Studies on cadmium toxicity in amoeba proteus

Abstract

In this thesis, cell survival, biochemical and electron microscopical techniques have been employed in the elucidation of the toxic effects of Cd on A. proteus. The toxicity demonstrated by lethality and ultrastructural changes was found to be time and concentration dependent. Amoebae were shown to be capable of accumulating Cd2+ to levels far greater than those of the surrounding media. Cd2 uptake was found to be of a biphasic pattern, i.e. initial rapid uptake - plateau - slow uptake - plateau. On return to Cd2+ free medium the Cd2+ treated amoeba were shown to retain about 50% of the total Cd4+ taken up. Within the subcellular fractions from Cd2+ treated amoebae, a high proportion of Cd2+ was always associated with the cytosol fraction, whereas, in the nuclear fraction there was an initial high proportion which then fell as the proportions in the microsomal and cytosol fractions continued to increase. Pretreatment of amoebae with a low dose of Cd2+ induced a short-lived protection, of unknown nature, against a subsequent higher dose of Cd2+.Gel filtration of the cytosol fractions demonstrated that Cd2+ was associated with two major peaks of Cd-binding proteins, Peak I at > 45,000 MW and peak II at 10,000-12,000 MW. Cd2+ treatment was shown to cause either a great increase in, or an induction of, the synthesis of peak II Cd-binding protein. Added cysteine increased the Cd + incorporated into both peaks I and II but it caused a disproportionate increase in that incorporated into peak II. The parallels between peak II Cd-binding protein and the mammalian metallo thioneins are discussed.E.M. examination of Cd2+ treated amoebae revealed extensive damage to the mitochondria, an increase of free detached ribosomes combined with an increase in the smooth membranes, and an increase of lipid found at particular doses. Continuous culturing of amoebae in the presence of low concentrations of Cd2+ yielded a population of 'Cd-adapted' amoebae. These amoebae demonstrated a recovery in ultrastructural Morphology and percent survivals and a change in the pattern of Cd + uptake from that of controls.

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