Significance of metallothioneins in metal accumulation kinetics in marine animals

Marine animals can induce metallothioneins (MTs) in their responses to exposure to certain trace metals in the environment. MTs generally function as metal storage/detoxification or homeostatic regulation of both essential and non-essential metals. This review discusses the important roles of MTs in metal biokinetics other than metal detoxification and homeostasis in marine animals. Recent studies have revealed the complicated cellular and biochemical processes involving intracellular ligands (cytosolic proteins and insoluble deposits) during metal uptake and elimination. The responses of metal biokinetics to MT induction are metal- and organism-specific. Depending on the different marine animals and metals, all biokinetic parameters such as dissolved metal uptake rate, dietary assimilation efficiency and elimination (efflux) rate can be significantly impacted by MT induction. Among the different metal biokinetic parameters, dietary assimilation efficiency and elimination rate appear to be most impacted by MT induction. MT turnover kinetics can also significantly affect metal uptake kinetics, but again, such influence is also dependent on the organism, particularly its predominant pathway of metal detoxification. Even though the total MT pool in aquatic animals may remain constant, the turnover of MTs, involving MT synthesis and breakdown, can potentially lead to a major change of metal accumulation biokinetics. We propose several issues that need to be further addressed in studying the interaction between MT induction and metal accumulation biokinetics.