2.3 Metabolism

Metabolism is a crucial process in toxicology, determining how our bodies handle harmful substances. It involves two main phases: Phase I reactions modify chemicals, while Phase II reactions help eliminate them. Understanding these processes is key to grasping how toxins affect us.

The body's metabolic system, particularly the liver, plays a vital role in breaking down and removing toxins. Factors like age, sex, and diet can influence metabolism, impacting how individuals respond to harmful substances. This knowledge is essential for predicting and managing toxic effects.

Phases of metabolism

• Metabolism plays a crucial role in determining the fate and toxicity of xenobiotics in the body
• Biotransformation reactions are divided into two main phases: Phase I and Phase II reactions
• These reactions modify the chemical structure of xenobiotics to facilitate their elimination from the body

Phase I reactions

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• Involve oxidation, reduction, or hydrolysis of the xenobiotic
• Catalyzed by enzymes such as cytochrome P450 (CYP) monooxygenases and flavin-containing monooxygenases (FMOs)
• Examples include hydroxylation of aromatic compounds (benzene) and oxidation of alcohols (ethanol)
• Phase I reactions often result in the formation of reactive intermediates that can be more toxic than the parent compound
• These reactions typically increase the polarity of the xenobiotic, making it more water-soluble

Phase II reactions

• Involve conjugation of the xenobiotic or its Phase I metabolite with endogenous molecules
• Catalyzed by transferase enzymes such as UDP-glucuronosyltransferases (UGTs) and sulfotransferases (SULTs)
• Examples include glucuronidation of morphine and sulfation of acetaminophen
• Phase II reactions generally increase the molecular weight and polarity of the xenobiotic, facilitating its excretion
• Conjugation reactions typically detoxify the xenobiotic, although some exceptions exist (acetaminophen-glucuronide)

Cytochrome P450 system

• The cytochrome P450 (CYP) system is a superfamily of heme-containing enzymes that play a central role in xenobiotic metabolism
• CYP enzymes are predominantly expressed in the liver but are also found in extrahepatic tissues (intestine, lung, brain)
• These enzymes catalyze a wide range of Phase I reactions, including oxidation, reduction, and hydrolysis

Structure and function

• CYP enzymes are membrane-bound proteins located in the endoplasmic reticulum
• They contain a heme group that binds molecular oxygen and a substrate-binding site that determines substrate specificity
• The catalytic cycle involves the activation of oxygen, leading to the insertion of one oxygen atom into the substrate and the formation of water
• Examples of CYP-mediated reactions include the hydroxylation of steroids (testosterone) and the epoxidation of polycyclic aromatic hydrocarbons (benzo[a]pyrene)

Genetic polymorphisms

• CYP enzymes exhibit significant genetic variability, resulting in interindividual differences in drug metabolism and toxicity
• Polymorphisms can lead to altered enzyme activity, ranging from complete loss of function to increased activity
• Examples include CYP2D6 polymorphisms affecting the metabolism of antidepressants (fluoxetine) and CYP2C19 polymorphisms influencing the response to proton pump inhibitors (omeprazole)
• Genetic testing can help predict an individual's metabolic capacity and guide personalized drug therapy

Induction and inhibition

• The activity of CYP enzymes can be modulated by various factors, including drugs, environmental pollutants, and dietary components
• Enzyme induction involves an increase in the expression or activity of CYP enzymes, leading to enhanced metabolism of substrates
• Examples of CYP inducers include rifampicin (CYP3A4) and polycyclic aromatic hydrocarbons (CYP1A1)
• Enzyme inhibition results in decreased metabolism of substrates, potentially leading to drug-drug interactions and toxicity
• Examples of CYP inhibitors include grapefruit juice (CYP3A4) and fluconazole (CYP2C9)

Conjugation reactions

• Conjugation reactions are Phase II biotransformation reactions that involve the covalent attachment of endogenous molecules to xenobiotics or their Phase I metabolites
• These reactions are catalyzed by transferase enzymes and require the presence of cofactors (UDP-glucuronic acid, 3'-phosphoadenosine-5'-phosphosulfate)
• Conjugation reactions generally increase the polarity and molecular weight of the xenobiotic, facilitating its excretion via urine or bile

Glucuronidation

• Catalyzed by UDP-glucuronosyltransferases (UGTs) in the endoplasmic reticulum
• Involves the transfer of glucuronic acid from UDP-glucuronic acid to the xenobiotic or its Phase I metabolite
• Examples include the glucuronidation of morphine and acetaminophen
• Glucuronidation typically detoxifies xenobiotics, although some exceptions exist (morphine-6-glucuronide)

Sulfation

• Catalyzed by sulfotransferases (SULTs) in the cytosol
• Involves the transfer of a sulfonate group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to the xenobiotic or its Phase I metabolite
• Examples include the sulfation of steroid hormones (estradiol) and phenolic compounds (acetaminophen)
• Sulfation can lead to the formation of reactive intermediates (N-sulfooxy-2-acetylaminofluorene)

Acetylation

• Catalyzed by N-acetyltransferases (NATs) in the cytosol
• Involves the transfer of an acetyl group from acetyl-coenzyme A to the amino group of the xenobiotic or its Phase I metabolite
• Examples include the acetylation of isoniazid and sulfonamides
• Acetylation exhibits genetic polymorphism, leading to slow and fast acetylator phenotypes

Methylation

• Catalyzed by methyltransferases in the cytosol
• Involves the transfer of a methyl group from S-adenosylmethionine (SAM) to the xenobiotic or its Phase I metabolite
• Examples include the methylation of catecholamines (dopamine) and arsenic
• Methylation can lead to the formation of toxic metabolites (monomethylarsonic acid)

Glutathione conjugation

• Catalyzed by glutathione S-transferases (GSTs) in the cytosol
• Involves the conjugation of the tripeptide glutathione to the xenobiotic or its Phase I metabolite
• Examples include the conjugation of electrophilic compounds (aflatoxin B1) and reactive oxygen species
• Glutathione conjugation is an important detoxification pathway, although some conjugates can be toxic (S-(1,2-dichlorovinyl)-L-cysteine)

Factors affecting metabolism

• Various factors can influence the rate and extent of xenobiotic metabolism, leading to interindividual variability in toxicity and drug response
• These factors include age, sex, nutritional status, disease states, and drug interactions
• Understanding the impact of these factors is crucial for personalized medicine and risk assessment

Age and development

• Metabolic capacity changes throughout the lifespan, with significant differences between neonates, children, adults, and the elderly
• Neonates have immature enzyme systems, leading to reduced metabolism of certain drugs (theophylline)
• Elderly individuals may have decreased liver mass and blood flow, resulting in altered drug clearance (benzodiazepines)
• Developmental changes in enzyme expression can influence the susceptibility to toxicants (chlorpyrifos)

Sex differences

• Sex-related differences in xenobiotic metabolism can be attributed to hormonal influences and genetic factors
• Some CYP enzymes (CYP3A4) exhibit higher activity in females, while others (CYP1A2) have higher activity in males
• Sex differences in metabolism can affect drug efficacy and toxicity (zolpidem)
• Hormonal fluctuations during the menstrual cycle and pregnancy can alter metabolic capacity

Nutritional status

• Diet and nutritional status can modulate the activity of drug-metabolizing enzymes
• High-protein diets can induce CYP enzymes (CYP3A4), while malnutrition can lead to decreased enzyme activity
• Specific dietary components can inhibit or induce enzyme activity (grapefruit juice, cruciferous vegetables)
• Nutritional deficiencies (vitamin B6, iron) can impair the function of certain enzymes (cysteine conjugate β-lyase)

Disease states

• Various disease states can alter the expression and activity of drug-metabolizing enzymes
• Liver diseases (cirrhosis, hepatitis) can reduce the metabolic capacity of the liver
• Inflammation and infection can downregulate CYP enzymes through the action of cytokines (interleukin-6)
• Genetic diseases (Gilbert's syndrome) can lead to impaired conjugation reactions (glucuronidation)

Drug interactions

• Drug-drug interactions can occur when one drug alters the metabolism of another drug
• Enzyme induction by one drug can lead to increased metabolism and reduced efficacy of another drug (rifampicin and oral contraceptives)
• Enzyme inhibition by one drug can result in decreased metabolism and increased toxicity of another drug (ketoconazole and midazolam)
• Herbal supplements and dietary components can also interact with drugs by modulating enzyme activity (St. John's wort, grapefruit juice)

Metabolic activation vs detoxification

• Xenobiotic metabolism can result in either detoxification or bioactivation of the parent compound
• Detoxification involves the conversion of the xenobiotic into less toxic or inactive metabolites, facilitating their elimination from the body
• Bioactivation, or metabolic activation, involves the formation of reactive intermediates that can be more toxic than the parent compound
• The balance between detoxification and bioactivation determines the ultimate toxicity of a xenobiotic

Bioactivation of toxicants

• Many toxicants require metabolic activation to exert their toxic effects
• Examples include the bioactivation of benzo[a]pyrene to diol epoxides that form DNA adducts and the activation of acetaminophen to N-acetyl-p-benzoquinone imine (NAPQI) that causes liver toxicity
• Bioactivation often involves Phase I reactions catalyzed by CYP enzymes, leading to the formation of electrophilic intermediates
• These reactive intermediates can bind to cellular macromolecules (DNA, proteins) and disrupt cellular function

Detoxification pathways

• Detoxification pathways convert xenobiotics into less toxic or inactive metabolites that can be readily excreted from the body
• Examples include the glucuronidation of morphine and the sulfation of acetaminophen
• Detoxification often involves Phase II conjugation reactions that increase the polarity and water solubility of the xenobiotic
• Glutathione conjugation is an important detoxification pathway for electrophilic compounds and reactive oxygen species

Balance and consequences

• The balance between bioactivation and detoxification determines the net toxicity of a xenobiotic
• Factors that influence this balance include the relative activities of Phase I and Phase II enzymes, the availability of cofactors, and genetic polymorphisms
• When bioactivation overwhelms detoxification, the accumulation of reactive intermediates can lead to cellular damage and toxicity (acetaminophen overdose)
• Interindividual differences in the balance between bioactivation and detoxification can contribute to variability in susceptibility to toxicants (aflatoxin B1)

Toxicokinetics of metabolism

• Toxicokinetics describes the absorption, distribution, metabolism, and excretion (ADME) of xenobiotics in the body
• Metabolism plays a crucial role in determining the fate and toxicity of xenobiotics
• Understanding the toxicokinetics of metabolism is essential for predicting the exposure, bioavailability, and clearance of xenobiotics

Absorption and distribution

• Absorption refers to the entry of a xenobiotic into the systemic circulation from the site of exposure (oral, dermal, inhalation)
• Factors influencing absorption include the physicochemical properties of the xenobiotic (lipophilicity, molecular weight) and the route of exposure
• Distribution describes the reversible transfer of the xenobiotic from the systemic circulation to various tissues and organs
• The extent of distribution depends on factors such as plasma protein binding, tissue affinity, and the presence of tissue-specific transporters

Metabolic clearance

• Metabolic clearance refers to the rate at which a xenobiotic is irreversibly removed from the body through metabolism
• Clearance is determined by the activity of drug-metabolizing enzymes and the blood flow to the metabolizing organs (liver, kidneys)
• High metabolic clearance can lead to rapid elimination of the xenobiotic, while low clearance can result in accumulation and prolonged exposure
• Interindividual differences in metabolic clearance can contribute to variability in drug response and toxicity

Elimination half-life

• The elimination half-life is the time required for the plasma concentration of a xenobiotic to decrease by 50% during the elimination phase
• The half-life is determined by the clearance and the volume of distribution of the xenobiotic
• Xenobiotics with short half-lives are rapidly eliminated from the body, while those with long half-lives can accumulate with repeated exposure
• Knowledge of the half-life is important for determining dosing intervals and assessing the risk of toxicity

Bioavailability and bioaccumulation

• Bioavailability refers to the fraction of the administered dose that reaches the systemic circulation unchanged
• Factors influencing bioavailability include the extent of absorption, first-pass metabolism, and presystemic elimination
• Bioaccumulation occurs when the rate of uptake of a xenobiotic exceeds the rate of elimination, leading to an increase in the body burden over time
• Xenobiotics with high lipophilicity and low metabolic clearance are more likely to bioaccumulate (PCBs, dioxins)

Organ-specific metabolism

• While the liver is the primary site of xenobiotic metabolism, other organs and tissues also possess metabolic capabilities
• Organ-specific metabolism can influence the local toxicity of xenobiotics and contribute to the overall metabolic fate of the compound
• Understanding the role of extrahepatic metabolism is important for predicting tissue-specific toxicity and drug-drug interactions

Liver as primary site

• The liver is the principal organ responsible for xenobiotic metabolism due to its high expression of drug-metabolizing enzymes and its central role in blood circulation
• Hepatocytes, the main cell type in the liver, express a wide range of Phase I and Phase II enzymes (CYP enzymes, UGTs, GSTs)
• The liver's high metabolic capacity and blood flow make it a major site for first-pass metabolism and systemic clearance of xenobiotics
• Liver-specific toxicity can occur when reactive metabolites are formed or when detoxification pathways are overwhelmed (acetaminophen-induced hepatotoxicity)

Extrahepatic metabolism

• Extrahepatic tissues, such as the intestine, lung, kidney, and skin, also express drug-metabolizing enzymes
• The intestine is a major site of first-pass metabolism for orally administered xenobiotics due to the presence of CYP enzymes and UGTs in enterocytes
• The lung is exposed to inhaled xenobiotics and can metabolize compounds through CYP enzymes and phase II reactions (benzene, naphthalene)
• The kidney plays a role in the metabolism and excretion of xenobiotics and can be a target for tissue-specific toxicity (chloroform)

Blood-brain barrier

• The blood-brain barrier (BBB) is a selectively permeable barrier that regulates the entry of xenobiotics into the central nervous system (CNS)
• The BBB is formed by tight junctions between endothelial cells and the presence of efflux transporters (P-glycoprotein)
• Xenobiotics that are lipophilic and have low molecular weight can cross the BBB more readily (ethanol, caffeine)
• Metabolism of xenobiotics by CYP enzymes and phase II reactions in the brain can influence their CNS effects and toxicity (nicotine, polycyclic aromatic hydrocarbons)

Placental transfer

• The placenta is a selective barrier that regulates the transfer of xenobiotics from the maternal circulation to the fetal compartment
• Placental transfer depends on factors such as the physicochemical properties of the xenobiotic, the stage of pregnancy, and the presence of placental transporters
• Some xenobiotics can cross the placenta and expose the developing fetus to potential toxicity (thalidomide, alcohol)
• The placenta expresses drug-metabolizing enzymes (CYP enzymes, UGTs) that can influence the extent of fetal exposure to xenobiotics and their metabolites

Metabolic disorders and toxicity

• Metabolic disorders, whether inborn or acquired, can alter the body's ability to metabolize xenobiotics and endogenous compounds
• These disorders can lead to the accumulation of toxic metabolites or the impaired detoxification of xenobiotics
• Understanding the impact of metabolic disorders on toxicity is important for risk assessment and patient management

Inborn errors of metabolism

• Inborn errors of metabolism are genetic disorders that affect the synthesis, degradation, or transport of specific molecules
• Examples include phenylketonuria (PKU), which impairs the metabolism of phenylalanine, and galactosemia, which affects the metabolism of galactose
• These