Table 1 –

Relative quantity of molecules of biological significance containing the soft nucleophile groups thiol (-SH) or selenol (-SeH). These functional groups are the main targets of methylmercury (MeHg) and other soft electrophiles.

High molecular mass thiol molecules (thiol-containing proteins)
	Occurrence
–Cysteinyl residues (Cys)	in thousands of proteins
High molecular mass selenol molecules (selenoproteins)
	Occurrence
–Selenocysteinyl residues (Sec)	in few number of proteins
Low molecular mass thiol molecules (-SH)
	Occurrence
Endogenous	Cysteine glutathione (GSH)
exogenous (synthetic)	2,3 dimercaptopropanol (BAL) 2,3 dimercaptosuccinic acid (DMSA) 2,3 dimercapto sulfonic acid (DMPS)
Low molecular mass selenol molecules (-SeH )
	occurrence
Endogenous	None
exogenous (synthetic)	Unstable