Mechanism of Toxicity
The path to toxicity involves four main steps:
Toxic effects depend on the concentration of ultimate toxicant at the target tissue. The ultimate toxicant is the chemical that reacts with an endogenous molecule or alters the biological environment resulting in toxicity. The ultimate toxicant may be a metabolite or byproduct of the primary toxicant to which the organism is exposed. Biotransformation of xenobiotics into harmful products is known as toxication or metabolic activation.
Reations between the ultimate toxicant and endogenous molecules depend on the properties of the endogenous molecule. The most common targets are macromolexules like nucleic acids and proteins, although lipids are often involved. Target molecules must have the appropriate reactivity and be accesible in a sufficient concentration.
Reaction between the ultimate toxicant and target molecule may cause either destruction or dysfunctrion of the target. If the target molecule is involved in cellular regulation, dysregulation of gene expression and/or cell function may occur. If the target is mostly involved in cell maintenance, the injury may cause cell death, especially if they disrupt energy production and/or protein synthesis.
Toxic damage may be repaired at the molecular, cellular or tissue levels. Molecular repair involves either reversal of the damage or degradation and re-synthesys of the molecule. In most tissues injured cells die and are replace by cell division, except in nerve tissue were cell division is not possible and repair is mediated by macrophages and Schwann cells.
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Toxicant Delivery
Accumulation of the ultimate toxicant at the target tissue depends on its distribution, absorption, metabolism and elimination.
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Toxication
Often a toxicant becomes indiscriminately reactive towards endogenous molecules.
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