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Kidney zinc-thionein regulation of delta-aminolevulinic acid dehydratase inhibition by lead.

This study was undertaken to evaluate the ability of kidney Zn-thionein to regulate Zn availability to the Zn-dependent enzyme, delta-aminolevulinic acid dehydratase (ALAD), and mediate the effect of Pb on this enzyme. Male CD rats were pretreated with 200 mumol Zn/kg, sc, 48 and 24 h prior to assay of renal ALAD, which resulted in activation of renal ALAD and increased the resistance of this enzyme to inhibition by Pb in vitro. To determine the mechanism for this resistance, binding patterns of Zn and Pb in kidney cytosol were assessed. Rats were pretreated with Zn 48 and 24 h prior to injection of 203Pb (170 microCi/kg, ip). Kidneys were removed 4 h later and cytosol was fractionated on a Sephadex G-75 gel filtration column. Both 203Pb and Zn coeluted with the Zn-thionein fraction. Zn-thionein-I and -II, purified previously by DEAE anion-exchange chromatography, bound 203Pb in vitro. In another experiment, addition of purified Zn-thionein to reaction mixtures increased activity of purified bovine liver ALAD twofold and reversed inhibition of ALAD by Pb. Addition of apo-thionein to reaction mixtures partially prevented the inhibition of purified ALAD by Pb, indicating the biological significance of 203Pb chelation. Gel filtration of ALAD assay incubates containing 65Zn-thionein demonstrated that Zn is transferred from Zn-thionein to ALAD. Gel filtration of incubates containing 203Pb demonstrated that the presence of Zn-thionein alters the cytosolic binding pattern of Pb, with less bound to ALAD and more bound to Zn-thionein. The results demonstrate a dual function for Zn-thionein in mediating Pb inhibition of ALAD by a mechanism involving both donation of Zn to this Zn-requiring enzyme and chelation of Pb. These results also suggest that Zn-thionein may serve to regulate ALAD activity in vivo and mediate the inhibition of this enzyme by Pb.