Effects of various trace metals on the binding of cadmium to rat hepatic metallothionein determined by the Cd/hemoglobin affinity assay.

The Cd/hemoglobin affinity assay is finding increasing use as an indicator of metallothionein in a variety of biological tissues. Because the assay relies on the ability of heat-stable proteins to bind Cd, it is important to know the relative affinities of different trace metals to bind to such proteins, relative to Cd. This study examines the ability of 15 trace metals to prevent the binding of Cd to metallothionein using the Cd/hemoglobin affinity assay for metallothionein. Cd, Cu, Hg, and Ag were the only metals tested which significantly inhibited the binding of a fixed concentration (2 microM) of 109Cd to a crude preparation of rat hepatic metallothionein. As, Co, Cr, Fe, Mn, Mo, Ni, Pb, Sn, Th, and Zn had no inhibitory effect at the highest concentrations tested. However, As, Sn, Th, and Zn prevented the precipitation of hemoglobin at relatively low concentrations and, thus, could not be fully tested for inhibitory potency. Cu was the most potent inhibitor, producing more than 90% inhibition at 5 microM, followed by Ag, Hg, and Cd, which produced 76, 72, and 65% inhibition of cadmium binding at 5 microM, respectively. These results suggest that caution should be taken in interpreting metallothionein concentrations obtained by the Cd/hemoglobin affinity assay in tissues which contain relatively a high concentration of Cu, Ag, or Hg, relative to that of Cd.