Mercury compounds biological effects

It is generally accepted that the basis of the biological activity of mercury compounds is their reaction with the thiol groups, but the biological action is rather more complicated. Frank (1955) showed that mercury compounds can influence the effect of enzymes which do not contain thiol groups. Mercury also reacts with the phosphoryl groups of the cell membranes (Bassow et a/., 1961) and with the amino and carboxyl groups of the enzymes (Lipscomb et al., 1968). Webb (1966) lists more than 40 enzymes inhibited by mercury compounds. 

The reaction between methylcobalamin and metal compounds plays a major role in biological transmethylation and may also be involved in the geochemical cycling of metals. The kinetics and mechanism of this reaction have been reported in detail for mercury(II) salts, [PdCU] , and chloroplatinum(IV) compounds. The effect of the chloride ion concentration on the reaction of Hg(OAc)2 with CH3B12 has now been investigated in detail. The reaction rate decreases markedly as the [Cr]/[Hg(II)]totai ratio is increased. 

It has been known since 1970 that inorganic mercury contained in sediments could be converted into methyl mercury compounds by biological processes (Wood et al. 1968, Jensen and Jernelov 1969). The significance of this process is still unclear however, it seems from recent reports (Billen 1974, Lindberg et al. 1975) that the rates of methylation are very low and are likely to be balanced by demethylation reactions. Thus, the amount of methyl mercury compounds present in natural waters is likely to be very small and of minor importance in terms of toxic effects to aquatic organisms. This is borne out by the fact that methyl mercury has never been detected in natural waters. 

Paracelsus, a Swiss physician of the sixteenth century, stated that everything is toxic, it is just the dose that matters. This statement still holds true 500 years after Paracelsus developed it to defend the use of toxic compounds such as lead and mercury in the treatment of serious diseases such as syphilis. Chemical compounds cause their toxic effects by inducing changes in cell physiology and biochemistry, and an understanding of cellular biology is a prerequisite if one wishes to understand the nature of toxic reactions. 

DE Boer J and Smedes F (1997) Effects of storage conditions of biological materials on the contents of organochlorine compounds and mercury. Mar Poll Bull 35 93-108. 

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