Induction and characterization of hepatic metallothionein expression from cadmium-induced channel catfish (Ictalurus punctatus)

Two low-molecular-weight proteins of approximately 10,000 daltons (Da) were isolated from the hepatic tissue of catfish intraperitoneally injected with CdCl₂ (5 mg/kg). Each protein was significantly induced following Cd treatment. Bound Cd, co-eluted from DEAE anion-exchange HPLC with rabbit hepatic metallothioneins I and II, did not possess UV absorption indicative of aromatic amino acids and corresponded to the expression of two mRNA transcripts of approximately 400 base pairs, which hybridized to a full-length cDNA probe to winter flounder metallothionein (MT). Because these data indicated that these two low molecular weight proteins were hepatic MT isoforms, induction of MT mRNA expression was studied in livers of channel catfish exposed to aqueous CdCl₂ utilizing the ribonuclease protection assay. Following a 7-d exposure to 10, 50, and 100 μg/L CdCl₂, expression of two MT mRNAs increased in a dose-dependent manner (MT-a, r² = 0.95; MT-b, r² = 0.98). Each transcript was maximally induced 48 h after exposure to 100 μg/L CdCl₂ and then gradually declined to approximately 20% above controls after 96 h and maintained this level of expression for 7 d. Although MT expression is rapidly induced in channel catfish by Cd, and MT mRNA measurement is a sensitive indicator of MT induction, the use of hepatic MT of channel catfish may not be a sensitive bioindicator of short-term, low-level aqueous exposures of Cd.