CHARACTERIZATION OF A CADMIUM-ZINC COMPLEX IN LETTUCE LEAVES

Vegetable food contributes a higher amount of daily cadmium (Cd) intak e in humans than food of animal origin. The bioavailability of plant C d depends on the content of plant zinc (Zn). The mechanism by which in creased plant Zn lowers the intestinal absorption of plant Cd could be mediated by changes in the chemical speciation of Cd or Zn in plant e dible tissues, including Zn-induced phytochelatin synthesis. To test t his hypothesis we investigated the chemical speciation of Cd and Zn in leaf extracts of lettuce grown under 10 mu M of Cd accompanied by 0.3 2 or 31.6 mu M Zn in nutrient solution. Gel filtration chromatography of the low- or high-Zn leaf extracts yielded a major low molecular wei ght Cd-Zn complex that eluted at similar elution volume. Compared to l ow-Zn leaf extracts, high-Zn leaf extracts contained a higher proporti on of Zn incorporated into high molecular weight components, and highe r content of the amino acids Cys, Glu, Gly, and Asp in the low molecul ar weight Cd-Zn complex. The peptides isolated by high performance liq uid chromatography (HPLC) of the Cd-Zn complex from the low- or high-Z n leaf extracts did not have an amino acid composition identical to ph ytochelatins. We concluded that 1. Sequestration of Cd or Zn via phyto chelatin does not occur in leaves of lettuce containing levels of thos e metals representatives of Zn-Cd or Cd-only contaminated crops; and 2 . Higher Cys, Glu, Gly, and Asp content in high-Zn than low-Zn leaves could lower Cd absorption in animals fed high-Zn crop diets, by enhanc ing metallothionein synthesis or changing Cd or Zn speciation in the a nimal gut.