Concept of Xenobiotics and Goals of Biotransformation
Xenobiotics are foreign substances that enter the body with food, through the skin, or via the lungs and are not used for energy or plastic purposes. As a rule, they are hydrophobic and toxic, and therefore subject to mandatory elimination. Since hydrophobic compounds are poorly excreted by the kidneys and bile, the body converts them into a water-soluble form via the biotransformation system, located primarily in the endoplasmic reticulum of hepatocytes (microsomal, or monooxygenase, oxidation). The process involves two sequential phases.
Phase I: Microsomal Oxidation and Cytochrome P450
The first phase is obligatory for most hydrophobic xenobiotics and most frequently involves hydroxylation. The key enzymes are cytochrome P450 and NADPH-cytochrome P450 reductase.
Reaction Mechanism:
- The reductase oxidizes NADPH to NADP⁺ and transfers two electrons to an oxygen molecule.
- One oxygen atom is reduced to water (accepting two electrons and two protons).
- The second oxygen atom is incorporated into the substrate to form a hydroxyl group (R–OH).
Overall equation: R–H + O₂ + NADPH + H⁺ → R–OH + H₂O + NADP⁺. The hydroxylated product becomes more hydrophilic and suitable for the second phase.
Cytochromes P450 possess broad substrate specificity: a single isoform (e.g., CYP2E1) can metabolize drugs, poisons, and ethanol alike. An important feature is inducibility: upon systematic exposure to toxic substances, the amount of cytochromes increases, and the rate of oxidation rises.
Phase II: Conjugation Reactions
In the second phase, conjugation of the hydroxyl group of the phase I product with polar molecules occurs, which sharply increases hydrophilicity. The reactions are catalyzed by transferase class enzymes. Conjugates used:
- glucuronic acid (UDP-glucuronate);
- sulfate;
- glycine;
- acetate;
- methyl groups;
- glutathione.
UDP-glucuronosyltransferase attaches a glucuronic acid residue to R–OH. Sulfotransferases transfer a sulfate group from the active donor PAPS (3′-phosphoadenosine-5′-phosphosulfate) to the hydroxylated substrate. The resulting conjugates are excreted in urine and bile.
Glutathione Transferases and the Role of Glutathione
Glutathione S-transferases (GSTs) are a large family of enzymes with diverse substrate specificity that utilize the tripeptide glutathione (γ-Glu–Cys–Gly) as a cofactor. They function in all animal and human tissues and are also active in erythrocytes.
GSTs are involved in the detoxification of:
- steroid hormone metabolites and prostaglandins;
- bilirubin and bile acids;
- lipid peroxidation products;
- drugs and xenobiotics.
Three main types of reactions: conjugation of substrates with glutathione, nucleophilic substitution, and reduction of organic hydroperoxides to alcohols. GSTs are induced by alcohol, drugs, and poisons in parallel with the enzymes of microsomal oxidation.
Ethanol metabolism
Normally, ethanol is oxidized in the hepatocyte cytosol by NAD⁺-dependent dehydrogenases:
- Alcohol dehydrogenase (ADH): ethanol → acetaldehyde.
- Acetaldehyde dehydrogenase (ALDH): acetaldehyde → acetic acid (acetate).
With systematic alcohol consumption, the microsomal ethanol-oxidizing system (MEOS) is activated, with the cytochrome CYP2E1 isoform playing the primary role. CYP2E1 oxidizes ethanol to acetaldehyde, consuming NADPH and oxygen.
Since CYP2E1 also metabolizes hypnotics and other medicinal drugs, its induction in chronically drinking individuals leads to accelerated drug destruction and the need to increase doses to achieve a therapeutic effect. A similar induction of MEOS develops with long-term medication use.
Chemical carcinogenesis
Carcinogenesis is the damage to DNA and RNA structure followed by tumor transformation of the cell. The main groups of chemical carcinogens:
- Nitrates and nitrites are found in water, sausage products, and preservatives. Nitrate metabolism produces intermediates up to hydroxylamine, capable of oxidizing hemoglobin (forming methemoglobin), respiratory chain cytochromes, and MSO enzymes.
- Secondary amines react with nitrites to form N-nitrosamines, causing alkylation of DNA nitrogenous bases. Deamination of cytosine leads to its conversion into uracil, which during replication yields an erroneous complementary pair and point mutations.
- Nitrates additionally reduce the activity of the antioxidant system, promoting the accumulation of reactive oxygen species and enhancing lipid peroxidation.
- Aflatoxin B1 from molds of the genus Aspergillus is converted during biotransformation by cytochrome P450 into an epoxide, a reactive electrophile that covalently binds to DNA and causes hepatocellular carcinoma.
Key points
- Xenobiotic biotransformation proceeds in two phases and converts hydrophobic toxicants into water-soluble conjugates.
- Phase I is hydroxylation involving cytochrome P450, NADPH reductase, O₂, and NADPH.
- Cytochromes P450 have broad substrate specificity and are induced by alcohol, drugs, and poisons.
- Phase II is conjugation with glucuronate, sulfate, glycine, acetate, a methyl group, or glutathione under the action of transferases.
- Glutathione S-transferases detoxify electrophiles and reduce lipid hydroperoxides.
- Ethanol is oxidized by ADH and ALDH to acetate; chronic consumption engages MEOS with CYP2E1.
- Induction of CYP2E1 accelerates the metabolism of hypnotics and other drugs, necessitating higher doses.
- The carcinogenic effect of nitrosamines is associated with DNA alkylation; aflatoxin B1 is activated by P450 into an epoxide and causes liver cancer.