Influence of dietary levels of protein and sulfur amino acids on the fate of methylmercury in mice.

We previously reported that the fate of methylmercury (MeHg) in mice was affected by dietary protein levels. To study the mechanism of this alteration, we investigated the effect of sulfur amino acid supplement for a lowered protein diet on the fate of MeHg. C57BL/6N male mice were fed on a 24.8% protein diet (normal protein diet, NPD), a 7.5% protein diet (low protein diet, LPD), or LPD supplemented by methionine and cystine so as maintain the normal levels (amino acid supplemented diet, ASD) for 5 days. NPD-fed mice were used as controls. The mice were orally administered MeHg chloride (20 mumol/kg), and were examined after 24 h distribution and excretion of Hg. The Hg level in brain increased with LPD feeding and was further enhanced by ASD feeding. The hepatic Hg level increased only with ASD feeding. Although Hg levels in kidney, blood and plasma did not change with LPD feeding, these decreased with ASD feeding. The urinary Hg level that decreased with LPD feeding was recovered and exceeded by far the control levels with ASD feeding. When mice were intravenously injected with MeHg-bovine serum albumin, the Hg uptake rate in the brain increased in LPD-fed mice and was further enhanced in ASD-fed mice. The brain uptake of intravenously injected L-[14C]phenylalanine was also accelerated with LPD or ASD feeding, which indicated that LPD or ASD feeding increased activity of neutral amino acid transport in the brain. This would cause increased Hg uptake in the brain, since MeHg reaches the brain through this transport system. Hg ratio in plasma low molecular weight fraction increased in ASD-fed mice, but not in LPD-fed mice. This might contribute to the further enhanced Hg uptake in the brain with ASD feeding. Analysis of thiol compounds in plasma and urine revealed increased levels with ASD feeding. The present results suggest that insufficiency of sulfur amino acids in LPD is one reason for the alteration in the fate of MeHg induced by LPD feeding. It is also suggested that the change in neutral amino acid transport caused by LPD feeding is involved in the alteration in the fate of MeHg.