| Abstract: | Relative synthesis of malic enzyme is stimulated 25-to 100-fold by feeding neonatal ducklings or by incubating embryonic chick hepatocytes in culture with triiodothyronine. Synthesis of the enzyme is almost completely blocked when fed birds are starved or when triiodothyronine-treated hepatocytes in culture are also treated with glucagon. Cytoplasmic poly(A)+ RNA was isolated from livers of intact ducklings or hepatocytes in culture treated as described above and translated in an mRNA-dependent rabbit reticulocyte lysate. The identity of malic enzyme synthesized in the cell-free system was confirmed by virtue of its antigenicity, subunit molecular weight, and proteolytic peptide pattern. Translatable levels of malic enzyme mRNA paralleled changes in relative synthesis of malic enzyme in vivo and in hepatocytes in culture. Translatable levels od albumin mRNA were either unaffected or changed in a direction opposite to that of malic enzyme mRNA. Thus, both nutritional and hormonal regulation of malic enzyme synthesis involves regulation of cytoplasmic translatable malic enzyme mRNA levels. The hepatocyte culture system is ideally suited for future studies on the regulation of malic enzyme mRNA synthesis and/or degradation by thyroid hormone and glucagon. |
