Mechanism for the decrease in the accumulation of cadmium (Cd) in Cd-resistant Chinese hamster V79 cells

Abstract

The mechanism for the decrease in the accumulation of cadmium (Cd) in Cd-resistant Chinese hamster V79 (Cd^r) cells in culture was investigated in a comparison with Cd-sensitive (Cd^s) cells. Both Cd^r and Cd^s cells took up Cd in a time-dependent manner but the rate of uptake of Cd by Cd^r cells was about 15% of that by Cd^s cells. Kinetic studies of the uptake of Cd showed that the V_max values for Cd^r and Cd^s cells were 0.31 and 0.46 pmol Cd/h per mg protein, respectively. The K_m values were 31.95 μM for Cd^r cells and 3.15 μM for Cd^s cells. Mersalyl acid, a sulfhydryl (SH) blocker to which cells are impermeable, inhibited the uptake of Cd by Cd^s cells at subtoxic concentrations while Cd^r cells were insensitive to inhibition by mersalyl acid, suggesting that SH groups in the plasma membrane play a role in the uptake of Cd. Uptake of Cd by Cd^s cells was dependent on the pH of the incubation medium and the rate of uptake was very high at pH 7.4 and pH 8.0 relative to the rates at pH 6.0 and pH 6.8. By contrast, the uptake of Cd by Cd^r cells was lower at all pH values than that by Cd^s cells. The decrease in the rate of uptake of Cd by Cd^r cells could not be ascribed to an increase in the efflux of Cd. A Cd-blotting technique was used to detect plasma membrane proteins with high affinity for Cd. Two major differences in terms of Cd-binding proteins (Cd-BPs) were observed between Cd^r and Cd^s cells. A 110-kDa Cd-BP, detected in Cd^s cells, was found at a reduced level in Cd^r cells, while an 82-kDa Cd-BP, which was not observed in Cd^s cells, was detected in Cd^r cells.