[Ligand and isotope exchange in the system: serum proteins--zinc--polyaminopolycarbonic acids].

In vitro-studies show that the Ca-chelates of EDTA and DTPA are equally effective in removing Zn from the proteins and that the Zn-protein pool is composed of several fractions with different stabilities. Only a small fraction of the protein-bound Zn can be labelled by 65Zn in vitro and, as to the mobilization of 65Zn, Ca-DTPA was found to be 20 times more effective than equimolar Ca-EDTA. The isotope exchange between Zn-DTPA and protein-bound 65Zn is an extremely fast reaction whereas in the case of Zn-EDTA a sluggish exchange takes place. The analysis of the results led to conclusion that the ligand and isotope exchange reactions in the case of EDTA proceed via the free Zn2+-ion; with the high-dentate DTPA, however, the formation of ternary and mixed complexes plays an important role. The implications of the findings as related to the toxicity of the Ca-chelates are discussed.