Administration of a sublethal dose of lead acetate to rats induced the simultaneous synthesis of a Pb-metallothionein (Pb-MT)-like protein (Pb-BP) and Zn-thionein (Zn-BP) in the liver. The Pb-BP had an apparent molecule mass of 6900 Da and seemed to bind preferentially to lead in the liver cytosol. The Zn-BP was identified by comparison of the Mr, elution profiles from Sephadex G-75 and DEAE-Sephadex A-25 columns, and polyacrylamide-gel-electrophoretic mobility, with those of rat liver Zn-MT-II. The Pb-BP accumulated in the liver to a maximum 6 h after the intraperitoneal injection of lead acetate and accounted for about 60% of the lead in the liver cytosol at this stage. However, after that, it gradually decreased in the liver, until it was close to the basal amount 24 h after the induction. In contrast, the amount of Zn-MT increased gradually, reached a maximum 12 h after the administration of lead acetate and maintained a constant value until at least 24 h after the induction. Amino acid analysis of the Pb-BP indicated that it contained about 280% half-cysteine. These results strongly suggest that lead acetate induces the synthesis of Pb-MT as well as Zn-MT in rat liver.

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Wepreviously reported that a 7.6-kb DNAfragment from Streptomyces kasugaensis M338-M1 , a kasugamycin (KSM) producer, included KSMacetyltransferase gene (kac33S) and some other genes possibly involved in KSMbiosynthesis. As an extension of that study, a 10-kb Sacl-Kpnl DNAfragment, located 5~15-kb upstream of kac?3%, was cloned and a 4.2-kb Sacl-EcoRl fragment there from was sequenced, revealing one incomplete (designated ORFJ) and three complete open reading frames (designated kasK, kasL and kasM). The coding frames of kasK, L and Moverlap one another with terminator/initiator ATGAsequence. RT-PCRanalysis of a DNAregion including kasKLMindicated the presence of one transcript that is long enough to span the three genes. The kasK gene potentially encodes an ATP-binding protein of the ATPbinding cassette (ABC) transporter super family. Homology search for the deduced KasK protein shows similarity to other ABCtransporters involved in self-resistance of a mithramycin and possibly doxorubicin producer strain. The kasL and kasM genes encode different integral membraneproteins, both having six putative transmembrane helices. Anexpression plasmid for kasKLM(pTV^KLM) was constructed and these genes were expressed in E. coli JM109, which had been sensitive to KSM.The transformant acquired resistance to KSM,suggesting that KasK, L and Mproteins as a set in S. kasugaensis M338-M1pump out KSMto protect the producer from its toxic metabolite. Organisms producing potentially autotoxic antibiotics possess basically three types of self-resistance mechanisms to avoid suicide: (1) modification of the target site that the antibiotic acts on, (2) intracellular inactivation of the antibiotic and (3) exclusion of the antibiotic from the cell1T he last resistance mechanism is designated "membraneassociated system" consisting of two classes. In one class, resistance is mediated by membrane proteins, which are believed to energize export of antibiotic molecules by proton-dependent transmembrane electrochemical gradients. The other class belongs to the ABCtransporter superfamily2) comprising many membrane-associated export and import systems, which are present both in prokaryotic and in eukaryotic cells. ABC transporters

To investigate the mechanism by which zinc suppresses mercury toxicity, the effects of zinc and mercury on glutathione (GSH) metabolism in the rat kidney were studied. When the time course of GSH level in the rat kidney was examined at 2, 6, and 12 h after treatment of rats with both metals, an increase of GSH was found and was apparently related to the activation of some GSH-associated enzymes. In the kidney of rats treated with both metals, the response of the protective function involving GSH and GSH-associated enzymes depended on the magnitude of mercury toxicity but appeared to be independent of the zinc dosage. The administration of diethyl maleate (DEM), which depletes GSH, increased lipid peroxidation and mercury toxicity concomitantly with a decrease of GSH level in the kidney of rats treated with zinc and mercury. In conclusion, the data suggest that an increased GSH level in the kidney resulting from the activation of GSH-associated enzymes plays a role in the protective effect of zinc against mercury toxicity.

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