Studies on the Induction of a Low-Molecular-Weight Zinc-Binding Protein in Rat Liver

than 30pg/g fresh wt., only a small proportion of the zinc was associated with the lowmolecular-weight components of fraction 111. Increases in liver zinc concentration above this value were largely accounted for by an increase in the zinc content of this fraction. No similar modifications of the zinc content of the higher-molecular-weight components (fractions I and 11) occurred in response to the above dietary treatments. Further confirmation of the effects of food restriction on liver zinc distribution was obtained in a separate study with younger (70g) rats killed 24h after suffering minor or severe restriction of intake (namely 7.8 or 2.1 g of food/day). The contributions of zinc in fraction 111 to total hepatic zinc were 11 and 19% respectively. Copper was present in the liver supernatants mainly in fraction 11, usually about 2pg/g of liver or 70-80% of the soluble copper. Only 5 % of the soluble copper was in fraction 111, except where liver zinc concentrations were high, as in food-restricted rats (16%) and ZnS0,-injected rats (up to 30%). The Cu/Zn ratio in the supernatant determined to some extent the Cu/Zn ratio in fraction 111. These findings suggest that fraction I11 may serve as a temporary store for zinc or be involved in some detoxication mechanism. However, the fate of zinc after its incorporation into the protein has not been established, and it is not known whether the zinc is subsequently excreted or is redistributed to other tissues. The association of copper with fraction I11 does not necessarily imply that it has a similar role in copper metabolism.