Degradation and metal composition of hepatic isometallothioneins in rats

Recently, our laboratory demonstrated that metallothionein-1 (MT-1) is degraded faster than metallothionein-2 (MT-2) in liver of Zn-treated adult rats; however, it is not clear whether this phenomenon is unique to Zn treatment or the age of the animal. Furthermore, many investigators maintain that the degradation of MT is regulated by its metal composition. The objective of this study was twofold: (1) to determine if MT-1 is more susceptible than MT-2 to proteolytic breakdown regardless of age or chemical pretreatment and (2) to examine the hypothesis that the amount and type of metals bound to MT influences its resistance to degradation. Pulse-labeling experiments were conducted to determine the half-lives of MT-1 and MT-2 in liver of adult rats (75-day-old), immature rats (1-day-old), and mature rats treated with single dosages of Zn (1 mmol/kg, sc), Cd (10 mumol/kg, sc), or ethanol (109 mmol/kg, po). Atomic absorption spectrometry was utilized to measure the Zn, Cu, and Cd contents of MT-1 and MT-2 obtained in selected experimental groups. MT-1 had a shorter half-life than MT-2 in Zn-treated adults (21 vs 33 hr) and in nontreated immature rats (49 vs 73 hr). In contrast, the half-life values of MT-1 and MT-2 were identical in nontreated adults (4 hr) and ethanol-treated adults (9 hr) and nearly identical in Cd-treated adults (58 and 61 hr, respectively). Both isoforms obtained from immature rats and adults treated with Zn or ethanol contained approximately 6.0 g atoms Zn/mol MT, trace levels of Cu, and nondetectable quantities of Cd. In Cd-treated rats, both isoforms contained approximately equal amounts of Zn and Cd (3.2 g atoms metal/mol MT) and trace levels of Cu. These results indicate that MT-1 is either as susceptible or more susceptible than MT-2 to intracellular degradation depending on age or chemical pretreatment. Furthermore, factors unrelated to the metal composition of MT appear to regulate the degradation of MT-1 and MT-2.