Zinc transport into endothelial cells is a facilitated process

The kinetics of zinc transport were examined by measuring the uptake of 65Zn into cultured endothelial cells. This served as a suitable model for characterizing the transport of zinc across a biological membrane (i.e., the plasma membrane). The transport process was saturable under physiological conditions, which indicates a facilitating transport mechanism. Within the physiological range of zinc concentrations, the maximum zinc transport rate was 27 pmoles zinc/(min x mg protein) and it was half maximal at 4.1 microM zinc. Cadmium competitively inhibited zinc transport (Ki = 6.5 microM), while equimolar concentrations of copper and manganese were ineffectual. The rate of zinc transport was substantially reduced at lower temperatures and in the presence of sulfhydryl blockers (sodium iodoacetate and N-ethylmaleimide). Inhibitors of energy metabolism (2,4-dinitrophenol and sodium azide) failed to disrupt zinc transport. These results demonstrate that zinc transport into endothelial cells is a facilitated process (i.e., it is carrier mediated and energy-independent).