Regulation of malic enzyme synthesis by thyroid hormone and glucagon: inhibitor and kinetic experiments.

Synthesis of malic enzyme was rapidly and markedly stimulated by the addition of triiodothyronine to chick embryo liver cells in culture. Alpha-Amanitin, an inhibitor of DNA-dependent RNA polymerase II, blocked induction. The kinetics of induction and de-induction of malic enzyme synthesis suggested that the most stable event in triiodothyronine induction had a half-life of 18 to 20 h. However, malic enzyme synthesis decayed with a half-life of 2,4 h when transcription was inhibited with alpha-amanitin. Thus a long-lived event in thyroid hormone stimulation of malic enzyme synthesis occurred prior to transcription of a specific messenger RNA (mRNA), presumably malic enzyme mRNA. Malic enzyme synthesis decayed with a half-life of about 2 h when glucagon was added to pre-induced liver cells. The similarity of decay rates after inhibition of transcription with alpha-amanitin or inhibition of malic enzyme synthesis by glucagon suggests that glucagon may inhibit the transcription or processing of a specific mRNA required for malic enzyme synthesis.