Properties and binding forms of cadmium and nickel in protein extracts from bean seeds (Phaseolus vulgaris L.)

Abstract

A special ultrafiltration procedure in combination with photometry and atomic absorption spectrometry is described. This technique allows not only the determination of the qualitative distribution patterns of metals but also of the quantitative binding parameters of these metals to functional groups of organic compounds. The technique was applied to protein extracts of bean seeds (Phaseolus vulgaris L.). The results show that Cd and Ni have different binding mechanisms in these seeds. Both metals differ not only in their preferred binding behaviour to certain molecular size fractions but also in their potential binding sites and the binding properties. Whereas Cd in seeds from Cd-treated plants prefers binding partners with molecular weights >30 kD and <0.5 kD, Ni is preferably bound to compounds with molecular weights <5 kD. Cd exhibits a tendency to bind more selectively to sulfhydryl groups, but Ni more to non-specific binding sites. Sulfhydryl groups account for 71% of the total capacity for Cd whereas for Ni it is only 36%.