| Summary: | 碩士 === 國立臺灣大學 === 微生物與生化學研究所 === 95 === Schizosaccharomyces pombe ATCC 2476 is tolerant to environmental cadmium. ATCC 2476 produced neither metallothionein nor phytochelatin for the sequestration of invading cadmium. However, ATCC 2476 did produce an intracellular cadmium binding complex (ICBC) as the chelating agent responding to the invading cadmium. Molecular weight of ICBC was determined by gel filtration chromatography as 9.6 kDa. No thiol group can be detected by Ellman’s reagent, and there was no effect on the molecular weight or its cadmium binding ability after lipid extraction or protease and nuclease treatment. There was no detectable glucan in ICBC by Dubois’s method. By infrared spectroscopy and effect of the pH on its cadmium binding ability, we proposed that carboxylic group might play a central role in chelating cadmium ions. This mechanism was different essentially from that of metallothionein and phytochelatin. When ATCC 2476 was cultured on a square agar plate, then a narrow strip of filter paper soaked with high concentration of CdCl2 was placed in the center of the plate. The surface of the cell near the Cd strip produced metal luster after several days of incubation. The metal luster was supposed to be caused by the accumulation of cadmium binding complex. The cells with luster were collected, and the surface cadmium binding complex was isolated and denoted as the extracellular cadmium binding complex (ECBC). ECBC shared similar properties with ICBC, e.g. having no thiol group, no change in molecular weight and cadmium binding ability after enzyme treatments. The concentration of cadmium accumulated in the cell increased when ATCC 2476 exposed to cadmium. If the environmental cadmium were then relieved, ATCC 2476 rejected cadmium from cells. The amount of cadmium chelated by ECBC increased corresponding to the cadmium released by ICBC. These results suggested a new cadmium chelating complex which sequestered cadmium ions by carboxylic groups in the cells, and then excluded the complex out of the cells by an unknown mechanism.
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