What stresses are you subjecting the cells too? Selenometheonine and selenocysteine (which can be inter-converted in many cells) can function as antioxidants protecting cells from oxidant stress (e.g., selenocysteine is an essential amino acid in glutathione peroxidase etc,), but its spontaneous degradation products are strongly pro-oxidant subjecting cells to high levels of oxidant stress. This would be particularly significant at low cell densities, were the spontaneous pro-oxidant effects would dominate.