Influence of Ni(II)-Binding Ligands on the Cellular Uptake and Distribution of Ni^2+

Abstract

The effect on Ni^2+ uptake of human serum albumin (HSA), sodium diethyldithiocarbamate (DOC), D-penicillamine (O-Pen) , ethylenediaminetetra-acetic acid [di sodiurn salt] (EDTA) , L-aspartic acid (L-Asp) , L-Lysine (L-Lys) , and L-histidine (L-His) was examined in three cell lines: (1) human red blood cells (RBCs), (2) cultured human B-lymphoblasts and (3) rabbit alveolar macrophages. It was found that EDTA, L-His, HSA, and O-Pen were good inhibitors of ^63Ni^2+ uptake by cells and each was able to remove ^63Ni^2+ previously associated with the cells. In contrast L-Lys and L-Asp, which do not bind Ni^2+ well, were both poor inhibitors of Ni uptake and poor sequestering agents for cell-associated Ni^2 +. Thus it seems that at physiological concentrations , L-His and HSA play a major role in regulating the association of Ni^2+ with cells. DOC enhanced cellular uptake of ^63Ni^2+, but was not very effective in removing ^63Ni^2+ from cells. An increase in pH enhanced ^63Ni^2+ uptake in the lymphoblasts, macrophages and human peripheral lymphocytes. This dependence was interpreted to indicate the existence of either: (1) an increase in membrane permeability with an increase in pH; (2) the development of a proton gradient across the cell membrane favouring the antiport transport of H^+ and Ni^2+; or (3) Ni^2+;proton competition for cellular binding groups. The cellular uptake of Ni^2+ is interpreted in terms of an "equilbrium" model of metal-ion transport. It is concluded that since HSA and L-His can control cellular uptake and removal of Ni^2+, they may play a role in regulating the cellular toxicity of this ion. It was found that L-His and O-Pen acted similarly such that at various concentrations both inhibited cellular uptake of ^63Ni^2+ but did not change the normal distribution of Ni^2+ within the cell. Conversely, DOC enhanced Ni^2+ uptake by cells while simultaneously shifting the distribution of Ni^2+ from the cell lysate to the cellular membranous pellet. Furthermore, DOC caused Ni^2+ to become more lipophilic as shown by the increase of ^63Ni^2+ in a chloroform extract. DOC also caused a change in Ni^2+ distribution in whole blood by enhancing Ni^2+ association with RBCs and lymphocytes and decreasing serum-associated Ni^2+. The different responses produced by O-Pen, L-His and DOC are ascribed to the hydrophilicity of the [Ni(D-Pen)_2]^2- and Ni(His)_2 complexes and the lipophilicity of the Ni(DDC)_2 complex, and allow a rationalization of the contrasting therapeutic effects of O-Pen and DDC.