Abstract
Metallothioneins (MT), small molecular weight metal binding proteins are known to play an important protective role against heavy metal toxicity, either as antioxidants or pre-oxidants. However, the mode of metabolic fate of MTs in various metal complexes is not clearly understood. This study was carried out to better understand the mode of selective turnover rate of various form of MT in complexes with different metals. The degradation of in vitro translated mouse 35S-cysteine-MT was examined in lysosomal or cytosolic fractions from mouse liver by gel electrophoresis and autoradiography. Overnight incubations of MT showed extensive proteolysis in the lysosomal fraction but not in cytosolic fractions. However, Cu2+-MT was found to be stable under the same experimental condition. In contrast, Zn did not interfere with MT degradation. These results suggest that lysosomes are chiefly responsible for MT removal and appears to be selective on the metals involved in the MT complex. In vitro, translated, radiolabeled MT provides a suitable substrate for investigating the characteristics of MT degradation.
Highlights
Metallothioneins (MT) are a group of cysteine-rich low molecular weight intracellular proteins that avidly bind heavy metals
We demonstrated that degradation of MT occurs in the lysosome and that copper interferes with the degradation of MT while Zn showed no effect
When 35Slabeled MT was incubated with cytosolic fractions, its molecular weight was slightly increased, with smearing of the band
Summary
Metallothioneins (MT) are a group of cysteine-rich low molecular weight (approximately 6000 to 7000 D) intracellular proteins that avidly bind heavy metals. The half-lives of MT-I and MT-II in the liver cytosol of Cu2+ injected rats were only 15.4 ± 1.5 and 18.2 ± 1.1 h, respectively, but in in vitro studies of copper-metallothionein in the rat liver, no significant degradation of the Cu-MT complex was observed (Mehra et al, 1985). In order to better understand a possible mode of cellular turnover rate of MT in various state of metal complexes, we have prepared radiolableled mouse MT by translation in in vitro reticulocyte lysate system and used it in the proteolysis experiments containing various mouse liver cellular fractions.
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