| Summary: | 碩士 === 國立臺灣大學 === 農業化學研究所 === 83 === Trace metal element (e.g. Zn or Cu) were essential to cell metabolism and hence its growth . If these trace nutrient elements or some other heavy metal (e.g. Cd, Hg or Pb) exceeded normal requirement or tolerance, cell might face fetal damages. To eliminate excess metal ions , some chelating agent were developed in the cell.
Animal cell produced metallothions , and plants as well as some fungi enzymatically synthesized small peptide phytochelatin which sequestered cadmium very efficiently in the vacuole . In this study , Schizosaccharomyces pombe (ATCC 2476) was found produced neither metallothionein nor phytochelatin when exposed to cadmium . Instead three groups of protein (W1 , W2 and W3) were were identified as the chelating agent responding invading cadmium . They contained few Cys , but had very high percentage of Asx , Glx and His , indicating that the carboxylic or imidazole group might play the central role in chelating Cd ions . This mechanism was different essentially from that oetallothioneins
or phytochelatin.Molecular weights of W1, W2 and W3 as etermined
by gel-permeation chromatography were 76 kD , 56 kD and 49 kD ,respectively . The subunit molecular weights estimated from SDS-PAGE were 24 kD (W1) and 25 kD (W2),and W3 contained two peptides two peptides (17.4 kD and 7 kD). It was also found that above 70% of cadmium in the cells was sequestered in the vacuoles of ATCC 2476. And if the environmental Cd was relieved,ATCC 2476 extruded cadmium and clean up the cell. These results showed that we might have found a new class of heavy metal chelating protein which sequestered cadmium in the vacuole of fission yeast ATCC 2476 by a novel mechanism not observed before.
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