Disposition: Biotransformation

Biotransformation

• principle mechanism for maintaining homeostasis during exposure of an organism to a toxicant or drug

• chemical properties of xenobiotic are changed from those favoring absorption to those favoring excretion

  • convert lipid-soluble, non-polar, non-excretable forms of chemicals to water-soluble, polar forms that are excretable in bile and urine

• without biotransformation, lipid-soluble xenobiotics would be excreted at such a slow pace that they would eventually overwhelm and kill an organism

Purpose of biotransformation

• converts lipophilic to hydrophilc

• facilitates excretion

Consequences of biotransformation

• changes in pharmacokinetic characteristics

• detoxification

• metabolic activation

Phase I Reactions

• enzymatic reactions that add or expose functional groups to xenobiotics (-OH, -SH, -NH₂, -COOH)

• functional groups serve as sites for conjugation of endogenous chemicals

• include hydrolysis, reduction, and oxidation

• does not change water solubility typically

• may result in metabolic activation

Phase II Reactions

• enzymatic reactions that result in the conjugation of large, water-soluble, charged biomolecules to xenobiotics

• functional group must be present on parent compound or phase I product

• includes conjugation

• polar compound added to functional group

• water solubility greatly increased

phases of xenobiotic transformation (benzene)

benzene (xenobiotic) → phenol (phase I product) → phenyl glucaronide (phase II)

organs involved in biotransformation

liver (most important): hepatocytes

lung: Clara cells, Type II alveolar cells

kidney: proximal tubular cell

intestine: mucosa lining cells (enterocyte, gut flora)

• can have presystemic elimination occur here

• gut flora can reverse a conjugation reaction (contribute to entero-hepatic circulation/recycling)

skin: epithelial cells

gonads: seminiferous tubules, sertolis cells

entero-hepatic recycling

• xenobiotic is metabolized in the liver (phase I and phase II)

• excreted into bile

• passed into the intestinal lumen

• reactivated by intestinal bacteria (deconjugate xenobiotic)

• xenobiotic metabolite is reabsorbed across the intestinal mucosa

  • increases the half-life of the xenobiotic

  • can enhance toxicity or be utilized for therapeutic reasons

phase I primary goal

inactivate most xenobiotics and drugs, initializing the process of increasing the polarity of the chemical to facilitate excretion

phase I alternative outcomes

• activation of prodrugs (activate an inactive compound)

• bioactivation of xenobiotic (produce more toxic metabolite)

prodrug examples

• acetylsalicylic acid (aspirin) activated to salicylic acid via BChE, PAFAH

• codeine activated to morphine via O-dealkylation

hydrolysis

• water is used to cleave a molecule, one part gains H+ other part gains hydroxyl group

• typically targets cleavage of ester (carboxylic acid and alcohol) or amide bonds (carboxylic acid and amine)

• also targets thioesters, hydrazides, and epoxides

reduction

• chemical species decreases its oxidation number by gaining electrons, often under low oxygen levels

• typically adds hydrogen or removes an oxygen

• carbonyl → alcohol

• nitro (NO₂) → amine (NH₂)

• azo (-N=N-) → amine

• reductive dehalogenation

oxidation

• most common reaction

• addition of an oxygen or removal of a hydrogen

• monooxygenases are main group of enzymes, use NADPH and O₂ as substrates

• aromatic hydroxylation (addn of hydroxyl to aromatic rings)

• aliphatic hydroxylation (oxidation of carbon atoms on side chains)

• N-dealkylation

• O-dealkylation

• epoxidation

monoxygenases

• enzyme systems that utilize a single molecular oxygen for addition of a single oxygen atom into a substrate and reduction of a single oxygen atom into water

• capable of oxygenation and reduction

• detoxification or bioactivation

• requires energy and/or reducing equivalents

• may be saturated

• can metabolize many xenobiotics

• liver is main organ but also widely distributed among tissues

flavin monooxygenases (FMO or FMN)

• protein family of enzymes that catalyze chemical reactions utilizing flavin as cofactor

• membrane-bound enzymes localized in the cytosolic face of endoplasmic reticulum

• important for oxidation of amines and other compounds containing nitrogen, sulfur, or phosphorus

• not inducible by xenobiotics

• may preferentially act on some xenobiotics less readily attacked by CYP450

CYP450

• protein family of enzymes that catalyze chemical reactions using heme as a cofactor

• membrane-bound enzymes localized in the cytosolic face of endoplasmic reticulum

• use a cyclical transfer of electrons between oxidizes (Fe³⁺) or reduced (Fe²⁺) forms of iron

• processes 90% of all xenobiotics and drugs in phase I

• inducible xenobiotics

• acts on wide range of xenobiotics

CYP450 functions

• regulation and induction potential

• genetic variation (difference in expression determines metabolism of xenobiotics)

• lipid/cholesterol biosynthesis

• fatty acid metabolism

• bile acid biosynthesis

• vitamin D degradation

• steroid hormone biosynthesis

basic CYP reaction

1. NADPH CYP450-reductase forms complex with CYP

2. 1 electron is transferred from NADPH to NADPH reductase to CYP

3. O₂ is added to cofactor (heme), potential for superoxide production

4. 1 electron is transferred from NADPH to NADPH reductase to CYP, negative charge on oxygen

5. Reductive oxidation splits O-O bond, water is produced, potential for hydrogen peroxide production

6. Oxygen atom is added to xenobiotic to form XOH (now has functional group) and other oxygen atom forms water

7. cytochrome b₅ can also serve as electron donor (sometimes)

Cyp1A1

• member of Cyp1 family

• induced by polycyclic hydrocarbons

• can bioactivate procarcinogens

• expression in GI tract is linked to colon cancer

• main enzyme in detoxification and bioactivation of aromatic hydrocarbons

Cyp1A2

• member of Cyp1 family

• induced by polycyclic hydrocarbons

• can bioactivate procarcinogens

• expression in GI tract is linked to colon cancer

• involved in detoxification of aromatic hydrocarbons

• involved in metabolism of ~15% of all pharmaceuticals used today

Cyp3A4

• most abundant CYP in human liver (up to 55% of the complement of hepatic CYP enzymes)

• most common CYP found in the intestinal mucosa and responsible for much of pre-systemic elimination in liver and intestines

• metabolizes over 120 xenobiotics and involved in biotransformation of ~70% of all pharmaceuticals

• since many drugs are substrates for Cyp3A4, this can lead to drug-drug interactions where inhibition can lead to toxic levels of drug or inhibition can cause treatment failures

• inhibited by grapejuice which leads to higher concentration of drugs

• powerful ROS producer

Cyp2D6

• metabolizes 25% of all drugs today and 50% of commonly used antipsychotics

• 5-10% of caucasians and 2-8% of African Americans are poor metabolizers

Cyp2C9

• most abundant CYP2 family member

• ~10-20% of CYP enzymes in liver

• biotransformation of NSAIDS and warfarin

• 1-3% of caucasians and 1-2% of Asians or African Americans are poor metabolizers

Cyp2C19

• ~10% of CYP enzymes in the liver

• biotransformation of several important protein pump inhibitors

CYP2E1

• 7% of the total CYP enzyme content in the human liver

• induced by a number of xenobiotics (ethanol, pheobarbital, isoniazid, phenytoin, cigarette smoke)

• induction associated with increased liver injury by reactive metabolites of CCl₄ and PhBr

• actively produces free radicals and other reactive metabolites associated with adduct formation and lipid peroxidation

• powerful ROS producer

CYP1 family

• primarily regulated by aryl hydrocarbon receptor

CYP2 and CYP3 families

• primarily regulated by constitutively active receptor and pregnane X receptor