Differential effect of Cd2+ and Ni2+ on amino acid metabolism in soybean seedlings

In 10-d-old soybean seedlings, the growth of roots and shoots was significa ntly inhibited at 50 and 100 mu M and more Cd2+, respectively, and by 50 CI M or more Ni2+. Although total protein content of roots exposed to 200 mu M Cd2+ or Ni2+ was similarly decreased compared to the control, the activity of nitrate reductase was much more inhibited by Cd2+. Ni2+-treatment (200 mu M) induced an accumulation of all free amino acids in roots associated w ith a decrease in alanine aminotransferase (ALT) and aspartate aminotransfe rase (AST) activities reflecting the accumulation of both alanine and aspar tic acid, respectively. Cd2+-treatment (200 mu M) decreased the amount of a ll free amino acids. In addition, cysteine which is the main amino acid con sisting the phytochelatin complexes constituted about 17.5% of total free a mino acids. The activities of both ALT and AST in Cd2+-treated roots were h igher than in Ni2+-treated roots suggesting higher conversion of alanine an d aspartate to pyruvate and oxaloacetate. Primary leaves excised from eithe r Cd2+ or Ni2+-treated seedlings showed similar pattern of enzyme activitie s as roots.