Effects of zinc and cadmium on HgCl2-delta-ALA-D inhibition and Hg levels in tissues of suckling rats.

The effects of CdCl2 and ZnCl2 pretreatments on the inhibition of delta-ALA-D (delta-aminolevulinic acid dehydratase) activity and Hg contents in liver and kidneys of suckling rats intoxicated with HgCl2 were investigated. Zn-pretreatment prevented the effects of mercury at a higher magnitude than CdCl2. Hepatic and renal delta-ALA-D activities were significantly inhibited by HgCl2 and prior exposure to CdCl2 partially prevented the renal effect of mercury but not altered the mercury levels in both tissues. Pretreatment with ZnCl2 abolished mercury-induced delta-ALA-D-inhibition in kidneys and liver and induced an increase in renal (about three times) and a decrease in hepatic (to one-third) Hg contents when compared to the group injected only with mercury. In face of zinc effects to prevent Hg-delta-ALA-D inhibition and to alter Hg-deposition levels in kidney and liver, these results suggest that these effects may be partially due to the synthesis of metallothioneins (MT). In fact, liver MT content presented by animals pretreated with zinc was significantly greater than control and Hg-treated groups, but the increase showed by renal tissue (about 60%) was not significant. Although the MT is rich in cysteine (-SH) and consequently can form a great number of MT-Hg complex, other mechanisms should be also involved in zinc protection on mercury toxicity.