Phase I Reactions
Phase I biotransformation reactions are simple reactions as compared to Phase II reactions. In Phase I reactions, a small polar group (containing both positive and negative charges) is either exposed on the toxicant or added to the toxicant. The three main Phase I reactions are oxidation, reduction, and hydrolysis.
Oxidation is a chemical reaction in which a substrate loses electrons. There are a number of reactions that can achieve the removal of electrons from the substrate. Addition of oxygen was the first of these reactions discovered and thus the reaction was named oxidation. However, many of the oxidizing reactions do not involve oxygen. The simplest type of oxidation reaction is dehydrogenation, that is the removal of hydrogen from the molecule. Another example of oxidation is electron transfer that consists simply of the transfer of an electron from the substrate.