. O T / 0 60 60 24 v 3 1 4 Ju l 2 00 6 Bistable equilibrium points of mercury body burden

In the last century mercury levels in the global environment have tripled as a result of increased pollution from industrial, occupational, medicinal and domestic uses [1]. Glutathione is known to be the main agent responsible for the excretion of mercury (we refer to [38], [36] and [34]). It has also been shown that mercury inhibits glutathione synthetase (an enzyme acting in the synthesization of Glutathione), therefore leading to decreased glutathione levels (we refer to [38], [32], [12] and [14]). Mercury also interferes with the production of heme in the porphyrin pathway [26]. Heme is needed for biological energy production and ability to detox organic toxins via the P450 enzymes [19]. The purpose of this letter is to show that when this negative feedback of mercury on its main detoxifying agents is strong enough then mercury body burden has two points of equilibrium: one with normal abilities to detoxify and low levels of mercury and one with inhibited abilities to detoxify and high levels of mercury. Furthermore, a small increase of body’s mercury burden may not be sufficient to trigger observable neurotoxic effects but it may be sufficient to act as a switch leading to an accumulation of mercury in the body through environmental exposure until its toxicity becomes manifest. Writing C the mercury body burden a simple model for the evolution of C with respect to time can be described by the following ordinary differential equation.