Synthesis of phosphoenolpyruvate carboxykinase (guanosine triphosphate) by isolated liver polyribosomes

Phosphoenolpyruvate carboxykinase guanosine triphosphate (GTP) (EC 4.1.1.32) was synthesized by postmitochondrial supernatants of rat liver in the presence of appropriate salts, an energy supply and [³H]leucine. Synthesis of enzyme released from polyribosomes was detected by immunoprecipitation with specific antibody followed by electrophoresis of the dissolved antibody antigen precipitates on sodium dodecyl sulphate polyacrylamide gels in the presence of a ¹⁴C labeled enzyme marker. Enzyme synthesis in vitro occurs predominantly on free rather than bound polyribosomes. Starved animals in which deinduction of phosphoenolpyruvate carboxykinase (GTP) had been initiated by refeeding for 2 hr had a markedly decreased rate of enzyme synthesis, whether the measurements were made after injection of radioactive leucine into the intact animal or if synthesis was determined in vitro. The low rate of enzyme synthesis by liver polyribosomes from refed animals was not due to the absence of soluble factors, nor could it be increased by the addition of cyclic adenosine monophosphate to the protein synthesis system. Phosphoenolpyruvate carboxykinase (GTP) synthesis in vitro is diminished relative to total protein synthesis when the postmitochondrial supernatant is kept at 0°C for several hr before measurement of protein synthesis. Since this effect is blocked by heparin, it is probably caused by selective ribonuclease attack on enzyme messenger RNA (mRNA). Deinduction of phosphoenolpyruvate carboxykinase (GTP) is tentatively explained as being due to a transcriptional block in specific mRNA synthesis, followed by rapid degradation of existing message.