Role of metallothionein in biliary metal excretion

We have studied the effect of acute induction of metallothionein on the biliary excretion of a bolus of metal in the rat. Female Sprague-Dawley rats were administered zinc chloride (6.5 mg/kg) or dexamethasone (2 mg/kg), ip in saline daily for 3 d; control animals received saline alone. Zinc causes a 17-fold induction in hepatic metallothionein levels, while dexamethasone caused a 5-fold induction. A bolus of metal chloride, either zinc, mercury, or cadmium, 1 mg/kg iv, was administered, and bile and plasma samples were collected and analyzed for metal content. At 3 h the rats were killed, livers excised, and both metallothionein and the metals associated with metallothionein estimated. Cadmium was excreted into the bile in inverse proportion to the hepatic metallothionein content, while metallothionein content did not appear to bear any relationship to biliary excretion of mercury or zinc. Metallothionein from rats pretreated with zinc contained twice as much zinc per molecule of metallothionein as that found in control rats. Cadmium, which exhibits a very high affinity for metallothionein, replaced this zinc found in association with metallothionein. Conversely, mercury only partially replaced the zinc associated with metallothionein, and a bolus of zinc was completely unable to bind to the already zinc-saturated metallothionein. Consequently, the bolus zinc was found associated with alternative cytosolic proteins, here termed the high-molecular-weight fraction. These findings support the hypothesis that the inhibitory effect of metallothionein on the biliary excretion of metals is dependent on the ability of the metal in question to replace any existing metals associated with metallothionein. Biliary metal excretion was directly proportional to the free metal content, regardless of the metal studied. Consequently, acute induction of metallothionein was inhibitory to biliary cadmium excretion, slightly inhibitory to biliary mercury excretion, and without effect on biliary zinc excretion.