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

The mechanism for the decrease in the accumulation of cadmium (Cd) in Cd-resistant Chinese hamster V79 (Cdr) cells in culture was investigated in a comparison with Cd-sensitive (Cds) cells. Both Cdr and Cds cells took up Cd in a time-dependent manner but the rate of uptake of Cd by Cdr cells was about 15% of that by Cds cells. Kinetic studies of the uptake of Cd showed that the Vmax values for Cdr and Cds cells were 0.31 and 0.46 pmol Cd/h per mg protein, respectively. The Km values were 31.95 microM for Cdr cells and 3.15 microM for Cds cells. Mersalyl acid, a sulfhydryl (SH) blocker to which cells are impermeable, inhibited the uptake of Cd by Cds cells at subtoxic concentrations while Cdr 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 Cds 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 Cdr cells was lower at all pH values than that by Cds cells. The decrease in the rate of uptake of Cd by Cdr 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 Cdr and Cds cells. A 110-kDa Cd-BP, detected in Cds cells, was found at a reduced level in Cdr cells, while an 82-kDa Cd-BP, which was not observed in Cds cells, was detected in Cdr cells.