Biotransformation is the process whereby a substance is changed from one chemical to another (transformed) by a chemical reaction within the body. Metabolism or metabolic transformations are terms frequently used for the biotransformation process. However, metabolism is sometimes not specific for the transformation process but may include other phases of toxicokinetics.
Biotransformation is vital to survival in that it transforms absorbed nutrients (food, oxygen, etc.) into substances required for normal body functions. For some pharmaceuticals, it is a metabolite that is therapeutic and not the absorbed drug. For example, phenoxybenzamine (Dibenzyline®), a drug given to relieve hypertension, is biotransformed into a metabolite, which is the active agent. Biotransformation also serves as an important defense mechanism in that toxic xenobiotics and body wastes are converted into less harmful substances and substances that can be excreted from the body.
If you recall, toxicants that are lipophilic ('lipid-loving', dissolve easily in lipids), non-polar, and of low molecular weight are readily absorbed through the cell membranes of the skin, gastrointestinal (GI) tract, and lung. These same chemical and physical properties control the distribution of a chemical throughout the body and its penetration into tissue cells. Lipophilic toxicants are hard for the body to eliminate and can accumulate to hazardous levels. However, most lipophilic toxicants can be transformed into hydrophilic ('water-loving', dissolve easily in water) metabolites that are less likely to pass through membranes of critical cells. Hydrophilic chemicals are easier for the body to eliminate than lipophilic substances. Biotransformation is thus a key body defense mechanism.
Fortunately, the human body has a well-developed capacity to biotransform most xenobiotics as well as body wastes. An example of a body waste that must be eliminated is hemoglobin, the oxygen-carrying iron-protein complex in red blood cells. Hemoglobin is released during the normal destruction of red blood cells. Under normal conditions hemoglobin is initially biotransformed to bilirubin, one of a number of hemoglobin metabolites. Bilirubin is toxic to the brain of newborns and, if present in high concentrations, may cause irreversible brain injury. Biotransformation of the lipophilic bilirubin molecule in the liver results in the production of water-soluble (hydrophilic) metabolites excreted into bile and eliminated via the feces.
The biotransformation process is not perfect. When biotransformation results in metabolites of lower toxicity, the process is known as detoxification. In many cases, however, the metabolites are more toxic than the parent substance. This is known as bioactivation. Occasionally, biotransformation can produce an unusually reactive metabolite that may interact with cellular macromolecules (e.g., DNA). This can lead to very serious health effects, for example, cancer or birth defects. An example is the biotransformation of vinyl chloride to vinyl chloride epoxide, which covalently binds to DNA and RNA, a step leading to cancer of the liver.
Chemical reactions are continually taking place in the body. They are a normal aspect of life, participating in the building up of new tissue, tearing down of old tissue, conversion of food to energy, disposal of waste materials, and elimination of toxic xenobiotics. Within the body is a magnificent assembly of chemical reactions, which is well-orchestrated and called upon as needed. Most of these chemical reactions occur at significant rates only because specific proteins, known as enzymes, are present to catalyze them, that is, accelerate the reaction. A catalyst is a substance that can accelerate a chemical reaction of another substance without itself undergoing a permanent chemical change.
Enzymes are the catalysts for nearly all biochemical reactions in the body. Without these enzymes, essential biotransformation reactions would take place slowly or not at all, causing major health problems. An example is the inability of persons that have phenylketonuria (PKU) to use the artificial sweetener, aspartame (in Equal®). Aspartame is basically phenylalanine, a natural constituent of most protein-containing foods. Some persons are born with a genetic condition in which the enzyme that can biotransform phenylalanine to tyrosine (another amino acid), is defective. As the result, phenylalanine can build up in the body and cause severe mental retardation. Babies are routinely checked at birth for PKU. If they have PKU, they must be given a special diet to restrict the intake of phenylalanine in infancy and childhood.
These enzymatic reactions are not always simple biochemical reactions. Some enzymes require the presence of cofactors or co-enzymes in addition to the substrate (the substance to be catalyzed) before their catalytic activity can be exerted. These co-factors exist as a normal component in most cells and are frequently involved in common reactions to convert nutrients into energy (vitamins are an example of co-factors). It is the drug or chemical transforming enzymes that hold the key to xenobiotic transformation. The relationship of substrate, enzyme, co-enzyme, and transformed product is illustrated below:
Most biotransforming enzymes are high molecular weight proteins, composed of chains of amino acids linked together by peptide bonds. A wide variety of biotransforming enzymes exist. Most enzymes will catalyze the reaction of only a few substrates, meaning that they have high "specificity". Specificity is a function of the enzyme's structure and its catalytic sites. While an enzyme may encounter many different chemicals, only those chemicals (substrates) that fit within the enzymes convoluted structure and spatial arrangement will be locked on and affected. This is sometimes referred to as the "lock and key" relationship. As shown in Figure 1, when a substrate fits into the enzyme's structure, an enzyme-substrate complex can be formed. This allows the enzyme to react with the substrate with the result that two different products are formed. If the substrate does not fit into the enzyme, no complex will be formed and thus no reaction can occur.
The array of enzymes range from those that have absolute specificity to those that have broad and overlapping specificity. In general, there are three main types of specificity:
For example, formaldehyde dehydrogenase has absolute specificity since it catalyzes only the reaction for formaldehyde. Acetylcholinesterase has absolute specificity for biotransforming the neurotransmitting chemical, acetylcholine. Alcohol dehydrogenase has group specificity since it can biotransform several different alcohols, including methanol and ethanol. N-oxidation can catalyze a reaction of a nitrogen bond, replacing the nitrogen with oxygen.
The names assigned to enzymes may seem confusing at first. However, except for some of the originally studied enzymes (such as pepsin and trypsin), a convention has been adopted to name enzymes. Enzyme names end in "ase" and usually combine the substrate acted on and the type of reaction catalyzed. For example, alcohol dehydrogenase is an enzyme that biotransforms alcohols by the removal of a hydrogen. The result is a completely different chemical, an aldehyde or ketone.
The biotransformation of ethyl alcohol to acetaldehyde is depicted below:
ADH = alcohol dehydrogenase, a specific catalyzing enzyme NAD = nicotinamide adenine dinucleotide, a common cellular reducing agent
By now you know that the transformation of a specific xenobiotic can be either beneficial or harmful—perhaps both depending on the dose and circumstances. A good example is the biotransformation of acetaminophen (Tylenol®), a commonly used drug to reduce pain and fever. When the prescribed doses are taken, the desired therapeutic response is observed with little or no toxicity. However, when excessive doses of acetaminophen are taken, hepatotoxicity can occur. This is because acetaminophen normally undergoes rapid biotransformation with the metabolites quickly eliminated in the urine and feces.
At high doses, the normal level of enzymes may be depleted and the acetaminophen is available to undergo reaction by an additional biosynthetic pathway, which produces a reactive metabolite that is toxic to the liver. For this reason, a user of Tylenol® is warned not to take the prescribed dose more frequently than every 4-6 hours and not to consume more than four doses within a 24-hour period. Biotransforming enzymes, like most other biochemicals, are available in a normal amount and in some situations can be "used up" at a rate that exceeds the bodies ability to replenish them. This illustrates the frequently used phrase, the "Dose Makes the Poison."
Biotransformation reactions are categorized not only by the nature of their reactions, e.g., oxidation, but also by the normal sequence with which they tend to react with a xenobiotic. They are usually classified as Phase I and Phase II reactions. Phase I reactions are generally reactions which modify the chemical by adding a functional structure. This allows the substance to "fit" into the Phase II enzyme so that it can become conjugated (joined together) with another substance. Phase II reactions consist of those enzymatic reactions that conjugate the modified xenobiotic with another substance. The conjugated products are larger molecules than the substrate and generally polar in nature (water-soluble). Thus, they can be readily excreted from the body. Conjugated compounds also have poor ability to cross cell membranes.
In some cases, the xenobiotic already has a functional group that can be conjugated and the xenobiotic can be biotransformed by a Phase II reaction without going through a Phase I reaction. A good example is phenol that can be directly conjugated into a metabolite that can then be excreted. The biotransformation of benzene requires both Phase I and Phase II reactions. As illustrated below, benzene is biotransformed initially to phenol by a Phase I reaction (oxidation). Phenol has the functional hydroxyl group that is then conjugated by a Phase II reaction (sulphation) to phenyl sulfate.
The major transformation reactions for xenobiotics are listed below:
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.
Examples of these types of oxidizing reactions are illustrated below:
The specific oxidizing reactions and oxidizing enzymes are numerous and several textbooks are devoted to this subject. Most of the reactions are self-evident from the name of the reaction or enzyme involved. Listed are several of these oxidizing reactions.
- alcohol dehydrogenation
- aldehyde dehydrogenation
- alkyl/acyclic hydroxylation
- aromatic hydroxylation
Reduction is a chemical reaction in which the substrate gains electrons. Reductions are most likely to occur with xenobiotics in which oxygen content is low. Reductions can occur across nitrogen-nitrogen double bonds (azo reduction) or on nitro groups (NO2). Frequently, the resulting amino compounds are oxidized forming toxic metabolites. Some chemicals such as carbon tetrachloride can be reduced to free radicals, which are quite reactive with biological tissues. Thus, reduction reactions frequently result in activation of a xenobiotic rather than detoxification. An example of a reduction reaction in which the nitro group is reduced is illustrated below:
There are fewer specific reduction reactions than oxidizing reactions. The nature of these reactions is also self-evident from their name. Listed are several of the reducing reactions.
- azo reduction
- disulfide reduction
- nitro reduction
- N-oxide reduction
- sulfoxide reduction
Hydrolysis is a chemical reaction in which the addition of water splits the toxicant into two fragments or smaller molecules. The hydroxyl group (OH-) is incorporated into one fragment and the hydrogen atom is incorporated into the other. Larger chemicals such as esters, amines, hydrazines, and carbamates are generally biotransformed by hydrolysis. The example of the biotransformation of procaine (local anesthetic) which is hydrolyzed to two smaller chemicals is illustrated below:
Toxicants that have undergone Phase I biotransformation are converted to metabolites that are sufficiently ionized, or hydrophilic, to be either eliminated from the body without further biotransformation or converted to an intermediate metabolite that is ready for Phase II biotransformation. The intermediates from Phase I transformations may be pharmacologically more effective and in many cases more toxic than the parent xenobiotic.
Phase II Reactions
A xenobiotic that has undergone a Phase I reaction is now a new intermediate metabolite that contains a reactive chemical group, e.g., hydroxyl (-OH), amino (-NH2), and carboxyl (-COOH). Many of these intermediate metabolites do not possess sufficient hydrophilicity to permit elimination from the body. These metabolites must undergo additional biotransformation as a Phase II reaction.
Phase II reactions are conjugation reactions, that is, a molecule normally present in the body is added to the reactive site of the Phase I metabolite. The result is a conjugated metabolite that is more water-soluble than the original xenobiotic or Phase I metabolite. Usually the Phase II metabolite is quite hydrophilic and can be readily eliminated from the body. The primary Phase II reactions are:
- glucuronide conjugation - most important reaction
- sulfate conjugation - important reaction
- amino acid conjugation
- glutathione conjugation
Glucuronide conjugation is one of the most important and common Phase II reactions. One of the most popular molecules added directly to the toxicant or its phase I metabolite is glucuronic acid, a molecule derived from glucose, a common carbohydrate (sugar) that is the primary source of energy for cells. The sites of glucuronidation reactions are substrates having an oxygen, nitrogen, or sulfur bond. This includes a wide array of xenobiotics as well as endogenous substances, such as bilirubin, steroid hormones and thyroid hormones. Glucuronidation is a high-capacity pathway for xenobiotic conjugation. Glucuronide conjugation usually decreases toxicity, although there are some notable exceptions, for example, the production of carcinogenic substances. The glucuronide conjugates are generally quite hydrophilic and are excreted by the kidney or bile, depending on the size of the conjugate. The glucuronide conjugation of aniline is illustrated below:
Sulfate conjugation is another important Phase II reaction that occurs with many xenobiotics. In general, sulfation decreases the toxicity of xenobiotics. Unlike glucuronic acid conjugates that are often eliminated in the bile, the highly polar sulfate conjugates are readily secreted in the urine. In general, sulfation is a low-capacity pathway for xenobiotic conjugation. Often glucuronidation or sulfation can conjugate the same xenobiotics.
Biotransforming enzymes are widely distributed throughout the body. However, the liver is the primary biotransforming organ due to its large size and high concentration of biotransforming enzymes. The kidneys and lungs are next with 10-30% of the liver's capacity. A low capacity exists in the skin, intestines, testes, and placenta. Since the liver is the primary site for biotransformation, it is also potentially quite vulnerable to the toxic action of a xenobiotic that is activated to a more toxic compound.
Within the liver cell, the primary subcellular components that contain the transforming enzymes are the microsomes (small vesicles) of the endoplasmic reticulum and the soluble fraction of the cytoplasm (cytosol). The mitochondria, nuclei, and lysosomes contain a small level of transforming activity.
Microsomal enzymes are associated with most Phase I reactions. Glucuronidation enzymes, however, are contained in microsomes. Cytosolic enzymes are non-membrane-bound and occur free within the cytoplasm. They are generally associated with Phase II reactions, although some oxidation and reduction enzymes are contained in the cytosol. The most important enzyme system involved in Phase I reactions it the cytochrome P-450 enzyme system. This system is frequently referred to as the "mixed function oxidase (MFO) " system. It is found in microsomes and is responsible for oxidation reactions of a wide array of chemicals.
The fact that the liver biotransforms most xenobiotics and that it receives blood directly from the gastrointestinal tract renders it particularly susceptible to damage by ingested toxicants. Blood leaving the gastrointestinal tract does not directly flow into the general circulatory system. Instead, it flows into the liver first via the portal vein. This is known as the "first pass" phenomena. Blood leaving the liver is eventually distributed to all other areas of the body; however, much of the absorbed xenobiotic has undergone detoxication or bioactivation. Thus, the liver may have removed most of the potentially toxic chemical. On the other hand, some toxic metabolites are in high concentration in the liver.
Modifiers of Biotransformation
The relative effectiveness of biotransformation depends on several factors, including species, age, gender, genetic variability, nutrition, disease, exposure to other chemicals that can inhibit or induce enzymes, and dose levels. Differences in species capability to biotransform specific chemicals are well known. Such differences are normally the basis for selective toxicity, used to develop chemicals effective as pesticides but relatively safe in humans. For example, malathion in mammals is biotransformed by hydrolysis to relatively safe metabolites, but in insects, it is oxidized to malaoxon, which is lethal to insects.
Safety testing of pharmaceuticals, environmental and occupational substances is conducted with laboratory animals. Often, differences between animal and human biotransformation are not known at the time of initial laboratory testing since information is lacking in humans. Humans have a higher capacity for glutamine conjugation than laboratory rodents. Otherwise, the types of enzymes and biotransforming reactions are basically comparable. For this reason, determination of biotransformation of drugs and other chemicals using laboratory animals is an accepted procedure in safety testing.
Age may affect the efficiency of biotransformation. In general, human fetuses and neonates (newborns) have limited abilities for xenobiotic biotransformations. This is due to inherent deficiencies in many, but not all, of the enzymes responsible for catalyzing Phase I and Phase II biotransformations. While the capacity for biotransformation fluctuates with age in adolescents, by early adulthood the enzyme activities have essentially stabilized. Biotransformation capability is also decreased in the aged. Gender may influence the efficiency of biotransformation for specific xenobiotics. This is usually limited to hormone-related differences in the oxidizing cytochrome P-450 enzymes.
Genetic variability in biotransforming capability accounts for most of the large variation among humans. The Phase II acetylation reaction in particular is influenced by genetic differences in humans. Some persons are rapid and some are slow acetylators. The most serious drug-related toxicity occurs in the slow acetylators, often referred to as "slow metabolizers". With slow acetylators, acetylation is so slow that blood or tissue levels of certain drugs (or Phase I metabolites) exceeds their toxic threshold.
Examples of drugs that build up to toxic levels in slow metabolizers that have specific genetic-related defects in biotransforming enzymes are listed below:
Poor nutrition can have a detrimental effect on biotransforming ability. This is related to inadequate levels of protein, vitamins, and essential metals. These deficiencies can decrease the ability to synthesize biotransforming enzymes. Many diseases can impair an individual's capacity to biotransform xenobiotics. A good example, is hepatitis (a liver disease), which is well known to reduce hepatic biotransformation to less than half normal capacity.
Enzyme inhibition and enzyme induction can be caused by prior or simultaneous exposure to xenobiotics. In some situations exposure to a substance will inhibit the biotransformation capacity for another chemical due to inhibition of specific enzymes. A major mechanism for the inhibition is competition between the two substances for the available oxidizing or conjugating enzymes, that is the presence of one substance uses up the enzyme that is needed to metabolize the second substance.
Enzyme induction is a situation where prior exposure to certain environmental chemicals and drugs results in an enhanced capability for biotransforming a xenobiotic. The prior exposures stimulate the body to increase the production of some enzymes. This increased level of enzyme activity results in increased biotransformation of a chemical subsequently absorbed. Examples of enzyme inducers are alcohol, isoniazid, polycyclic halogenated aromatic hydrocarbons (e.g., dioxin), phenobarbital, and cigarette smoke. The most commonly induced enzyme reactions involve the cytochrome P-450 enzymes.
Dose level can affect the nature of the biotransformation. In certain situations, the biotransformation may be quite different at high doses versus that seen at low dose levels. This contributes to the existence of a dose threshold for toxicity. The mechanism that causes this dose-related difference in biotransformation usually can be explained by the existence of different biotransformation pathways. At low doses, a xenobiotic may follow a biotransformation pathway that detoxifies the substance. However, if the amount of xenobiotic exceeds the specific enzyme capacity, the biotransformation pathway is "saturated". In that case, it is possible that the level of parent toxin builds up. In other cases, the xenobiotic may enter a different biotransformation pathway that may result in the production of a toxic metabolite.
An example of a dose-related difference in biotransformation occurs with acetaminophen (Tylenol®). At normal doses, approximately 96% of acetaminophen is biotransformed to non-toxic metabolites by sulfate and glucuronide conjugation. At the normal dose, about 4% of the acetaminophen is oxidized to a toxic metabolite; however, that toxic metabolite is conjugated with glutathione and excreted. With 7-10 times the recommended therapeutic level, the sulphate and glucuronide conjugation pathways become saturated and more of the toxic metabolite is formed. In addition, the glutathione in the liver may also be depleted so that the toxic metabolite is not detoxified and eliminated. It can react with liver proteins and cause fatal liver damage.
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