1. The main mechanisms involved in heavy metal cation homeostasis in m
arine invertebrate cells are described. 2. Metallothioneins are probab
ly the most important soluble compounds involved in heavy metal cation
homeostasis. The biochemical characteristics of these metalloproteins
and the relationship between their amino acid composition and heavy m
etal binding capacity are elucidated. Moreover data are reported conce
rning the physiological role of metallothioneins e.g. heavy metal deto
xification, cellular metal redistribution, free radical scavenger acti
on, etc. The possible meaning of different soluble compounds in heavy
metal cation homeostasis is also discussed. 3. The biochemical role of
lysosomes in heavy metal cation compartmentalization and the involvem
ent of these organelles in metallothionein accumulation and sequestrat
ion are shown. 4. Data are reported concerning the sequestration of he
avy metal cations in insoluble granules as a mechanism of metal detoxi
fication.