Alterations in hepatocytes after manipulation of the diet: copper, zinc and cadmium interactions

Roles of metallothionein and related proteins in metal metabolism and toxicity: problems and perspectives

This summary examines some of the known and hypothesized roles of metallothionein and related proteins in mediating the metal metabolism and toxicity from a chemical perspective. It attempts to examine in kinetic terms how such molecules may exert homeostatic control over the intracellular bioavailability of metal ions to essential enzymatic or other molecular systems. The concept of ongoing competition between metallothionein and related proteins with other intracellular metal-binding sites for various metals is also examined in relation to the thermodynamic stability of these proteins. Comparisons between mammalian metallothionein and analogous nonmammalian proteins demonstrate both similarities and great differences in types of metal-binding sites, metal-binding constants, amino acid composition, and secondary structures such that apparent diversity of these low molecular weight metal-binding molecules in nature appears to be growing ever wider. The potential value of these data rests both in delineating new hypotheses for metallothionein evolution and in suggesting new model systems for discovering the normal function of metallothionein and related proteins in cells.

On the sensitivity of metallothioneins to oxidation during isolation

It is demonstrated that the distribution of metals among the Sephadex G-75 fractions of rat liver and horse kidney supernatant is altered by exposure to oxidizing conditions. In particular, the metals bound to metallothionein are displaced into high-molecular-weight fractions and, to a lesser extent, into the low-molecular-weight forms, under aerobic conditions. In this process, metallothionein zinc is much more labile than cadmium. An appreciable proportion of the thionein is also found in the high-molecular-weight fractions and can be recovered along with the metals by treatment with mercaptoethanol. This result shows that the distributions obtained aerobically with large cadmium content in the high-molecular-weight fractions are an artefact due to metallothionein oxidation and suggests that 'spillage' of metals such as cadmium may be due in large part to oxidative processes rather than saturation effects. Evidence is presented that disulphide-bond formation occurs as thionein becomes bound in the high-molecular-weight region and that chemical reduction is necessary to restore its normal elution behaviour. Mercaptoethanol added to the homogenates maintains the reducing conditions normally found in the cellular milieu and prevents the oxidation of the metallothionein redistribution of the metals during isolation. Under these conditions the rat liver metallothionein isolated from animals exposed to chronic low concentrations of cadmium in drinking water contains appreciable quantities of copper as well as zinc and contains much of the zinc that is present in horse kidney supernatants. Metallothionein can also be extracted from a 40 000g pellet after sonication of the pellet. Thus careful analytical studies of the sites of cadmium deposition in rat liver indicate that greater than 95% is bound to metallothionein.