For CHO Cdr cells the presence of lead acetate in the media in concentrations above 1 mM leads to gradual cell death, as measured by the reduction of [3H]thymidine incorporation into DNA. These cells accumulate an increased amount of newly synthesized metallothionein. Typical 9S metallothionein mRNA could be detected by hybridization using metallothionein cDNA probes, with maximal accumulation occurring after 4-h exposure of cells to 2 mM lead acetate. The intracellular levels of metallothionein protein increase continually with time; metallothionein was identified by its richness in cysteine, chromatographic and electrophoretic behavior and reactiveness to carboxyamidomethylation. When separated by an anion-exchanger, both isospecies MT I and MT II could be observed, as they were identical in every respect tested to those induced by zinc chloride. The induction of metallothionein by lead was not due to an increase in intracellular zinc levels, as zinc uptake was unaffected by the presence of lead acetate in the media.
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