Concept of Xenobiotics and Substrates for Detoxification
The human body is constantly exposed to compounds that are neither used for energy nor as structural building blocks. These are known as xenobiotics (foreign chemical substances). They are typically hydrophobic, exert toxic effects, and must be eliminated.
Substrates for the detoxification systems fall into two main categories:
- Endogenous toxins: Generated within the body, including ammonia ($NH_3$), peptide and steroid hormones, catecholamines, heme breakdown products (bilirubin), and products of amino acid putrefaction in the gut (indole, phenol, skatole).
- Exogenous substances: Primarily pharmacological drugs that, after fulfilling their therapeutic role, must be cleared either in their original or modified form.
The primary goal of detoxification is to decrease the toxicity of these substances and render them water-soluble (hydrophilic) so they can leave the body.
Phase I Biotransformation: Modification
Phase I occurs in the liver and is essential for hydrophobic compounds. Without it, lipophilic toxins would accumulate in adipose tissue, cell membranes, and the nervous system.
The core of this phase is chemical modification, most frequently via hydroxylation (introduction of an -OH group). The reaction is represented as: $RH \rightarrow R-OH$. Reduction, hydrolysis, deamination, and sulfoxidation also occur. As a result, the substance becomes more polar, its toxicity decreases, and functional groups are created for the next stage.
Microsomal Oxidation System (MOS)
Phase I reactions are driven by the microsomal oxidation system (monooxygenase system). Under physiological conditions, it is localized in the endoplasmic reticulum (ER) membranes of most tissues, but it is most active in the liver. The term "microsomes" refers to closed vesicles formed from ER fragments during laboratory isolation of enzymes.
The core components of this system are:
- Cytochrome P450: A hemoprotein that binds the toxin (substrate) and molecular oxygen.
- Cytochrome P450 reductase: A flavoprotein (containing FAD and FMN) that transfers electrons from the donor (NADPH) to cytochrome P450.
During hydroxylation, cytochrome P450 binds the lipophilic substrate ($RH$) and oxygen ($O_2$). NADPH supplies electrons via the reductase. One oxygen atom is reduced to water, accepting electrons and protons, while the other is inserted into the substrate molecule, forming a hydroxyl group.
Cytochrome P450 is characterized by broad substrate specificity. Its various isoforms interact with a wide range of structurally similar xenobiotics, including many hydrophobic drugs.
Phase II: Conjugation and Excretion Pathways
Phase II involves conjugation—the attachment of a new molecule or radical to the substrate. Transferase enzymes attach glucuronic acid (glucuronate), glutathione, sulfate, glycine, acetate, or a methyl group to the functional groups introduced or exposed in Phase I. The general scheme is: $R-OH \rightarrow R-OK$. This markedly increases the hydrophilicity of the metabolite and completely eliminates its toxicity.
Following biotransformation, the resulting soluble products leave the body via two main routes depending on their properties and molecular weight (MW):
- Via bile (intestine): Hydrophobic metabolites with a molecular weight greater than 300 Da.
- Via urine (kidneys): Hydrophilic metabolites with a molecular weight less than 300 Da, passing from the liver into the systemic circulation.