HIGH-AFFINITY LEAD BINDING-PROTEINS IN RAT-KIDNEY CYTOSOL MEDIATE CELL-FREE NUCLEAR TRANSLOCATION OF LEAD

  • 1 January 1985
    • journal article
    • research article
    • Vol. 232  (2) , 462-469
Abstract
Pb2+-binding proteins of rat kidney cytosol were investigated. Saturation and Scatchard analysis of 203Pb binding in whole cytosol and in 40% saturated H8N2O4S precipitated fractions disclosed a class of relatively high-affinity sites with an apparent Kd of approximately 50 nM and binding capacities of approximately 41 and 9 pmol/mg of protein, respectively. 203Pb binding proteins (2) with approximate MW of 63K [kilodaltons] and 11.5K daltons and a high MW component (> 200K) were isolated by Sepharose-6B column chromatography. The time course of association of 203Pb with cytosol and the 63K protein showed maximum binding at 18 h which was stable up to 25 h at 4.degree. C. The approximate half-time dissociation rate (T1/2) of specifically bound 203Pb to the 63K protein was 100 min at 4.degree. C whereas the 11.5K protein showed little dissociation of specifically bound ligand at this temperature. Saturation analysis of the 3 isolated proteins disclosed low capacity, high-affinity sites with similar apparent Kd values to the cytosol assay. Sucrose density gradient analysis of kidney cytosol showed approximate sedimentation coefficients of 2S, 4.6S and 7S for the 11.5K, 63K and the high MW proteins, respectively. Competitive binding studies with cytosol demonstrated displacement of 203Pb by Pb2+, Cd2+ and Zn2+ ions but not Ca2+ ions. Nuclear uptake studies showed both time- and temperature-dependent uptake of 203Pb from kidney cytosol and KCl extraction data indicated that these high-affinity lead binding proteins which act as the initial cytosolic Pb2+ ligands for Pb2+ in the kidney are capable of mediating the nuclear uptake of Pb2+. This may explain the kidney''s high susceptibility to the toxic effects of Pb.