Drug-metabolizing enzymes sulfotransferases

Busby WF, Ackermann JM, Crespi CL (1999) Effect of methanol, ethanol, dimethyl sulfoxide, and acetonitrile on in vitro activities of cDNA-expressed human cytochromes P-450. Drug Metab Dispos 27 246-249 Clohs L, Wong J (2002) Validation of a capillary electrophoresis assay for assessing the metabolic stability of verapamil in human liver microsomes. J Cap Elec Microchip Tech 7 113 Coughtrie MWH, Fisher MB (2003) The role of sulfotransferase and UDP-glucuronosyltransferases. In Lee JS, Obach RS, Fisher MB (eds) Drug Metabolizing Enzymes. Marcel Dekker, New York pp 541-575... 

Table 4 Drug metabolizing enzyme (P450 isoforms (CYP), UDP-glucuronosyl transferase (UGT) and phenol sulfotransferase (PST)) substrates that can be used for the in situ measurement of activity in the human hepatocyte enzyme induction assay.

Human CYPs involved in metabolism can be easily characterized because of the tools that are commercially available. However, similar tools are not available for other drug-metabolizing enzymes like FMOs, UGTs, and sulfotransferases. There are no specific inhibitors or monoclonal anitibodies identified for any of the commercially available expressed UGTs or sulfotransferases. Due to the lack of necessary tools it is difficult to quantitatively assess their contribution to the overall clearance of drugs that are metabolized by these enzymes. 

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. 

For exogenous compounds such as drugs, various enzymes involved in both phase I and phase II metabolic routes are present, e.g. various isoforms of cytochrome p450, cytochrome b5, glucuronyl transferase and sulfotransferase [15]. 

Since liver is the most important organ for metabolism investigations the procedures described here focus on liver cytosol exemplarily. Liver cytosol fraction contains soluble Phase I and Phase II enzymes which play an important role in drug metabolism (Brandon 2003). These are alcohol and aldehyde dehydrogenases, epoxide hydrolases, sulfotransferases, glutathione S transferase, N-acetyl transferases, and methyl transferases. Therefore, in cytosolic preparations these biotransformation steps can be studied. Cytosolic fractions are commercially available (BDGentest, Invitro Technologies, Xenotech and others) or easy to prepare, alternatively. 

Age. The pharmacokinetic and pharmacodynamic effects of a drug can be influenced by age, and drug metabolism plays an important role in understanding the differences observed. Differences between the levels of metabolism enzymes for the fetal and neonatal (first 4 weeks postpartum) liver versus the adult liver have been observed in both animal and human studies (125). At birth, total CYP levels are approximately 30%of adult levels and glucuronidation activity is at 10-30% of adult levels. Interestingly, sulfotransferase activity in neonates seems to be comparable with that in adults. 

Sulfotransferases (SULTs) are important for the metabolism of a number of drugs, neurotransmitters, and hormones, especially the steroid hormones. The cosubstrate for these reactions is 3 -phosphoadenosine 5 -phosphosulfate (PAPS) (Fig. 4.1). Like the aforementioned enzymes, sulfate conjugation typically renders the compound inactive and more water soluble. However, this process can also result in the activation of certain compounds, such as the antihypertensive minoxidil and several of the steroid hormones. Seven SULT isoforms identified in humans, including SULTs lAl to 1A3, possess activity toward phenolic substrates such as dopamine, estradiol, and acetaminophen. SULTIBI possesses activity toward such endogenous substrates as dopamine and triiodothyronine. SULTIEI has substantial activity toward steroid hormones, especially estradiol and dehydroepiandrosterone, and toward the anti-... 

---