Metallothionein and cadmium in human kidney cortex: influence of smoking

Post-mortem specimens of human kidney cortex of 47 individuals classified according to their smoking habits were analysed for tissue cadmium and cadmium bound to metallothionein. The cadmium content in the kidney cortex of all individuals was 5-99 micrograms/g wet wt. In smokers consuming more than 20 cigarettes/day the mean content of renal cortex cadmium was twice that of non-smokers and amounted to 33.3 +/- 12.5 micrograms/g wet wt. The amount of cadmium bound to metallothionein of all individuals was 0.3-66 micrograms/g wet wt. directly correlating with the cadmium content of the kidney cortex (r = 0.932). More than 50% of renal cortex cadmium was associated with the metallothionein fractions. Due to constant values of zinc and copper in metallothionein the relative amount of zinc and copper to cadmium in metallothionein decreased with increasing tissue cadmium. Together with the elevated binding of cadmium to renal cortex metallothionein, cadmium increasingly was bound to non-metallothionein ligands. These results suggest that, in the renal cortex of smokers with elevated cadmium, a major portion of cadmium is bound to metallothionein. However, it is unclear yet whether the binding of cadmium to metallothionein with subsequent liberation of the metal during degradation of the protein, the impairment of metallothionein functions by the binding of cadmium or the increased binding to non-metallothionein ligands contribute to the toxicity of cadmium in highly exposed individuals.