Abstract:To determine if catechol-O-methyltransferase (COMT) metabolizes catecholamines within cell lines used for heterologous expression of plasmalemmal transporters and alters the measured characteristics of 3H-substrate transport, the uptake of monoamine transporter substrates was assessed in three cell lines (C6 glioma, L-M fibroblast, and HEK293 cells) that had been transfected with the recombinant human transporters. Uptake and cellular retention of 3H-catecholamines was increased by up to fourfold by two COMT inhibitors, tropolone and Ro 41-0960, with potencies similar to those for inhibition of COMT activity, whereas the uptake of two transporter substrates that are not substrates for COMT, [3H]serotoninand [3H]MPP~, was unaffected. Direct measurement of monoamine substrates by HPLC confirmed that tropolone (1 mM) increased the retention of the catecholamines dopamine and norepinephrine, but not the retention of serotonin in HEK293 cells. Saturation analysis of the uptake of [3H]dopamineby C6 cells expressing the dopamine transporter demonstrated that tropolone (1 mM)decreased the apparent Km of transport from 0.61 biM to 0.34~iMwithout significantly altering the maximal velocity of transport. These data suggest that endogenous COMT activity in mammalian cells may alter neurotransmitter deposition and thus the apparent kinetic characteristics of transport. Key Words: Dopamine transporter -cDNA -Catechol -O -methyltransferase-Tropolone. J. Neurochem. 69, 1459-1466 (1997).We (Eshleman et al., 1994(Eshleman et al., , 1995 and others (Kitty et al., 1991;Shimada et at., 1991;Giros et at., 1992;Pristupa et al., 1994; Watt et at., 1995) have ctoned and expressed the dopamine (DA) transporter in several cell lines for the purposes of characterizing transporter-mediated uptake and release of DA, and differentiating between the interactions of abused and therapeutic drugs with the DA transporter. The issue of the appropriate cettutar environment for expression of the DA transporter is particutarty retevant because of the observation that DA is apparently taken up tess avidly by the recombinant DA transporter (Km