I understand that 2 mercaptoethanol is a thiol which may bind to mercury ion. Since the function of mercuric reductase is to bind with the metal and reduce it into Hg(0), is the addition of mercaptoethanol might affect the reaction rate?
The actual substrate for mercuric reductase is Hg bound to sulfhydryl groups, or RS-Hg-SR. It is not the free form of Hg++. There is a JBC paper I got from a google search. Sorry, forgot the reference. But the BME binds to the mercury metal to form the disulfide complex, which in turn allows the mercuric reductase to use it as a substrate and reduce the metal.