Glutathione S-conjugate transport by cultured human cells

Abstract

Elimination of the products of xenobiotic metabolism is an important step in cellular detoxification and involves a specific transport system or “export pump”. ATP-dependent transport of glutathione S-conjugates has previously been demonstrated in a variety of tissues, mainly from rat. However, the characteristics of this pump have not been fully explored in human cells. This study investigated transport of a glutathione S-conjugate, 2,4-dinitrophenyl glutathione (GS-DNP), by a variety of cultured human cell lines. GS-DNP was generated intracellularly after treatment of cells with 1-chloro-2,4-dinitrobenzene and subsequent transport of the conjugate into the extracellular medium was measured spectrophotometrically at 340 nm. Calculation of the initial transport rates at 37°C revealed considerable variation in GS-DNP secretion between cell lines which was statistically significant in some cases. A 2-fold increase in GS-DNP efflux was observed between Jurkat and HL-60 cells (11.360 ± 3.893 vs. 5.662 ± 2.263 nmol/10 6 cells/h, P < 0.007). The highest rate of transport was found in HepG2 cells (14.171 ± 4.790 nmol/10 6 cells/h) whereas the 5637 cell line had the lowest level with a transport rate of 1.475 ± 0.631 nmol/10 6 cells/h. For each cell line, transport of GS-DNP was almost totally inhibited or markedly reduced on ice. Pre-incubation of cells at 42°C also lowered the initial transport rates compared with cells maintained at 37°C but these were not significantly different except in the case of HeLa cells. ATP levels ranged from 30.5 to 89.3 nmol/mg protein and there was variation in the glutathione content and glutathione S-transferase activities of the cells. This report demonstrates firstly that transport of glutathione conjugates is a feature of many cell types in vitro and secondly that the basal levels of GS-DNP secretion vary significantly between human cells.