The ultimate toxicant is the chemical species that reacts with the endogenous target
molecule (e.g., receptor, enzyme, DNA, microfilamental protein, lipid) or critically alters the
biological (micro) environment, initiating structural and/or functional alterations that result is
toxicity. Often the ultimate toxicant is the original chemical to which the organism is exposed
(parent compound). In other cases, the ultimate toxicant is a metabolite of the parent
compound or reactive oxygen or nitrogen species (ROS or RNS) generated during the
biotransformation of the toxicant. Occasionally, the ultimate toxicant is an unchanged or
altered endogenous molecule