ABSTRACT
Metallothioneins or metallothionein like proteins protect the biological system by encountering the influx of metal ions. These proteins are present in the contaminated as well as uncontaminated organisms. Studies show that the native metallothioneins or metallothionein like proteins of O. mossambicus is predominantly zinc bound. Increased influx of zinc causes a redistribution of this essential metal to other proteins requiring zinc as co-factor. Influx of mercury is encountered by the displacement of zinc bound to the protein and also by de novo of synthesis of metal binding proteins and subsequent binding of the incoming mercury. Data available show that "spill over" hypothesis is too simple to explain the mechanism of heavy metal toxicity.