Metallothionein in plasma and urine of cadmium-exposed rats determined by a single-antibody radioimmunoassay

Abstract

A single-antibody radioimmunoassay for determining nanogram quantities of rat metallothionein has been developed. This assay has advantages over the double-antibody radioimmunoassay for metallothioneins in terms of accuracy and expedition. Metallothionein obtained from rat liver, human kidney and even rabbit liver completely cross-reacted with the antiserum, whereas metallothionein of crab hepatopancreas did not show any cross-reactivity. With the use of this assay metallothionein was detected in plasma and urine of rats exposed to 5 mumol CdCl2/kg/day, 5 days a week for up to 15 weeks. The sensitivity of the radioimmunoassay, the 50 percent intercept of standard curves, for metallothionein in rat plasma and urine was 11.5 and 11.6 ng, respectively. Upon cadmium exposure the metallothionein level in plasma increased progressively during the first 4 weeks, and remained unchanged thereafter. Metallothionein was detected in urine as early as 4 days after the exposure. The urinary excretion of the protein was significantly elevated by 11 weeks. By this time the kidney cortex cadmium level exceeded 200 micrograms/g wet weight, the critical concentration for the onset of renal dysfunction. These results suggest that the appearance of metallothionein in the body fluids is an early effect of cadmium exposure and that the marked increase in urinary metallothionein excretion may be a useful index of renal tubular dysfunction. Toxicological implications of the observation on complete cross-reactivity of the rabbit anti-rat metallothionein serum with rabbit metallothionein are also discussed.