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Metabolic Detoxification

Detoxification (“detox”) has broad connotations ranging from the spiritual to the scientific, and has been used to describe practices and protocols that embrace both complementary (fasting, colonic cleaning) and conventional (chelation or antitoxin therapy) schools of medical thought -- as well as some that push the boundaries of scientific plausibility (such as ionic foot detoxification).

In the context of human biochemistry (and this protocol), detoxification can be described with much more precision; here it refers to a specific metabolic pathway, active throughout the human body, that processes unwanted chemicals for elimination. This pathway (which will be referred to as metabolic detoxification) involves a series of enzymatic reactions that neutralize and solubilize toxins, and transport them to secretory organs (like the liver or kidneys), so that they can be excreted from the body. This type of detoxification is sometimes called xenobiotic metabolism, because it is the primary mechanism for ridding the body of xenobiotics (foreign chemicals); however, detoxification reactions are frequently used to prepare unneeded endobiotics (endogenously-produced chemicals) for excretion from the body.

Excess hormones, vitamins, inflammatory molecules, and signaling compounds, amongst others, are typically eliminated from the body by the same enzymatic detoxification systems that protect the body from environmental toxins, or clear prescription drugs from circulation. Metabolic detoxification reactions, therefore, are not only important for protection from the environment, but central to homeostatic balance in the body.

This protocol describes nutritional approaches for general optimization of metabolic detoxification; it is designed to provide a foundation for proper function of this critical system. Specific health concerns may require supplementary detoxification “intervention” protocols (such as heavy metal detoxification, or alcohol-induced hangover prevention).

Toxin and Toxicant Exposure

Toxins are poisonous compounds produced by living organisms; sometimes the term “biotoxin” is used to emphasize the biological origin of these compounds. Man-made chemical compounds with toxic potential are more properly called toxicants. Toxins and toxicants can exert their detrimental effects on health in a number of ways. Some broadly act as mutagens or carcinogens (causing DNA damage or mutations, which can lead to cancer), others can disrupt specific metabolic pathways (which can lead to dysfunction of particular biological systems such as the nervous system, liver, or kidneys).

The diet is a major source of toxin exposure. Toxins can find their way into the diet by several routes, notably contamination by microorganisms, man-made toxicants (including pesticides, residues from food processing, prescription drugs and industrial wastes), or less frequently, contamination by toxins from other “non-food” plant sources.1,2 Some of the toxic heavy metals (lead, mercury, cadmium, chromium), while not “man-made,” have been released/redistributed into the environment at potentially dangerous levels by man, and can find their way into the diet as well. Microbial toxins, secreted by bacteria and fungi, can be ingested along with contaminated or improperly prepared food.

Even the method of food preparation has the potential for converting naturally-occurring food constituents into toxins.3 For instance, high temperatures can convert nitrogen-containing compounds in meats and cereal products into the potent mutagens benzopyrene and acrylamide, respectively. Smoked fish and cheeses contain precursors to toxins called N-nitroso compounds (NOCs), which become mutagenic when metabolized by colonic bacteria.

Outside of the diet, respiratory exposure to volatile organic compounds (VOCs) is a common risk which has been associated with several adverse health effects, including kidney damage, immunological problems, hormonal imbalances, blood disorders, and increased rates of asthma and bronchitis.4

One of the greatest sources of non-dietary toxicant exposure is the air in the home.5 Building materials (such as floor and wall coverings, particle board, adhesives, and paints) can “off-gas” releasing several toxicants that can be detected in humans.6 For example, a toxic benzene derivative commonly used in disinfectants and deodorizers was detected in 98% of adults in the Environmental Protection Agency’s (EPA) “TEAM” study.7 In another EPA study, three additional toxic solvents were present in 100 percent of human tissue samples tested across the country.8

Newly built or remodeled buildings can have substantial amounts of chemical “off-gassing”, giving rise to what has been called “sick building syndrome.”9 Carpet is an especially big offender, potentially releasing several neurotoxins; in testing of over 400 carpet samples, neurotoxins were present in more than 90 percent of the samples, quantitatively sufficient in some samples to cause death in mice.10 Ironically, shortly after the TEAM report, seventy-one ill employees evacuated the new EPA headquarters in Washington DC complaining claiming health problems, which were eventually attributed to the 27,000 sq. feet of new carpet.11

Carpets also trap environmental toxins; the “Non-Occupational Pesticide Exposure Study” (NOPES) found an average of 12 pesticide residues per carpet sampled, and determined that this route of exposure likely provides infants and toddlers with nearly all of their non-dietary exposure to the notorious pesticides DDT, aldrin, atrazine, and carbaryl.12

Avoiding toxin/toxicant exposure

While it is not possible to completely eliminate toxin/toxicant exposure from all sources, there are ways to minimize it:

  • Limit the introduction of VOCs in the home by using VOC-free cleaning products, low-VOC paints, and choosing throw rugs instead of new carpeting13;
  • Store food in bisphenol A (BPA)-free or phthalate-free containers, and avoid reheating foods in plastic containers;
  • Look for organic produce, which is grown without pesticides, and will contain less residue than conventionally-produced fruits and vegetables (although be aware that organic produce isn’t necessarily “pesticide free”)14
  • Washing fruits or vegetables can decrease some pesticide residue, although it is not effective against all pesticide types15, and commercial fruit and vegetable wash solutions may not be any more effective than water alone.16 Peeling skins off of produce may help to further lower pesticide levels;
  • Limit intake of processed foods. Even ones that are free of synthetic preservatives may contain detectable amounts of toxic compounds that were introduced (by chemical transformation) during processing. For example, numerous toxins are produced by the high temperatures used to manufacture some processed food ingredients.17
  • Although the risk of acute toxicity from undercooking meat (food poisoning) is likely a greater risk than toxin exposure from overcooking it, there are ways to reduce toxin production during meat preparation: avoid direct exposure of meat to open flame or hot metal surfaces; cook meat at or below 250◦ F via stewing, braising, crockpot cooking (slow food preparation methods that utilize liquid); turn meat often during cooking, avoid prolonged cooking time at high temperatures, and refrain from consuming charred portions.18

Overview of Xenobiotic Metabolism

The driving force in the evolution of sophisticated metabolic detoxification systems was actually fairly straight forward and dependent on the ability of water to act as a “solvent” to dissolve substances.

Since cellular membranes are primarily lipid based and impermeable to most water soluble (scientifically: “polar”) substances, the transport of water-soluble compounds into a cell requires specialized transport proteins. By placing the appropriate transport proteins on the cell membrane, a cell will only allow desirable water-soluble molecules to enter, and will prevent entry of water-soluble toxins. This same paradigm also applies when the cell needs to excrete unwanted water soluble compounds (like cellular wastes); they exit the cell by a similar mechanism.

In contrast to water-soluble compounds, the lipid cell membrane presents little barrier to lipid-soluble compounds, which can freely pass through it. Potentially damaging lipid-soluble toxins can therefore gain free access to cellular interiors, and are much more difficult to remove.

The metabolic detoxification systems address this problem by converting lipid-soluble toxins into inactive water-soluble metabolites. The “solubilization” of a toxin is accomplished by enzymes which attach (conjugate) additional water-soluble molecules to the lipid-soluble toxin at specific attachment points. If the toxin does not contain any of these attachment points, they are first added by a separate set of enzymes which chemically transform the toxin to include these molecular “handles”. Following the solubilization reactions, the chemically-modified toxin is transported out of the cell and excreted.

These three steps or phases of removing undesirable or harmful lipid-soluble compounds are performed by three sets of cellular proteins or enzymes, called the phase I (transformation) and phase II (conjugation) enzymes, and the phase III (transport) proteins.

Phase I, II, and III metabolisms have different biochemical requirements and respond to different metabolic signals, but must work in unison for proper removal of unwanted xenobiotics (such as toxins or drugs) or endobiotics (such as excess hormones). Enzymes of the phase I, II, and III pathways have several characteristics that make them well suited for their important roles.19 Unlike most other enzymes, detoxification enzymes; can react with many different compounds broadening the number of toxins a single enzyme can metabolize; are more concentrated in areas of the body that are most directly exposed to the environment (like the liver, intestines, or lungs); are inducible, meaning that their synthesis can be increased in response to toxin exposure.

The liver is the primary detoxification organ; it filters blood coming directly from the intestines and prepares toxins for excretion from the body. Significant amounts of detoxification also occur in the intestine, kidney, lungs, and brain, with phase I, II, and III reactions occurring throughout the rest of the body to a lesser degree.

The Three Phases of Detoxification

Phase I Detoxification – Enzymatic Transformation: Under most circumstances, Phase I enzymes begin the detoxification process by chemically transforming lipid soluble compounds into water soluble compounds in preparation for phase II detoxification. The bulk of the phase I transformation reactions are performed by a family of enzymes called the cytochrome P450s (CYPs).

CYP enzymes are relatively non-specific, each has the potential to recognize and modify countless different toxins; after all, a mere 57 human CYPs must be able to detoxify any potential toxin that enters the body.20 However, the cost of this versatility is speed; CYPs metabolize toxins very slowly compared to other enzymes. For instance compare the predominant CYP3A4, which metabolizes 1-20 molecules per second,21 to superoxide dismutase (SOD), which metabolizes over a million molecules per second. Major sites of detoxification overcome the slower speed by producing large amounts of CYPs - CYPs may represent up to 5% of total liver proteins, and similar large concentrations can be found in the intestines. CYPs are amongst the most well studied and best characterized detoxification proteins due to their role in the metabolism of prescription drugs, and to their role in metabolizing endogenous biochemicals (for example, aromatase, which transforms testosterone to estradiol, is a CYP.)22

Several other enzymes contribute to the phase I process as well, notably: the flavin monooxygenases (FMOs; responsible for the detoxification of nicotine from cigarette smoke); alcohol and aldehyde dehydrogenases (which metabolize drinking alcohol), and monoamine oxidases (MAO’s; which break down serotonin, dopamine, and epinephrine in neurons and are targets of several older antidepressant drugs)23

Phase II Detoxification – Enzymatic Conjugation: Following phase I transformation, the original lipid-soluble toxin has been converted into a more water-soluble form, however, this reactive intermediate is still unsuitable for immediate elimination from the cell for a couple of reasons: 1) phase I reactions are not sufficient to make the toxin water-soluble enough to complete the entire excretion pathway; and 2) in many cases, products from the phase I reactions have been rendered more reactive then the original toxins, which makes them potentially more destructive than they once were. Both of these shortcomings are addressed by the activities of the phase II enzymes, which modify phase I products to both increase their solubility and reduce their toxicity. The activation of the phase II enzymes is responsible for the anti-mutagenic and anti-carcinogenic properties of the metabolic detoxification systems; it is widely accepted that phase II enzymes protect against chemical carcinogenesis, especially during the initiation phase of cancers.24

At the genetic level, the production of most phase II enzymes is controlled by a protein called nuclear factor erythroid-derived 2 (Nrf2), a master regulator of antioxidant response.25 Under normal cellular conditions, Nrf2 resides in the cytoplasm (the liquid inside cells within which the cells components are contained) of the cell in an inactive state.26 However, the presence of oxidative stress (triggered by metabolism of toxins by CYPs) activates Nrf2, allowing it to travel to the cell nucleus.27 In the cell nucleus, Nrf2 turns on the genes of many antioxidant proteins, including the phase II enzymes.28 In this way, Nrf2 “senses” oxidative stress or the presence of toxins in the cell, and allows the cell to mount an appropriate response. Nrf2 regulates the activity of genes involved in the synthesis and activation of important detoxification molecules including glutathione and superoxide dismutase (SOD). It also plays an important role in initiating heavy metal detoxification, and the recycling of CoQ10, a potent antioxidant.29,30,31

Certain dietary constituents (including sulforaphane from broccoli and xanthohumol from hops) may also directly activate Nrf2 and stimulate antioxidant enzyme activity; this may partially explain their beneficial effects on detoxification.32

There are several families of phase II enzymes that differ significantly in their activities and biochemistry. In several cases, phase II enzymes exhibit redundancy -- a particular xenobiotic or endobiotic can be detoxified by more than one phase II enzyme.

UDP-glucuronlytransferases (UGTs) catalyze glucuronidation reactions, the attachment of glucuronic acid to toxins to render them less reactive and more water-soluble. There are several different UGTs that are distributed throughout the body, with the liver being the major location. In humans, many xenobiotics, environmental toxicants, and 40-70% of clinical drugs are metabolized by UGTs.33 The plasticizer bisphenol A34 and benzopyrene (from cooked meats)35 are two notable examples of UGT substrates (a substrate is a molecule upon which an enzyme acts). Intestinal UGTs may affect oral bioavailability of several drugs and dietary supplements, and may be responsible for chemoprevention in this tissue.36

Glutathione S-transferases (GSTs) catalyze the transfer of glutathione (a significant cellular antioxidant) to phase I products. GSTs play a major role in the metabolism of several endobiotics, including steroids, thyroid hormone, fat-soluble vitamins, bile acids, bilirubin and prostaglandins.37 GSTs can also function as antioxidant enzymes, detoxifying free radicals38 and oxidized lipids or DNA.39 GSTs are soluble enzymes that are ubiquitous in nature and in humans, forming about 4% of the soluble protein in the human liver and present in several other tissues (including brain, heart, lung, intestines, kidney, pancreas, lens, skeletal muscle, prostate, spleen and testes).40,41 Products of GST conjugation can be excreted via bile, or can travel to the kidneys where they are further processed and eliminated in urine.

Sulfotransferases (SULTs) attach sulfates from a sulfur donor to endo- or xenobiotic acceptor molecules. This reaction is important both in detoxification reactions, as well as normal biosynthesis (the addition of sulfate to chondroitin and heparin, for example, is catalyzed by specific SULTs.42) SULTs play a major role in drug and xenobiotic detoxification, and the metabolism of several endogenous molecules (including steroids, thyroid and adrenal hormones, serotonin, retinol, ascorbate and vitamin D).43 SULTs in the placenta, uterus, and prostate are thought to play a role in the regulation of androgen levels.44 In contrast to other phase II enzymes, SULTs can convert a number of procarcinogens (such as heterocyclic amines from cooked meats) into highly reactive intermediates which may act as chemical carcinogens and mutagens.45

While the UGTs, GSTs, and SULTs catalyze the bulk of human detoxification reactions, several other phase II enzymes contribute to the process to a lesser, but still important extent, including:

Methyltransferase enzymes catalyze methylation reactions using S-adenosyl-L-methionine (SAMe) as a substrate. COMT (catechol O-methyltransferase) is a major pathway for eliminating excess catecholamine neurotransmitters (such as adrenaline or dopamine). Methylation reactions are one of the few phase II reactions that decrease water solubility46;

Arylamine N-acetyltransferases (NATs): NATs detoxify carcinogenic aromatic amines and heterocyclic amines47;

Amino acid conjugating enzymes: Acyl-CoA synthetase and acyl-CoA amino acid N-acyltransferases attach amino acids (most commonly glycine or glutamine) to xenobiotics. The food preservative benzoic acid is one example of a toxin metabolized by amino acid conjugation.48

Phase III Detoxification – Transport: Phase III transporters are present in many tissues, including the liver, intestines, kidneys, and brain, where they can provide a barrier against xenobiotic entry, or a mechanism for actively moving xenobiotics and endobiotics in and out of cells.49 Since water-soluble compounds require specific transporters to move in and out of cells, phase III transporters are necessary to excrete the newly formed phase II products out of the cell. Phase III transporters belong to a family of proteins called the ABC transporters (for ATP-Binding Cassette50), because they require chemical energy, in the form of ATP, to actively pump toxins through the cell membrane and out of the cell.51 They are sometimes called the Multidrug Resistance Proteins (MRPs), because drug-resistant cancer cells use them as protection against chemotherapy drugs52

In the liver, phase III transporters move glutathione, sulfate, and glucuronide conjugates out of cells into the bile for elimination. In the kidney and intestine, phase III transporters can remove xenobiotics from the blood for excretion from the body.53

Balance of Phase I and Phase II Reactions

The products of phase I metabolism are potentially more toxic than the original molecules, which does not present a problem if the phase II enzymes are functioning at a rate to rapidly neutralize the phase I products as they are formed. This, however, is not always the case. Factors which increase the ratio of phase I to phase II activity can upset this delicate balance, producing harmful metabolites faster than they can be detoxified, and increasing the risk of cellular damage. Some of the factors include: diet (some foods and supplements increase phase I enzyme activity), smoking and alcohol consumption (both induce phase I), age (which can decrease phase II UGT, GST, and SULT activity), sex (premenopausal women show 30-40% more phase I CYP3A4 activity than men or postmenopausal women), disease, and genetics (reviewed in 54).

An illustrative (and unfortunately common) example of the consequences of phase I/phase II imbalance is toxicity caused by overdose of the analgesic acetaminophen (paracetamol) – the active ingredient in Tylenol®. Acetaminophen toxicity is the most common cause of liver failure in the US.55 With a normal therapeutic dose of acetaminophen, the drug is predominantly detoxified by the phase II UGT and SULT enzymes. A small amount of the drug is detoxified by a third mechanism: it is first transformed into the toxic metabolite NAPQI (N-acetyl-p-benzoquinoneimine) by phase I CYP enzymes; and this intermediate is detoxified by conjugation with glutathione using the phase II enzyme GST.

During acetaminophen overdose, the UGT and SULT enzymes become quickly overwhelmed. Proportionally more of the drug undergoes the third detoxification mechanism (transformation to NAPQI and conjugation by GST). Eventually, activity of the phase II GST enzyme slows as glutathione stores become depleted56, and NAPQI is produced faster than it can be detoxified. Rising levels of NAPQI in the liver cause widespread damage, including lipid peroxidation, inactivation of cellular proteins, and disruption of DNA metabolism.57 Treatment for acetaminophen overdose involves the timely replenishment of glutathione stores through administration of the precursor amino acids for glutathione synthesis (most commonly N-acetyl cysteine58; see below).