Copper binding proteins in spinach tolerant to excess copper

Cu-complexes were isolated from cytosol of roots and leaves of spinach plants surviving in a medium containing a high concentration of copper without showing any symptoms of toxicity and possessing an efficiently acting photosynthetic apparatus. Most of the copper in cytosol is assumed to be bound to low molecular weight proteins. Two Cu-binding fractions, at a molecular weight of 12,500 and 9,500 respectively, were isolated from the roots. However, in the cytosol of the leaves only one fraction, at a molecular weight of 8,500, was isolated. It has been suggested that Cu-complexes are formed due to copper binding by preexisting proteins in cytosol and their synthesis seems to be stimulated by excess copper. They would be able to protect, among other things, the photosynthetic apparatus against the toxic effect of copper ions.