Dietary Protein Levels Cause Different Effects of Methionine Supplement on the Fate of Methylmercury in Mice

The effect of supplementing methionine (1%) to a 24.8% protein diet (normal protein diet, NPD) or a 7.5% protein diet (low protein diet, LPD) on the fate of methylmercury (MeHg) was investigated after oral administration of MeHg (20 μmol/kg). Hg concentration in the brain was increased by methionine supplement to LPD, but not to NPD. Methionine supplement to both NPD and LPD resulted in increased Hg concentration in the liver but decreased Hg concentration in the kidney. Hg concentrations in the blood and plasma were decreased only by methionine supplement to LPD. Urinary Hg excretion was increased by methionine supplement to both diets, whereas no marked difference in fecal Hg excretion was observed by the supplement. Hg concentration in the plasma low molecular weight (LMW) fraction 2 h after oral administration of MeHg (20 μmol/kg) was increased by methionine supplement to LPD, but not to NPD. This suggests that the ratio of availability of sulfur amino acids for the synthesis of protein to those for the synthesis of LMW thiol compounds may be affected by dietary levels of protein and methionine. As a result, the increased Hg concentration in the plasma LMW fraction would result in higher brain Hg concentration, since the brain uptake of 14C-L-phenylalanine was not affected by methionine supplement. The present results suggest that the effects of methionine supplement on the fate of MeHg depend, at least partly, on dietary protein levels.