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 (n ormal protein diet, NPD) or a 7.5% protein diet (low protein diet, LPD ) on the fate of methylmercury (MeHg) was investigated after oral admi nistration of MeHg (20 mu mol/kg). Hg concentration in the brain was i ncreased by methionine supplement to LPD, but not to NPD. Methionine s upplement to both NPD and LPD resulted in increased Hg concentration i n the liver but decreased Hg concentration in the kidney. Hg concentra tions in the blood and plasma were decreased only by methionine supple ment to LPD. Urinary Hg excretion was increased by methionine suppleme nt to both diets, whereas no marked difference in fecal Hg excretion w as observed by the supplement. Hg concentration in the plasma low mole cular weight (LMW) fraction 2 h after oral administration of MeHg (20 mu mol/kg) was increased by methionine supplement to LPD, but not to N PD. 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 fractio n would result in higher brain Hg concentration, since the brain uptak e of C-14-L-phenylalanine was not affected by methionine supplement. T he present results suggest that the effects of methionine supplement o n the fate of MeHg depend, at least partly, on dietary protein levels.