Abstract
The synthesis and intracellular transport of the mitochondrial matrix enzymes ornithine transcarbamylase (carbamoylphosphate: L-ornithine carbamoyltransferase, EC 2.1.3.3.) and carbamoyl-phosphate synthetase (ammonia) I [carbon-dioxide:ammonia ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.4.16] were studied in isolated rat hepatocytes. In pulse experiments at 37 degrees C, the larger precursors of the two enzymes appeared in the cytosol of the liver cells, where radioactivity levels of the precursors reached a plateau in 10-20 min after the pulse. The pulse-labeled mature enzymes appeared in the particulate fraction (containing mitochondria) after a time lag and increased almost linearly with time up to 40 min. The specific radioactivities of the precursors in the cytosol were much higher than those of the mature enzymes in the particulate fraction. In pulse--chase experiments, the labeled precursors disappeared from the cytosol with estimated half-lives of about 1-2 min. These results indicate that ornithine transcarbamylase and carbamoyl-phosphate synthetase I are initially synthesized as larger precursors and exist in a cytosolic pool from which they are transported into mitochondria and processed there to the mature enzymes concomitantly with or immediately after transport. Although the rates of synthesis, transport, and processing were decreased about 3-fold at 25 degrees C (as compared to incubation at 37 degrees C), the pool size of the precursors in the cytosol were somewhat larger at this temperature.