MECHANISM OF RENAL LEAD-BINDING PROTEIN REVERSAL OF DELTA-AMINOLEVULINIC-ACID DEHYDRATASE INHIBITION BY LEAD

  • 1 January 1985
    • journal article
    • research article
    • Vol. 234  (2) , 365-371
Abstract
The bioavailability of Pb in kidney is mediated in part by binding to endogenous high-affinity cytosolic lead-binding proteins (PbBP), which are not detectable in liver. Addition of semipurified 11,500 dalton PbBP to liver .delta.-aminolevulinic acid dehydratase (ALAD) reaction mixtures reverses inhibition of this enzyme by lead and thus provides an explanation for the relative insensitivity of renal ALAD to lead inhibition in vivo and in vitro. This effect results in part from a marked increase in binding of 203Pb to the PbBP relative to control liver cytosol (no PbBP) as demonstrated by Sephadex G-150 gel filtration chromatography. Zn is known to activate ALAD and is an endogenous component of the PbBP fraction (6 .mu.M in reaction mixtures). Zn activated hepatic and renal ALAD over a range of 1.5-50 .mu.M and also reversed the IC50 [mean inhibitory concentration] Pb-inhibited activity. Studies of Zn release and/or displacement from PbBP under ALAD assay conditions (37.degree. C, + glutathione, pH 6.8) were conducted utilizing Sephadex G-25 chromatography. Fifteen to twenty-five percent of the Zn in the PBBP fraction was released, and this value was not markedly influenced by addition of IC50 lead, temperature (4.degree. C) or absence of glutathione; Zn release was primarily dependent upon the pH of the reaction mixture. Apparently, the PbBP fraction attenuates lead inhibition of ALAD in vitro both by chelating PB and apparently serving as a Zn donor for this enzyme under optimal conditions of the ALAD assay.