Definition:-Bioactivation is defined as: Enzymaticallyformed metabolites, which are more reactive than themother substance and excreted metabolites. ORFormation of highly reactive metabolites (from relativelyinert chemical compounds) which interact with thetissues to precipitate one or more of the several formsof toxicities such as carcinogenesis is called asbioactivation or toxicological activation.
The most significant toxicological effects ofxenobiotics are reactive metabolites are:- can react with nucleophilic sites SH groups (glutathione, cysteine) NH2 and – COOH groups (DNA, RNA, proteins)
Imbalance between formation and detoxification ofreactive metabolites can arise from: enzyme induction (increased biotransformation and formation of reactive metabolites) high dose of xenobiotic depletion of cellular defence mechanisms. saturation of non-toxic pathways
Compounds Reactive pathway or Factors increasing intermediate product toxicityAcetaminophen N-hydroxylation Sulphate and GSH depletionAcetyl hydrazine N-hydroxylationAflatoxin B Epoxidation Further metabolismBenzene EpoxidationBenzo[a]pyrene EpoxidationPCB Epoxidation GSH depletionTetrachlorcarbon Free radicals Reductive metabolismHalothane Free radicals Reductive metabolismParathion Oxidation with sulphur formation
Electrophiles Free radicalsElectrophiles: - are species deficient in electron pair.The enzyme system through which they generated iscytochrome P-450. Carbon, nitrogen or sulphurcontaining compounds can be metabolically activated toyield electrophiles.
Important electrophiles are: Epoxides;-e.g., epoxide of benzo(a)pyrene present in cigarette smoke which causes cancer. Hydroxylamines, nitroso and azoxy derivatives, nitrenium ions and elemental sulphur.Mechanism:- The mechanism by which electrophiles precipitate toxicity is through covalent binding to nucleophilic tissue components such as macromolecules(proteins, nucleic acids, and lipids) or low molecular weight cellular constituents. Covalent binding to DNA is responsible for carcinogenicity and tumour formation.
The body’s defence against electrophiles is their inactivation by conjugation with glutathione, the most abundant cellular nucleophile with –SH group. An e.g. of tissue toxicity due to electrophiles is hepatotoxicity of paracetamol metabolites
Free radicals are species containing an odd number of electron. They may be positively charged (cation radical), negatively charged (anion radical) or neutral radical. R+ R- RCation radical Anion radical neutral radical Free radicals are generally formed via NADPH cytochrome P-450 reductase or other flavin containing reductases. Xenobiotics that on metabolic activation yield free radicals are Quinone’s, aryl amines, nitro aryls and carbon tetrachloride. Endogenous compounds such as epinephrine and DOPA can also generate free radicals. Most free radicals are organic. They provide toxicity by peroxidation of cellular components.
An important class of free radicals is inorganic free radicals such as hydrogen peroxide and superoxide anion. These oxidative moieties can cause tremendous tissue damage leading to mutations or cancer. The potential toxicity of free radicals is far greater than that of the electrophiles. Cellular defence mechanisms against free radicals include control imposed by membrane structure, neutralization by glutathione, control exerted by non- enzymatic antioxidant scavengers such as vitamin A, E and C and enzymatic inactivation of oxygen derived free radicals.
Generation of reactive metabolites is indicated by modification in enzyme activities, formation of glutathione conjugates and depletion in tissue levels of glutathione. Since the availability of glutathione in the body determines the threshold for toxic response, thiols(e.g., N-acetyl cysteine) can be used to treat poisoning by drugs such as paracetamol that yield reactive metabolites.