Mechanisms for removal of copper from metallothionein by tetrathiomolybdate.

Mechanisms for removal of copper (Cu) from metallothionein (MT) by tetrathiomolybdate (TTM) were examined in vivo and in vitro using the LEC rat, which accumulates Cu as MT owing to the hereditary disorder of this strain. In our previous experiment, repeated intraperitoneal injections of TTM were shown to remove approximately two-thirds of the Cu from the liver, and the Cu remaining in the liver changed from soluble MT-bound forms to nonsoluble unidentified forms. The present single intravenous injection of TTM changed only part of the distribution in the soluble fraction, and dimeric MT was assumed to be formed. The liver supernatant was treated in vitro with high and low doses of TTM. The former treatment removed all Cu bound to MT and the Cu distributed to high molecular weight proteins, while the latter treatment produced dimeric MT. The results indicate that Cu accumulated as MT can be removed differently by TTM according to its relative dose both in vivo and in vitro. Excess TTM removes Cu completely from MT, leaving apothionein, while lesser TTM than Cu removes Cu incompletely, leaving MT with unoccupied sulfhydryl groups which coordinate with Cu intermolecularly to form dimeric and polymeric MT through the -S-Cu-S- bridge.