A sensitive, high-capacity radioimmunoassay for metallothionein and its application to urine samples from cadmium-treated rats

A double-antibody radioimmunoassay (RIA) for metallothionein (MT) using she ep antiserum and its application to urine samples from rats chronically exp osed to cadmium (Cd) are described. The procedure can be completed in two r ather than the usual three days. The separation of the antigen-antibody com plex is carried out in 96-well plates equipped with membrane filters. This assay format makes it possible td process approximately 200 samples in quad ruplicate with relative ease. Antiserum raised in sheep against a mixture o f rat hepatic MT-I and MT-2 reacted equally with both isomers of MT with a detection limit of about 1 ng/50 mu L urine or tissue homogenate. The usefu lness of this high-capacity assay was further examined by analyzing the uri nary MT levels of control rats and rats maintained on drinking mater contai ning 100 mg Cd/L for up to 21 months. In the Cd-treated group, urinary MT g radually increased with the duration of exposure. After 6 months of exposur e urinary MT was significantly higher in the Cd-treated group than in the c ontrol group and at 21 months reached 1 mu g/mg creatinine. The results ind icate that the modified high-capacity RIA is a sensitive, convenient, and r eliable method for measuring MT in biological tissues and fluids. Applicati on of the method to urine specimens from rats chronically exposed to Cd con firms an earlier finding that urinary MT is an indicator of Cd exposure.