Degradation of rat liver metallothioneins in vitro

The degradation of zinc and cadmium-induced hepatic metallothionineins was investigated in vitro. Both zinc-thionein and cadmium-thionein were labeled in vivo with [35S]cystine. The labeled proteins were isolated and purified by gel filtration and DEAE-ion exchange chromatography. Purified zinc[35S]thionein and cadmium-[35S]thionein were incubated with trypsin, chymotrypsin and pronase for varying times up to 24 h. The rate of degradation of zinc-thionein was twice that of cadmium-thionein when the proteins were incubated with trypsin. Virtually no digestion occurred when the proteins were incubated with chymotrypsin, whereas the rates of degradation were about equal when they were incubated with pronase. In contrast, degradation of zinc-thionein was twice that observed with cadmium-thionein when the proteins were incubated at pH 5.0 with a purified lysosomal extract. Degradation of these proteins by the lysosomal proteases was 77 and 46% within 3 h for zinc-thionein and cadmium-thionein, respectively. Thionein, the metal-free form of metallothionein, was degraded extremely rapidly by both neutral and lysosomal proteases. Chromatography of the digestion products on Sephadex G-25 demonstrated that all three forms of metallothionein were degraded to species of approximately 100-300 daltons. These data indicate that metals stabilize thionein polypeptides and suggest that the degradation of metallothionein in vivo is regulated in part by the species of metal bound.