Sulfotransferases

Sulfotransferases (SULTs) are cytosolic phase II detoxification enzymes involved in sulfonation of various xenobiotics and endobiotics. There are also membrane-bound SULTs that are not involved in phase II metabolism but are involved in the sulfonation of proteins and polysaccharides. Substrates of cytosolic SULTs include alcohols (ethanol, 2-butanol, cholesterol, bile acids), phenols (phenol, naphthol, acetaminophen), aromatic amines and hydroxyamines (2-naphthylamine, A-hydroxy 2-naphthylamine). SULTs transfer sulfonate (S03) to a hydroxy or amino group of a substrates from the cofactor 3 -phosphoadenosine-5 -phosphosulfate (PAPS), generating highly water-soluble metabolites for elimination through the kidney and liver. 

The enterocyte expresses many of the metabolic enzymes that are expressed in the liver. These include UDP-glucuronyltransferases, sulfotransferases, esterases and cytochromes P450. 

UDP-glucuronyltransferases catalyze the addition of glucuronic acid onto phenol, hydroxyl and carboxylic acid functions of molecules. They are expressed in many tissues of the body, including the liver and intestine [2-5], Microsomes from human intestines have been shown to metabolize UDP-glucuronyltransferase substrates including p-nitrophenol [6], 1-naphthol, morphine, and ethinylestradiol [4]. The relative rates of metabolism of these substrates in liver and intestinal microsomes are shown in Table 13.1. 

Overall, the human intestine is capable of metabolizing UDP-glucuronyltransferase substrates, although the rates of metabolism are between 5- and 10-fold lower than those observed in human liver microsomes. However, the presence of a metabolic capacity towards UDP-glucuronyltransferase substrates at the level of the enterocyte can exert a significant gut wall first-pass extraction on oral administration. 

Sulfotransferases are cytosolic enzymes that add sulfate to phenolic and hydroxyl functions as well as certain amines. The relative rates of sulfation of several sulfo- 

Overall, the rates of sulfation in the intestine tend to be higher than the liver (up to 10-fold). In addition, for (+)-terbutaline, the relative rates of metabolism are reduced at sites lower down the GIT (i.e., activity values of 1195, 415 and 268 pmol min-1 mg 1 in the duodenum, ileum and colon, respectively). Thus, the intestine represents a considerable barrier to the oral bioavailability of sulfotrans-ferase substrate drugs. 

Sulfonation is a common Phase 11 conjugation reaction that occurs across species. Numerous endobiotics as well as xenobiotics are sulfonated in the human body. This process of sulfonation was discovered by Baumann with the 

FIGURE 3.5 The overall sulfonation reaction, and formation of the cosubstrate PAPS. PAPSS PAPS synthetase— fused bifunctional enzyme, varients PAPSSl and PAPSS2 found in humans. 

FIGURE 3.6 Common SULT substrates. Arrows indicate site of suUbnation for each substrate. 

Sulfonation is generally a detoxification pathway whereby the conjugated product has greater water solubility and is therefore excreted more readily from the body. However, several chemicals have been shown to form mutagenic and carcinogenic reactive electrophiles upon sulfonation (Glatt, 2000). Additionally, it is the sulfonated forms of minoxidil and cholecystokinin that elicit biologic activity (Weinshilboum et al., 1997). 

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The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

Sulfotransferases catalyze the transfer of sulfate from PAPS to wide-range xenobiotics that possess hydroxyl groups. Steroid alcohols are among the endogenous substrates. The sulfotransferases exist in different forms. 

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

Dajani R, Cleasby A, Neu M, Wonacott AJ, Jhoti H, Hood AM, et al. X-ray crystal structure of human dopamine sulfotransferase, SULT1A3. J Biol Chem 1999 53 37862-8. 

Lee KA, Fuda H, Lee YC, Negishi M, Strott CA, Pedersen LC. Crystal structure of human cholesterol sulfotransferase (SULT2Blb) in the presence of pregnenolone and 3 -phosphoadenosine 5 -phosphate. Rationale for specificity differences between prototypical SULT2A1 and the SULT2BG1 isoforms. I Biol Chem 2003 278 44593-9. 

King RS, Sharma V, Pedersen LC, Kakuta Y, Negishi M, Duffel MW. Structure-function modeling of the interactions of the A-alkyl-A-hydroxyanilines with rat hepatic aryl sulfotransferase IV. Chem Res Toxicol 2000 13 1251-8. 

Bidwell LM, McManus ME, Gaedigk A, Kakuta Y, Negishi M, Pedersen L, et al. Crystal structure of human catecholamine sulfotransferase. / Mol Biol 1999 293 521-30. 

HARRIS R M, WARING R H, KIRK c J, HUGHES p J (2000) Sulfation of oestrogenic alkylphenols and 17beta-oestradiol by human platelet phenol sulfotransferases. J Biol Chem. 275 159-66. 

Tosylate derivatives are not usually found in nature, but sulfate derivatives of alcohols are common (52). They are formed by the reaction of an alcohol with sulfate, catalyzed by a sulfotransferase enzyme. 

Seibert C, Cadene M, Sanhz A, Chait BT, Sakmar TP. Tyrosine sulfation of CCR5 N-terminal peptide by tyrosylprotein sulfotransferases 1 and 2 follows a discrete pattern and temporal sequence. Proc Natl Acad Sci U S A 2002 99(17) 11031— 11036. 

Easterbrook, J., Liu, C., Sakai, Y. and Li, A.P. (2001) Effects of organic solvents on the activities of cytochrome P450 isoforms, UDP-dependent glucuronyl transferase, and phenol sulfotransferase in human hepatocytes. Drug Metabolism and Disposition The Biological Fate of Chemicals, 29, 141-144. 

From the above, it is clear that the gut wall represents more than just a physical barrier to oral drug absorption. In addition to the requirement to permeate the membrane of the enterocyte, the drug must avoid metabolism by the enzymes present in the gut wall cell as well as counter-absorptive efflux by transport proteins in the gut wall cell membrane. Metabolic enzymes expressed by the enterocyte include the cytochrome P450, glucuronyltransferases, sulfotransferases and esterases. The levels of expression of these enzymes in the small intestine can approach that of the liver. The most well-studied efflux transporter expressed by the enterocyte is P-gp. 

Quinone oxidoreductase, Sulfotransferase Drug metabolism 6 out of 22 coding region SNPs gave amino acid substitutions 15... 

Iida A, Sekine A, Saito S, Kitamura Y, Kitamoto T, Osawa S, Mishima C, Nakamura Y. Catalog of 320 single nucleotide polymorphisms (SNPs) in 20 qui-none oxidoreductase and sulfotransferase genes. J Hum Genet 2001 46 225-240. 

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