1.6 Biotransformation of toxicants

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1.6 Biotransformation and elimination of toxicants : 

1.6 Biotransformation and elimination of toxicants Limiting the impact of a toxicant

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Advance Reading : 

Advance Reading Shibamoto, pp 35-47; pp 13-17 Hughes, pp 72-82 NLM Toxicology Tutor Biotransformation and Excretion http://www.sis.nlm.nih.gov/enviro/toxtutor/Tox2/a41.htm http://www.sis.nlm.nih.gov/enviro/toxtutor/Tox2/a51.htm The Liver and Liver Disease - American Liver Foundation http://www.yourliver.org/Liver-Facts.pdf TOXIC CHARACTERISTICS OF FLUOROCITRATE, THE TOXIC METABOLITE OF COMPOUND 1080 Peter J. Savarie Denver Wildlife Research Center, U.S. Fish and Wildlife Service http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1032&context=vpc11 3

Keywords : 

Keywords Anabolism Bioactivation Biological half-life (t1/2) Biotransformation Catabolism Conjugate Conjugation reactions Cytochrome p-450 Cytosolic enzymes Detoxication Enterohepatic loop Fecal excretion Glomerular filtration Glucuronidation Hydrolysis Intestinal excretion Ionized molecules Metabolism Microsomal enzymes Nephrons Oxidation Phase I biotransformation Phase II biotransformation Reabsorption Reduction Secretion Sulfate conjugation Toxication 4

Learning Objectives : 

Learning Objectives Eplain the role of biotransformation in toxicokinetics. Describe how biotransformation facilitates elimination of toxicants. Distinguish between Phase I and Phase II reactions. Define bioactivation or toxication. Identify tissues and factors involved in biotransformation. Summarize the role of elimination in toxicokinetics. Describe processes occurring in the kidney, liver and lung related to the elimination of toxicants. 5

Metabolism : 

Metabolism Sum of biochemical reactions occurring to a molecule within the body. Anabolism - build-up Catabolism - break-down Occurs in the cytoplasm or at specific organelles within the cell. Storage affects the body’s ability to biotransform and eliminate. Sequestration in bone, lipid reduces biotransformation; may keep toxicant away from other tissues. 6

Biotransformation : 

Biotransformation Phase 1 reactions Phase 2 reactions Examples 7

Biotransformation : 

Biotransformation Process that changes substances from hydrophobic to hydrophilic to aid in elimination (grease to salt). Hydrophilic molecules are less able to cross cellular membranes, hence fluid filterable (kidneys). Major elimination routes are fæces (biliary) and urine. Biological half-life, T½ , allows comparison of rates of removal of toxicants, nutrients or pharmaceuticals 8

Biotransformation reactions : 

Biotransformation reactions Grouped as Phase I (functional group modification) and Phase II (conjugation). Goals Produce water soluble metabolites. Activate natural/endogenous compounds for normal function. Some compounds undergo bioactivation. The biotransformed metabolite is more toxic than the original compound. 9

Results of biotransformation : 

Results of biotransformation Increase toxicity via a toxic metabolite. Decrease toxicity via metabolism of a toxic parent compound. No effect on toxicity. Metabolized endogenous compounds may produce toxins. When chemicals such as food additives are tested for toxicity, the metabolites as well as the original compound are identified and tested. 10

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Major Categories/Reactions Phase I Phase II Elimination oxidation reduction hydrolysis conjugation synthesis polar very polar 11

Enzymes of Biotransformation: Phase I enzymes : 

Enzymes of Biotransformation: Phase I enzymes Oxidation (most important). Add O, remove H, increase valence. Cytochrome P450, mixed function oxidase (MFO), alcohol dehydrogenase, oxidases, others. Reduction (less important). Remove O, add H, decrease valence. Reductases. Hydrolysis. Add water. Esterases, phosphatases, others. 12

Cytochrome P450 : 

Cytochrome P450 NADPH-cytochrome P450 reductase (CPR) is the electron donor protein for several oxygenase enzymes found on the endoplasmic reticulum of most eukaryotic cells.  These oxygenases include the cytochromes P450, a family of enzymes involved in the metabolism of many drugs and dietary substances, and in the synthesis of steroid hormones and other extracellular lipid signaling molecules as well as Phase I detoxification. 13

Herbicide resistance in plants : 

Herbicide resistance in plants Cytochrome P450 mono-oxygenases in plant microsomes is related to some herbicide resistance http://www.agnet.org/library/tb/159/ 14

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Phase I Reactions Hughes 15

Enzymes of Biotransformation: Phase II enzymes : 

Enzymes of Biotransformation: Phase II enzymes Conjugation reactions. Enzymes (tranferases) + cofactor. Enzyme catalyzes. Cofactor donates group. Glucuronic acid, glutathione, sulphate, acetyl group, methyl group. Tends to increase molecular size and polarity for excretion. 16

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Phase II Cofactors: GSH Glutathione 17

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Food Toxicology Phase II Cofactors: Acetyl-CoA Acetyl Coenzyme A Acetyl 18

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Food Toxicology Phase II Cofactors: PAPS 3’-Phosphoadenosine 5”-phosphosulfate 19

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Phase II Cofactors: UDPGA acid Uridine-5’- diphosphoglucuronic acid 20

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Benzene Metabolism 21

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Food Toxicology Aniline 22

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De-Alkylation 23

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Free Radical Generation A solvent 24

Case Study: Fluorocitrate and Kangaroos : 

Case Study: Fluorocitrate and Kangaroos Fluorocitrate found in legume pasture plants of Western Australia. Gastrolobium and Oxylobium. Leaf concentrations can be 2.6 g/kg. Highly lethal (TD 1 mg/kg) variable Inhibits TCA cycle by inhibiting aconitase - causes depletion of ATP and accumulation of citrate in the liver. http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1032&context=vpc11 Occurs in the rabbit poison 1080. The gray kangaroo of Western Australia has evolved resistance. In vivo defluorination with glutathione. Other kangaroos from areas without these plants are not tolerant. 25

Fluoroacetate and fluorocitrate : 

Fluoroacetate and fluorocitrate Diagram of the metabolism of fluoroacetate to fluorocitrate in the tricarboxylic acid cycle and inhibition of the enzyme aconitase by fluorocitrate (adapted from Egekeze and Oehme 1979). http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1032&context=vpc11 26

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Rodenticide: Fluoroacetic Acid Fluoroacetate Fluoroacetyl CoA Sodium Fluoroacetate Compound 1080 rodenticide predator control 27

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Deoxynivalenol, Vomitoxin Fusarium trichothecene mycotoxin found on corn and barley 28

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Aflatoxin B1 B1 Q1 = hepatic metabolite Aspergillus mycotoxin found on corn, peanuts and cottonseed 29

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Benzo[a]pyrene Polycyclic aromatic hydrocarbon. Environmental carcinogen. Cell cultures from rodents, fish and humans 30

Heavy Metal Toxicity 1: Pb : 

Heavy Metal Toxicity 1: Pb Absorbed via Ca channels as divalent ion. Capable of reacting with a variety of binding sites. Protein precipitation damages hormones, enzymes. Specific toxic effect depends on reactions with ligands that are essential for the living system. Metal ligands are formed with sulfhydryl groups, as well as amino, phosphate, imidazole, and hydroxyl groups of enzymes and essential proteins – variable effect. 31

Heavy metal toxicity 2: Pb : 

Heavy metal toxicity 2: Pb Sensitivity of a system and degree of interference determines clinical effects. Digestion/respiration  absorption. Liver  detoxification. Kidney  excretion. Antidotes compete as ligands EDTA 32

Heavy Metal Toxicity 3: Pb : 

Heavy Metal Toxicity 3: Pb Metallic lead absorbed most efficiently by the respiratory tract. Pb dust, leaded petrol exhaust products 10% of ingested lead is absorbed. Small intestine. Lead salts are soluble in gastric juices; easily absorbed. Plasma to blood cells – erythrocytes – affects oxygen transport. After oral ingestion: 60% bone (also hair, teeth). 25% liver (hepatocytes). 4% kidney (renal tubules). 3% intestinal wall. 33

Heavy Metal Toxicity 4: Pb : 

Heavy Metal Toxicity 4: Pb Some endpoints. Sulfhydral enzyme inhibition. K transport in red blood cells inhibited Anemia. Porphyrinuria. Excreted chiefly in fæces and urine. Chelating agents: Ca - EDTA. Penicillamine. Dimercaptrol (BAL). 34

Case Study: Elevated PbB Associated with Illicitly Distilled Alcohol, Alabama 1991 : 

Case Study: Elevated PbB Associated with Illicitly Distilled Alcohol, Alabama 1991 Food Toxicology The use of automobile radiators containing lead-soldered parts in the illicit distillation of alcohol ("moonshine") is an important source of lead poisoning among persons insome rural Alabama counties. In 1991, eight persons were diagnosed with elevated blood lead levels (BLLs) at a local hospital. 9 patients had been evaluated for alcohol-related medical conditions at the hospital. Manifestations included generalized tonic-clonic seizures (six), microcytic anemia (five) (hematocrit mean: 32.1%), encephalopathy (two), upper extremity weakness (one), and abdominal colic (one). BLLs ranged from 16 μg/dL to 259 μg/dL (median: 67 μg/dL ). MMWR (1992) 41(17);294-295 35

Case Study: “Moonshine” Lead Toxicity : 

Case Study: “Moonshine” Lead Toxicity Seven patients required hospitalization for 48 hours or longer (range: 2-18 days). Three of these received chelation therapy; initial BLLs were 67, 228, and 259 μg/dL . One patient, whose BLL was 67 μg/dL , died during hospitalization from alcohol-withdrawal syndrome complicated by aspiration pneumonia. Patients reported moonshine ingestion ranging from 0.2 L per day to 1.5 L per day. The lead contents of specimens of moonshine confiscated from two radiator-containing stills in the county in 1991 were 7400 μg/L and 9700 μg/L, compared with nondetectable amounts (less than 1.0 μg/L) in municipal water from the county. Consumption of 0.5 L per day of moonshine containing 9700 μg/L lead would result in a steady state BLL of approximately 190 μg/dL. 36

Elimination of toxicants : 

Elimination of toxicants Removing toxins and metabolites from the body 37

Elimination of Toxicants : 

Elimination of Toxicants Urinary. Fæcal. Respiratory. Other: Saliva. Sweat. Milk (transfer to child). Nails, Hair, Skin. Cerebrospinal fluid. 38

Kidney : 

Kidney Artery: blood away from heart Vein: blood towards heart Membrane filtration Passive or active Food Toxicology 39

Renal macrostructure : 

Renal macrostructure 40

Renal filtration microstructure : 

Renal filtration microstructure Red arterial blood into kidney Filtered in glomerulus. Small molecules pass membrane Water, some salts reabsorbed to blood in tubules Urine collects in medulla to ureter and bladder A nephron 41

Renal histology : 

Renal histology Tubules Glomerulus Microscopic 42

Urinary excretion : 

Urinary excretion Glomerular filtration Tubular secretion Tubular reabsorption 43

Fæcal elimination : 

Fæcal elimination Excretion in bile to intestine. Active transport of toxicant parent and metabolites. Highly soluble Phase II metabolites (large, ionized) Excretion into the lumen of the GI tract. Direct diffusion from capillaries. Bile emulsifies fat in intestine 44

Exhaled air : 

Exhaled air Gas phase xenobiotics. Passive diffusion from blood to alveolus via concentration gradient. The total alveolar epithelial surface area within an average adult human lung has been estimated to be as large as 100-140 m2. http://www.odec.ca/projects/2005/thog5n0/public_html/whatarelungs.html 45

Acknowledgement : 

Acknowledgement Professor Greg Möller University of Idaho 46

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