Abstract
Using radioimmunoassay, radiolabelling and subcellular fractionation techniques, further information as to the role of metallothionein in the disposition of cadmium in the rat and man, following exposure to this metal, was obtained. A radioimmunoassay for the specific measurement of mammalian metallothionein I and II was developed, using a sheep anti-(rat metallothionein) antibody. The assay had a large linear working range (6. 6ng - 1.0mg/ tube) and was capable of metallothionein measurement in the mammalian tissues and body fluids without the need for high dilutions. Chronic dietary administration of 75, 150 and 300ppm cadmium to rats lead to a dose- and time-related accumulation of this metal in the liver, kidney and small intestine. The highest cadmium concentrations were located in the kidney. This was accompanied by disturbances in zinc and copper concentrations in these tissues. Using the radioimmunoassay, metallothionein induction was seen to parallel cadmium accumulation in these tissues. Chromatographic separation of the cytosolic fractions of some of these tissue samples revealed that only 42% of the cadmium was associated with metallothionein in the small intestine, whereas about 90% of the cadmium was associated with this protein in the other tissues. Urinary excretion of cadmium was about 10 fold higher(3-4ug/day) in the 300ppm dose group than the lower dose groups. Copper and not cadmium was associated with metallothionein in the urine; cadmium was located in the lower molecular weight fraction. Urinary metallothionein appeared to be a good index of cadmium body burden and also a good indicator of recent exposure - being detected in the urine after 3 days cadmium exposure. Studies using the isolated perfused liver system, revealed the paralleled release of cadmium and metallothionein from the liver of rats exposed to cadmium. The profile of this release suggests an active mechanism. Using [109]Cd, [3]H-labelled metallothionein, an exponential uptake of [109]Cd by the liver of rats not previously exposed to cadmium, was observed. Tritium uptake was less rapid. Subcellular fractionation, revealed that most of the tritium was located in the membranous fraction of the liver, whereas [109]Cd was found mainly in the cytosol (associated with endogenous metallothionein). Intravenously injected [109]Cd,[3]H-labelled metallothionein into rats,showed that 30-40% of this protein was taken up by the renal cortex within 10 minutes. Subcellular fractionation studies indicated the lysosomal degradation of this protein. 109Cd was retained in the cortex- associated with endogenous metallothionein of the cytosol fraction. Tritium was lost from the cortex by 24 hours after injection. The biological half-life of the exogenous metallothionein was calculated to be between 10 and 30 minutes. A group of human volunteers repeatedly consumed "brown crabmeat" containing cadmium for up to 12 weeks. Plasma and urine cadmium concentrations showed a paralleled increase after 2 and 6 weeks suggesting a "cyclic" release of cadmium possibly from the liver to the kidney, bound to metallothionein. This is discussed in the light of the data obtained from the in vitro organ perfusion studies and animal studies in vivo. descibed previously.