Sulfotransferase drug metabolism studies

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. 

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. 

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