Mercury is a silvery metal that is liquid at room temperature. Having an atomic number of 80 and a specific gravity of 13.6, mercury is over thirteen times as heavy as water! Mercury can be absorbed through the skin and is very poisonous to humans, as it tends to settle in soft tissues and organs and interrupt the function of normal life processes. Hat-makers in the hatting industry used mercury to line the felt brims of their hats and came into contact with the element on an almost daily basis. Many of the hatters went insane as time went by and the mercury accumulated in their systems, hence, the term "mad as a hatter".
Mercury occurs natuarally in deposits throughout the world, mostly as cinnabar (mercuric sulfide), which is the source of the red pigment vermillion, and is mostly obtained by reuction from cinnabar. Cinnabar is highly toxic by ingestion or inhalation of the dust. Mercury poisoning can also occur from exposure to soluble forms of mercury, inhalation of mercury vapor, or eating seafood contaminated with mercury.
All this tends to suggest mercury would not be a naturally occuring part of a human cell. If mercury were present in a cell at all, it would most likely have a debilitating effect on the cell and the cell's functions.