Effect of sulfhydryl reagents on nucleoside transport in cultured mammalian cells

Incubation of Novikoff rat hepatoma cells; mouse L929, P388 and L1210 cells; and Chinese hamster ovary cells with sulfhydryl reagents, such as p-hydroxymercuribenzoate or p-hydroxymercuribenzenesulfonate, reduced the zero-trans influx of uridine in a concentration-dependent manner. The sensitivity of uridine transport to inhibition varied somewhat for the cell lines, Chinese hamster ovary cells being the most sensitive. Maximum inhibition by p-hydroxymercuribenzoate occurred in 10–20 min of incubation at 37 °C, and was associated with a decrease in maximum transport velocity without significant change in substrate affinity of the carrier. The development of inhibition of uridine influx correlated with binding of ¹⁴C]p-hydroxymercuribenzoate to the cells. Inhibition of transport also roughly correlated with a decreased binding of 6-nitrobenzylthioinosine to high-affinity binding sites on the cells (presumably representing the nucleoside transporter) without affecting binding affinity. Treatment of cells with p-hydroxymercuribenzenesulfonate reduced uridine influx and efflux to a similar extent. Inhibition of uridine transport and binding of ¹⁴C]p-hydroxymercuribenzoate were readily reversed by incubation of the cells with dithiothreitol. The results indicate that sulfhydryl groups are essential for the functioning of the nucleoside transporter, perhaps for the binding of substrate. Blockage of the sulfhydryl groups results in a reversible inactivation of the carrier. Treatment of the cells with the sulfhydryl reagents also caused a concentration-dependent increase in cell volume, which was readily reversed by incubation of the cells with dithiothreitol but seemed unrelated to the inhibition of nucleoside transport.