[Frontiers in Bioscience 2, d427-437, September 15, 1997]
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MULTIPLE TRANSPORT PROTEINS INVOLVED IN THE DETOXIFICATION OF ENDO- AND XENOBIOTICS 

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

4. FUTURE DIRECTIONS

Recent advances in molecular techniques have led to the identification of a number of genes for transporter proteins belonging to the ABC family both in prokaryotes and eukaryotes (89-92). These transporters are distinct from other ATP binding proteins such as inorganic cation pumps, and may be involved in active transport of structurally divergent xeno- and endobiotics. According to a recent study utilizing the expressed sequence tags database, the number of genes identified in the ABC gene family in humans now stands at 33 (92). Whether or not all these genes correspond to functional transporters of xenobiotics and are relevant to detoxification mechanisms remains to be determined. Whereas the primary structure of these transporters can be determined by approaches such as those described in (92), the usefulness of functional predictions based on sequence comparisons is limited. Transfection of cultured cells with cDNAs yields important functional information, but is not free from uncertainties, as discussed previously. It is therefore likely that a full functional characterization of the newly identified ABC transporters can be obtained only through a combination of molecular and transfection techniques with classical approaches of studying their catalytic and kinetic properties in isolated reconstituted systems. Recombinant proteins, especially expressed in bacterial systems, could be unsuitable for reconstitution studies because of posttranslational modifications that may be required for function. The usefulness of recombinant proteins needs therefore to be validated in each case, and in some situations fully functional, mature proteins can perhaps be obtained only by their purification from tissues. At present, concerted efforts are being made to characterize transporter proteins through molecular cloning, determination of mechanisms regulating their expression, and development of transgenic animals. These approaches were undoubtedly instrumental in an unparalleled advance in our knowledge of transport proteins. However, the molecular approaches have overshadowed the need for an understanding of the precise kinetic properties of these transporters. The latter information can be obtained by studies of reconstituted model systems obtained with purified transporters in the absence of interfering activities (93). Such knowledge, when coupled with information gained from cloning approaches, will synergistically enhance our understanding on the physiological roles of this important group of proteins.