Individual Difference of Methylmercury Metabolism in Animals and Its Significance in Methylmercury Toxicity

Abstract

Various factors affect the absorption, distribution, biotransformation, excretion of methylmercury (MeHg), and consequently its toxicity. The most important is the influence of genetically determined factors such as species, sex, and the molecular structure of binding substances. Environments, e. g. nutrition, coexisting metals, etc., are also important for MeHg toxicity. It is well known that the greater part of MeHg exists in erythrocytes in the form bound to hemoglobin. The molecular structures of hemoglobin shows the most remarkable effect on the binding of MeHg. The number and position of cysteinyl residues in the hemoglobin determine the affinity of hemoglobin to MeHg and the distribution of MeHg in an erythrocyte. The role of molecular structures of the binding substances of MeHg in the brain and other organs remains to be elucidated in the future.

Sex has a role on the tissue distribution and half-time of MeHg. Usually female mice showed higher tissue MeHg levels and longer half-times than male mice when they were given the same dose of MeHg per body weight. The role of the sex hormone to the organ distribution of MeHg has been reported by Hirayama et al. (1987). The age of the animals also has a role on the distribution of MeHg in mice. Usually young mice have a lower blood MeHg level than the older mice at the same dose.

Physiological conditions, such as local blood flow and erythrocyte count have some effect on the distribution and elimination of MeHg in the blood, brain and liver. Polyerythrocytemic mice due to long term exposure to CO, showed a higher organ MeHg distribution and a shorter half-time of MeHg. Thus local ischemia, anemia or polycytemia may affect the tissue distribution of MeHg.

The individual differences of MeHg metabolism in man remain to be studied (Al-Shahristani and Shihab, 1974).