THE PHYSIOLOGY OF MASSIVE ZINC ACCUMULATION IN THE LIVER OF FEMALE SQUIRRELFISH AND ITS RELATIONSHIP TO REPRODUCTION

It is well known that zinc is an essential micronutrient and, as a rul e, organisms keep relatively constant low levels of zinc to maintain c ellular functions. The squirrelfish family (Holocentridae) is the only known exception from this rule. Squirrelfish accumulate very high con centrations of zinc in the liver. In the present study, we demonstrate that, while female squirrelfish store large amounts of zinc in the li ver and ovaries, the males show zinc levels that are typical for verte brates. The zinc content of the diet is the same in males and females, and zinc is not lost from the liver during starvation. Thus, the diff erence between genders in zinc storage is not dependent upon the diet. Rather, there are at least two processes that contribute to the accum ulation in females. First, females possess high levels of two major zi nc-binding proteins: metallothionein (MT) and a novel female-specific zinc-binding protein (FZnBP). In females, but not in males, almost all MT is present in the hepatocyte nucleus, FZnBP is exclusively found i n the hepatocyte cytosol of females. Second, hepatocytes of female squ irrelfish have a high capacity to transport zinc across the plasma mem brane. In addition to the liver, only the gonads of females showed unu sually high concentrations of zinc. Administration of exogenous oestro gen to females decreases the hepatic zinc concentration while there is a matching increase in the zinc content of the ovaries. Thus, oestrog en may trigger a redistribution of zinc from liver to ovaries. Togethe r, our findings suggest that female squirrelfish may be uniquely adapt ed to detoxify zinc and to utilize it as a macronutrient for processes related to reproduction.