[Frontiers in Bioscience 2, d427-437, September 15, 1997]

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Yogesh C. Awasthi1,2, Sanjay Awasthi3, and Piotr Zimniak4

Departments of 2Human Biological Chemistry & Genetics and 3Internal Medicine, University of Texas Medical Branch, Galveston, Texas; and 4Department of Internal Medicine and Biochemistry & Molecular Biology, University of Arkansas for Medical Sciences, and McClellan VA Hospital, Little Rock, Arkansas

Received 9/3/97 Accepted 9/8/97


Living organisms defend themselves from the toxicants present in the environment through biotransformation of these compounds to relatively non-toxic metabolites and their subsequent elimination through transport. Most cells are equipped with a multitude of phase I and phase II biotransforming enzymes (1). In phase I, reactive groups such as -OH, -NH2, or >O are introduced/exposed on relatively hydrophobic xenobiotics so that they can be conjugated to hydrophilic-compounds such as GSH, glucuronate, sulfate, etc. by the phase II enzymes, and the resultant products (usually less toxic, more hydrophilic) can be excreted through active/facilitated transport processes across the cellular membrane. Likewise, electrophiles (both hydrophobic and water soluble) can be conjugated to the abundant nucleophiles such as GSH by phase II enzymes, and the conjugation products can be transported out of cells. Whereas extensive studies have been conducted on biotransformation enzymes resulting in the identification of numerous isozymes belonging to the superfamilies of phase I (2,3) and phase II enzymes (4-8) in mammalian tissues including humans, much less information is available on the enzymes which are involved in the active transport of xenobiotics and/or their metabolites. A concept that the transport of xenobiotic metabolites may be considered as phase III of the detoxification mechanisms has been propagated recently (9-11).

Cells resist chemical aggression of environmental toxins by, 1) warding off the aggression through repulsion by force, 2) capturing or converting the toxins into relatively harmless entities and, 3) expulsion of the converted entities in order to avoid any possible long term harmful effects. In general, all cells are equipped with defense mechanisms involving these three discrete steps. Bacteria and parasites are known to defend against toxins by exclusion mechanisms as exemplified by the acquisition of drug resistance by the malaria parasite, Plasmodium falciparum (12). In humans, the significance of these exclusion mechanisms was not fully recognized until the relatively recent discoveries of drug efflux pumps, including the P-glycoproteins (Pgp) and multidrug resistance associated proteins (MRP), which are overexpressed in multidrug-resistant cancer cells (13,14). Capturing of toxicants is carried out in cells through their binding to certain abundant proteins such as albumin and ligandin (15, review) which trap and often inactivate these compounds, while their conversion to relatively harmless molecules is catalyzed by a multitude of phase I (2,3) and phase II (4-8,15-18) enzymes. The efflux of the metabolites of xenobiotics is carried out by plasma membrane transporter proteins, thus ensuring a safe environment for the cellular components. As expected, all these steps require energy. The drug exclusion pumps require ATP while phase I reactions require the reducing equivalents of NADPH. Some of the phase II enzymes (e.g. glucuronosyl and sulfotransferases) require energy for the activation of substrates, while others, such as glutathione (GSH) S-transferases (GST), require energy for the synthesis of GSH. Phase I and phase II enzymes are often induced by the invading chemicals. Overexpression of the drug efflux pumps, Pgp (13,19) and MRP (14,20), in cancer cells exposed to gradually increasing drug concentrations suggests that the transporters are also induced by xenobiotics. When the toxicants evade or overwhelm the cellular defense mechanisms, they cause toxicity which may eventually result in cell death.

As pointed out above, studies on cellular defense mechanisms have largely focused on phase I and phase II biotransforming enzymes. Multiple forms of these enzymes (e.g. CYP450s, glutathione S-transferases, UDP-glucuronosyltransferases, and sulfotransferases) are expressed in mammals in a tissue-specific manner. The structural and functional properties of these enzymes have been extensively studied. In particular, a large number of isozymes belonging to the gene superfamilies of CYP450s (2,3) and GSTs (4,5,8,15-18) have been well characterized. The transport mechanisms which flank the biotransformation processes in the simplified description of cellular defense mechanisms given above are rather poorly understood at present. The importance of the transport mechanisms in cellular defense against xenobiotics has been underscored by the discoveries of drug transporters overexpressed in multidrug resistant cancer cells. These drug efflux pumps have been covered in detail in several excellent review articles (13,19,20). In this mini-review, our current understanding of the structure and function of transport mechanisms for xenobiotics and their metabolites in mammalian tissues including humans is summarized.