Analysis of hepatic copper, zinc, metallothionein and metallothionein-Ia mRNA in developing sheep.

The concentrations of zinc, copper, metallothionein and metallothionein-Ia mRNA in sheep livers during development was determined. It was found that early sheep foetuses (30-40 days gestation) had very high concentrations of hepatic zinc (2305 +/- 814 micrograms/g dry mass), and that these levels declined steadily to 644 +/- 304 micrograms/g near to term. The copper concentrations in the foetal livers were not higher than those in the adult. The concentrations of metallothionein and metallothionein-Ia mRNA were also very high in the foetal livers and declined steadily during gestation from 261 +/- 94 molecules/pg RNA to 71 +/- 18 molecules/pg near to term. Metallothionein-Ia mRNA concentrations were closely correlated with hepatic zinc concentrations but not with copper. Metallothionein concentrations also decreased during gestation: e.g. 3044 micrograms/g (wet mass) in one foetus on day 34 of gestation to 862 micrograms/g on day 125. After birth, however, the concentrations of metallothionein declined to less than 100 micrograms/g and this decline occurred despite the presence of significant quantities of mRNA. The ratio of metallothionein/metallothionein-Ia mRNA decreased from 1.3 to 3.2 x 10(5) molecules metallothionein/molecule of metallothionein-Ia mRNA during gestation to between 0.28-0.64 x 10(5) molecules/molecule in the postnatal animals. We conclude that the major function of metallothioneins in the foetal liver is protection of the liver against the potentially toxic accumulation of zinc. In the postnatal sheep there appears to be a decreased synthesis or increased degradation of metallothionein.