NMR studies of crab and plaice metallothioneins

Metallothioneins isolated from the hepatopancreas of the edible crab (Cancer paqurus) and the plaice (Pleuronectes platessa) after cadmium injection are predominantly cadmium proteins containing only small amounts of zinc and traces of copper. The removal of metal ions from the two metallothioneins by EDTA was studied using proton NMR spectroscopy. The rates of removal of cadmium and zinc were monitored directly from the intensity of the resonances due to the cadmium and zinc-EDTA complexes. Nearly all the zinc present in the protein was extracted by EDTA relatively rapidly, whereas only 10 to 20% of the total cadmium was removed in at least three steps. The total (Cd + Zn) metal removed at equilibrium was 1.2 to 1.8 g-ions/mole protein. Information on conformational changes in the protein were also obtained from studying alterations in the proton resonances of the protein. This was directly correlated with removal of metal from the protein. The coordination environments of the cadmium ions in crab metallothionein were investigated by using 113Cd-NMR, and compared with 113Cd-NMR spectra of rabbit liver MT-II and Scylla serrata MT-I.