Molecular mechanisms which regulate the intracellular bioavailability of metals such as lead and cadmium have been studied. High affinity cytosolic lead-binding proteins (PbBP) of 63,000 (63K) and 11,500 (11.5K) daltons from kidneys of rats have been partially purified by gel and anion exchange chromatography, electrophoresis, and sucrose density gradient analysis. These molecules were found to exhibit dissociation constants (Kd) for lead of 10minus8M. Competitive binding studies on sucrose density gradients with cytosol showed displacement of 203Pb by Pb11, Cd11, and Zn11 ions but not Ca11 ions. Cell-free nuclear translocation studies showed both time- and temperature-dependent uptake. Cd11 and Zn11 ions effectively blocked the nuclear uptake of 203Pb. In vivo Pb-injection studies showed a close temporal relationship between formation and loss of Pb intranuclear inclusions in renal proximal tubule cell nuclei and marked changes in renal 2-D gel 35S-labelled protein synthesis patterns. The data indicate that these high affinity PbBP, which act as the initial cytosolic ligands for Pb in the kidney, are capable of mediating the intranuclear translocation of Pb and that the presence of Pb within renal nuclei is temporally associated with marked changes in renal gene expression. The 11.5K dalton protein, but not the 63K protein, was also found to regulate the inhibitory effects of Pb on the heme biosynthetic pathway enzyme -aminolevelinic acid dehydratase (ALAD). Reversal of Pb-induced inhibition of hepatic ALAD activity was dependent on the concentration of ll.5K dalton PbBP added to the reaction mixture. The data indicate that the ll.5K dalton protein confers partial resistance to Pb inhibition of liver ALAD in vitro and suggests a similar role for this protein in kidney with respect to the resistance of renal ALAD to Pb inhibition. PbBP chelation of Pb and donation of Zn to the ALAD were found to be the mechanisms of this effect.
