Nats

GSTs

Others

Others

FIGURE 3-2 The fraction of clinically used drugs metabolized by the major phase 1 and phase 2 enzymes. The relative size of each pie section represents the estimated percentage of drugs metabolized by the major phase 1 (panel A) and phase 2 (panel B) enzymes. In some cases, more than a single enzyme is responsible for metabolism of a single drug. CYP, cytochrome P450; DPYD, dihydropyrimidine dehydrogenase; GST, glutathione-S-transferase; NAT, N-acetyltransferase; SULT, sulfotransferase, TPMT, thiopurine methyltransferase; UGT, UDP-glucuronosyltransferase.

SULTs

UGTs

FIGURE 3-2 The fraction of clinically used drugs metabolized by the major phase 1 and phase 2 enzymes. The relative size of each pie section represents the estimated percentage of drugs metabolized by the major phase 1 (panel A) and phase 2 (panel B) enzymes. In some cases, more than a single enzyme is responsible for metabolism of a single drug. CYP, cytochrome P450; DPYD, dihydropyrimidine dehydrogenase; GST, glutathione-S-transferase; NAT, N-acetyltransferase; SULT, sulfotransferase, TPMT, thiopurine methyltransferase; UGT, UDP-glucuronosyltransferase.

GLUCURONIDATION UGTs catalyze the transfer of glucuronic acid from the cofactor UDP-GA to a substrate to form b-D-glucopyranosiduronic acids (glucuronides), metabolites that are sensitive to cleavage by b-glucuronidase. The generation of glucuronides can be formed through alcoholic and phenolic hydroxyl groups, carboxyl, sulfuryl, and carbonyl moieties, as well as through primary, secondary, and tertiary amine linkages. Examples of glucuronidation reactions are shown in Table 3-2. The broad specificity of UGTs assures that most clinically used drugs are excreted as glucuronides. There are 19 human genes that encode the UGT proteins; nine are encoded by the UGT1 locus on chromosome 2; ten are encoded by the UGT2 gene cluster on chromosome 4. Both families of proteins are involved in the metabolism of drugs and xenobiotics, while the UGT2 family appears to have greater specificity for the glucuronidation of endogenous substances such as steroids.

UGTs are expressed in a tissue-specific and often inducible fashion, with the highest concentration in the GI tract and liver. Based upon their physicochemical properties, glucuronides are excreted by the kidneys into the urine or through active transport processes through the apical surface of the liver hepatocytes into the bile ducts and thence to the duodenum with bile. Many drugs that are glucuronidated and excreted in the bile reenter the circulation by "enterohepatic recirculation": b-D-glucopyranosiduronic acids are targets for b-glucuronidase activity found in strains of

Glucoronidation

FIGURE 3-3 Routes of SN-38 transport and exposure to intestinal epithelial cells. SN-38 is transported into the bile following glucuronidation by liver UGT1A1 and extrahepatic UGT1A7. Following cleavage of luminal SN-38 glu-curonide (SN-38G) by bacterial b-glucuronidase, reabsorption into epithelial cells can occur by passive diffusion (indicated by the dashed arrows entering the cell) as well as by apical transporters. Movement into epithelial cells may also occur from the blood by basolateral transporters. Intestinal SN-38 can efflux into the lumen through P-glycoprotein (P-gp) and multidrug resistance protein 2 (MRP2) and into the blood via MRP1. Excessive accumulation of the SN-38 in intestinal epithelial cells, resulting from reduced glucuronidation, can lead to cellular damage and toxicity.

FIGURE 3-3 Routes of SN-38 transport and exposure to intestinal epithelial cells. SN-38 is transported into the bile following glucuronidation by liver UGT1A1 and extrahepatic UGT1A7. Following cleavage of luminal SN-38 glu-curonide (SN-38G) by bacterial b-glucuronidase, reabsorption into epithelial cells can occur by passive diffusion (indicated by the dashed arrows entering the cell) as well as by apical transporters. Movement into epithelial cells may also occur from the blood by basolateral transporters. Intestinal SN-38 can efflux into the lumen through P-glycoprotein (P-gp) and multidrug resistance protein 2 (MRP2) and into the blood via MRP1. Excessive accumulation of the SN-38 in intestinal epithelial cells, resulting from reduced glucuronidation, can lead to cellular damage and toxicity.

bacteria that are common in the lower GI tract; the result is the liberation of free drug into the intestinal lumen; free drug is transported by passive diffusion or through apical transporters back into the intestinal epithelial cells, and enters the portal circulation (Figure 3-3).

UGTIAI is of great importance in drug metabolism. For instance, the glucuronidation of bilirubin by UGTIAI is the rate-limiting step in assuring efficient bilirubin clearance; this rate can be affected by both genetic variation and competing substrates (drugs). Bilirubin is the breakdown product of heme, 80% of which originates from circulating hemoglobin and 20% from other heme-containing proteins such as the CYPs. Bilirubin must be metabolized further by glucuronidation to assure its elimination. The failure to efficiently metabolize bilirubin by glucuronidation leads to elevated serum levels (hyperbilirubinemia). There are more than 50 genetic lesions in the UGT1A1 gene that can lead to inherited unconjugated hyperbilirubinemia. Two UGTIAI deficiencies are Crigler-Najjar syndrome type I, diagnosed as a complete lack of bilirubin glucuronidation, and Crigler-Najjar syndrome type II, differentiated by the detection of low amounts of bilirubin glucuronides in duodenal secretions. These rare syndromes result from mutations in the UGT1A1 gene and the consequent production of little or no functional UGTIAI protein.

Gilbert's syndrome is a generally benign condition, present in up to I0% of the population, that is diagnosed clinically because circulating bilirubin levels are 60—70% higher than those in normal subjects. The most common genetic polymorphism associated with Gilbert's syndrome is a mutation in the UGT1A1 gene promoter, which leads to reduced expression of UGTIAI. Subjects with Gilbert's syndrome may be predisposed to adverse drug reactions resulting from a reduced capacity to metabolize drugs by UGTIAI. In these patients, there is competition for drug metabolism with bilirubin glucuronidation, resulting in pronounced hyperbilirubinemia as well as reduced formation of the glucuronide metabolites of drugs. Gilbert's syndrome alters patient responses to irinotecan. Irinotecan, a prodrug used in chemotherapy of solid tumors (see Chapter 5I), is metabolized to its active form SN-38 by serum carboxylesterases. SN-38, a potent topoisomerase inhibitor, is inactivated by UGTIAI and excreted in the bile (Figure 3—3). Once in the lumen of the intestine, the SN-38 glucuronide undergoes cleavage by bacterial [-glucuronidase and reenters the circulation through intestinal absorption. Elevated levels of SN-38 in the blood lead to hematological toxicities characterized by leukopenia and neutropenia, and damage to the intestinal epithelial cells, resulting in severe diarrhea. Patients with Gilbert's syndrome who are receiving irinotecan therapy are predisposed to hematological and GI toxicities resulting from elevated serum levels of SN-38, the net result of insufficient UGT1A activity and consequent accumulation of a toxic drug in the GI epithelium.

SULFATION The sulfotransferases (SULTs) are located in the cytosol and conjugate sulfate derived from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to the hydroxyl groups of aromatic and aliphatic compounds. In humans, 11 SULT isoforms have been identified. SULTs metabolize a wide variety of endogenous and exogenous substrates and play important roles in normal human homeostasis. For example, SULT1B1 is the predominant form expressed in skin and brain, carrying out sulfation of cholesterol and thyroid hormones; cholesterol sulfate is an essential regulator of keratinocyte differentiation and skin development. SULT1A3 is highly selective for cate-cholamines, while estrogens are sulfated by SULT1E1 and dehydroepiandrosterone (DHEA) is sulfated by SULT2A1; as a consequence, significant fractions of circulating catecholamines, estrogens, iodothyronines, and DHEA exist in the sulfated form.

The SULT1 family isoforms are the major SULT forms involved in drug metabolism, with SULT1A1 being the most important. SULT1C2 and SULT1C4 are expressed abundantly in fetal tissues and decline in abundance in adults; little is known about their substrate specificities. SULT1E catalyzes the sulfation of endogenous and exogenous steroids, and has been found localized in liver as well as in hormone-responsive or producing tissues such as the testis, breast, adrenal gland, and placenta.

Metabolism of drugs through sulfation often leads to the generation of chemically reactive metabolites, where the sulfate is electron withdrawing and may be heterolytically cleaved, leading to the formation of an electrophilic cation. Examples of the generation by sulfation of a carcinogenic or toxic response in mutagenicity assays occur with chemicals derived from the environment or from food mutagens generated from well-cooked meat. Thus, it is important to understand whether human SULT polymorphisms are associated with cancers related to environmental exposure. Since SULT1A1 is the most abundant form in human tissues and displays broad substrate specificity, the polymorphic profiles associated with this gene and the onset of various human cancers are of considerable interest.

GLUTATHIONE CONJUGATION The glutathione-S-transferases (GSTs) catalyze the transfer of glutathione to reactive electrophiles, a function that serves to protect cellular macro-molecules from interacting with electrophilic heteroatoms (-O, -N, and -S). The cosubstrate in the reaction is the tripeptide glutathione(g-glutamic acid, cysteine, and glycine (see Figure 3-4). Cellular glutathione may be oxidized (GSSG) or reduced (GSH), and the ratio of GSH:GSSG is critical in maintaining a cellular environment in the reduced state. In addition to affecting xenobiotic

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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