LEAD-BINDING PROTEINS IN BRAIN-TISSUE OF ENVIRONMENTALLY LEAD-EXPOSEDHUMANS

This study reports the partial purification and characterization of cy tosolic lead binding proteins (PbBPs) in human brain tissue of environ mentally Pb-exposed subjects. The isolated proteins were initially cha racterized based upon the presence of endogenously associated Pb. Foll owing partial purification (Sephadex G-75 and A-25 DEAE anion-exchange chromatography), the isolated PbBPs (contained within a single DEAE p eak) showed a single class of high affinity binding sites with an appa rent K-d Of 10(-9) M, based upon competition assays using radioactive Pb-203 and Hill and Scatchard analysis. The presence of endogenously b ound Pb with the isolated proteins indicated the association of Pb wit h the protein(s) in vivo in these environmentally Pb-exposed subjects, since the samples were prepared in an ultraclean lead analysis labora tory. Moreover, the persistence of Pb-protein binding throughout the i nitial two steps (Sephadex G-75 and A-25 DEAE) of the purification sch eme is consistent with the high affinity and stability of binding meas ured with the radiolead competition assays. The DEAE isolated PbBPs we re further purified by denaturing reversed-phase HPLC analysis, result ing in the isolation of two proteins, thymosin beta(4) (5 kDa, pi 5.1) and a second as yet unidentified protein with an approximate molecula r mass of 20 kDa and a pi of 5.9. Qualitative Pb-203-binding analysis of these HPLC purified proteins suggested that they may be primarily r esponsible for the observed Pb binding in the single DEAL peak. Nearly identical results were obtained in brain cytosols from male and femal e, and young and adult individuals, although further quantitative anal yses are needed to investigate possible sex and age relationships. The se data are significant because they contribute to a better understand ing of the presence of PbBPs in a sensitive target organ for Pb toxici ty in humans, suggesting a possible role of these or similar proteins as sensitive biomarkers of Pb exposure and toxicity.