Coupling of conjugating enzymes and efflux transporters: Impact on bioavailability and drug interactions

Conjugating enzymes are traditionally recognized as one of the major biological barriers to the entry of xenobiotics/drugs into systemic circulation and represent one of the main pathways for their elimination. Similar to drugs that undergo extensive phase I metabolism, drugs that undergo extensive conjugation have poor bioavailability and are more prone to metabolism-based drug interactions. Previously, enterohepatic recycling is used to explain why certain xenobiotics have half-lives that are much longer than expected from intravenous injection studies. In addition, changes in expression levels of metabolic enzymes due to chemical induction or suppression are often recognized as the source of drug interaction or toxicity of pollutants and carcinogens. These traditional approaches, whereas yielding highly valuable information, fail to recognize the fact that many conjugates (especially hydrophilic ones) cannot permeate the cell membrane. In the present review, we will focus on the coupling process that involves both conjugating enzymes and efflux transporters. We will briefly review conjugating enzymes capable of producing highly hydrophilic metabolic products. The other focus of this review is on various transporters capable of moving negatively charged hydrophilic conjugates across the cellular membrane. Evidence will support the hypothesis that efficient coupling of the conjugating enzymes and efflux transporters enables enterohepatic recycling and enteric recycling processes. Termed as a "revolving door" theory, the hypothesis focuses on the role played by efflux transporter capable of modulating the cellular excretion of hydrophilic metabolites. Coupling process in intestine, liver and kidney will be discussed with an emphasis on the intestinal coupling process, since we have just begun to understand it. Biological consequence and new insights into how coupling process can impact bioavailability of xenobiotics, biological functions of drugs and carcinogens, and drug interactions will be discussed.