Protein binding of mercury in milk and plasma from mice and man - a comparison between methylmercury and inorganic mercury

Inorganic mercury has previously been shown to be excreted to milk from pla sma to a higher extent than methylmercury. Protein binding of mercury as me thylmercury and inorganic mercury in whey and plasma from mouse and man was studied in order to get a better understanding of the transport of mercury into milk. Mice were administered a single i.v. dose of 0.25 mg Hg/kg body weight labelled with (CH3HgCl)-Hg-203 or (HgCl2)-Hg-203, resulting in 11 n g Hg/g milk and 38 ng Hg/g milk after 1 h, respectively. Milk and plasma fr om mice and man were also incubated with the respective radiolabelled compo und (150 ng Hg/g milk or plasma). Casein, fat and whey fractions in milk fr om methylmercury treated mice were found to contain 11, 39 and 34%, respect ively, and from inorganic mercury treated mice 31, 15 and 41%, respectively , of the total amount of mercury in milk. Serum albumin was a major mercury binding protein in whey and plasma from mice for both methylmercury and in organic mercury, as demonstrated by FPLC gel filtration and anion-exchange chromatography and further characterised by SDS-PAGE for whey. In addition, anion-exchange chromatography indicated that inorganic mercury, but not me thylmercury, in whey from mouse milk formed a dimer of serum albumin. The u nbound fraction of mercury in whey and plasma from mice was very small (<0. 7%), and somewhat higher in plasma and whey from man. It is concluded, that the unbound fraction in plasma cannot be a determining factor for the obse rved differences in milk excretion between the two mercury compounds. Inste ad, it is suggested that methylmercury and to some extent inorganic mercury are transferred from plasma into milk using albumin as a passive carrier. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.