SPECIES VARIATION IN HEPATIC METALLOTHIONEIN

Metallothionein (MT) is a low-molecular-weight protein involved in the homeostasis of endogenous metals and in the detoxication of heavy met als. In humans, the levels of hepatic MT have been shown to be up to 1 00 times the levels found in rat and mouse liver. In order to further investigate this species difference in hepatic MT levels, hepatic MT w as quantified in 15 species (human, monkey, dog, cat, cow, pig, sheep, goat, rabbit, chicken, hamster, rat, mice, guinea pig, and frog). Fre sh liver was obtained from each species and MT was quantified try 2 di fferent metal-saturation assays. Result from the Cd-heme and Ag-heme a ssays showed that human, dog, cat, pig, and goat had the highest hepat ic MT levels (400-700 mu g/g liver). Monkey, cow and sheep had moderat e hepatic MT levels (about 200 mu g/g liver), while rodent (mouse, rat , hamster, guinea pig, and rabbit) had low hepatic MT levels (2-10 mu g/g liver). Hepatic MT levels in non-mammals (chick and frog) were sli ghtly higher than rodents (about 20 mu g/g liver). Sephadex G-75 colum n elution volumes ranged from 1.7 to 1.8, which implies that MT from a ll species had approximately the same molecular weight and similar str ucture. Copper and zinc concentration in the cytosols were measured by atomic absorption spectrophotometry. Dog and cat had the highest leve ls of Cu (86 and 50 mu g/g liver, respectively), and pig and hamster w ere lowest (about 10 mu g/g liver). Human, dog, cat, and goat had the highest levels of zinc (approximately 40-50 mu g/g liver) while hamste r and guinea pig were fewest (approximately 15 mu g/g liver). The resu lts show that there is a marked species difference in hepatic MT conce ntrations with dog, cat, and human having the highest levels.