Mouse Monocytes (RAW CELLS) and the Handling of Cysteine and Homocysteine S-Conjugates of Inorganic Mercury and Methylmercury

Abstract

Although there is evidence indicating that mononuclear phagocytes can take up mercury by some forms of endocytosis, very little is known about the potential for the uptake of mercuric species by carrier-mediated processes. Thus, we hypothesized that monocytes also possess mechanisms allowing these cells to take up inorganic mercury (Hg2+) and/or methylmercury (CH3Hg+) as cysteine (Cys) and/or homocysteine (Hcy) S-conjugates by certain membrane transport proteins. The specific thiol S-conjugates were chosen for study because our laboratory and those of some other investigators have demonstrated that these species of mercury are indeed transportable substrates for several membrane transport proteins in certain types of epithelial cells. We chose to use RAW 264.7 cells for our experiments. These cells represent an adherent line of mouse monocytes. Kinetic analyses for the uptake of Cys-Hg-Cys, CH3Hg-Cys, Hcy-Hg-Hcy, and CH3Hg-Hcy revealed that uptake occurred by a saturable, concentration-dependent mechanism, displaying Michaelis-Menten properties. Interestingly, in the cells exposed to the Cys or Hcy S-conjugate of Hg2+, significantly more Hg2+ was taken up in the presence of 140 mM sodium chloride (NaCl) than in the presence of 140 mM N-methyl-D-glucamine (NMDG), indicating that Na-dependent processes play more of a role in the uptake of these species of Hg2+ than sodium-independent ones. With respect to the uptake of CH3Hg+, rates of uptake of the Cys and Hcy S-conjugates of CH3Hg+ were similar under both Na-dependent and Na-independent conditions, although the levels of uptake of these mercuric species far exceeded the levels of uptake of the corresponding S-conjugate of Hg2+. Uptake of Hg2+ and CH3Hg+, as the Cys or Hcy S-conjugates, was also time-dependent. We also showed that when the temperature in the bathing medium was reduced to 4 degrees C, uptake of the Cys S-conjugates Hg2+ or CH3Hg+ was for the most part reduced to negligible levels in the RAW cells; indicating that the preponderance of uptake at 37 degrees C was not due primarily to simple diffusion and/or non-specific binding. Overall, the present findings strongly suggest that the uptake of the Cys and Hcy S-conjugates of Hg2+ and/or CH3Hg+ occurs in monocytes by one or more mechanisms involving carrier proteins.

Ecogenetics of mercury: From genetic polymorphisms and epigenetics to risk assessment and decision‐making

“…It has been speculated that methionine synthase could be involved with MeHg‐associated changes to DNA methylation status ; methionine synthase is inhibited by inorganic and ethylated Hg , and the same may be true for MeHg. Inorganic Hg and MeHg may also bind with homocysteine, and these Hg‐homocysteine conjugates are biologically transpor . An epidemiological analysis of US National Health and Nutrition Examination Survey (NHANES) data suggest that blood Hg levels may be inversely associated with plasma homocysteine levels in younger males .…”

Metallomics of Mercury: Role of Thiol‐ and Selenol‐Containing Biomolecules

“…Produced from the intracellular metabolism of methionine, it can undergo transsulfuration to CysSH, or remethylation back to methionine (Selhub, 1999). Similar to its CysSH counterparts, experimental studies have shown that HcySH complexes of Hg 2+ and CH 3 Hg + can be transported across biological membranes via certain amino acid transporters and organic anion transporters Zalups, 2004, 2010;Zalups et al, 2007). Although no structural or thermodynamic data are available for these complexes, they are expected to be similar to the corresponding Hg 2+ or CH 3 Hg + complexes with CysSH.…”