The sensitivity of MTGase toward other cations in the absence of reducing agents has also been investigated[25]: Cu2+, Zn2+, Pb2+ and Li+ significantly inhibited MTGase. Because heavy metals such as Cu2+, Zn2+ and Pb2+ bind the thiol group of the single cysteine residue, this strongly supports the idea that the cysteine residue could be part of the active site of MTGase.