alkylate N- or S-nucleophiles, such as glutathione and glycine. 

PAHs are known to undergo covalent binding to macromolecules. They are potent inducers of Cyt P450. [17]. They can also be activated twice by CYP to dihydrodiol peroxides. Carcinogenicity is affected by both the number of rings and the methyl substitution, so that the toxicity pattern for the series below is as shown below [18], with toxicity increasing to the right (two separate series of molecules) ... 

Furans can also undergo covalent binding to macromolecules [20]. Furans, such as 8-methoxypsoralen, are epoxidized by Cyt P450 to yield a highly electrophilic species, which is so reactive that it covalently modifies an amino acid residue of CYP itself and thus inactivates the enzyme irreversibly (hence it is also in the class of suicide inhibitors) [21]. 

Nitrosamines Dimethyl nitrosamine Covalent binding to macromolecules [23]. 

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