Processing of a putative precursor of rat liver ornithine transcarbamylase, a mitochondrial matrix enzyme.

A putative precursor of rat liver ornithine transcarbamylase [EC 2.1.3.3] which was about 3,400 daltons larger than the subunit of the mature enzyme (36,000 daltons) was synthesized in a rabbit reticulocyte cell-free system and immunoprecipitated using an antibody against the bovine enzyme and fixed Staphylococcus aureus cells. The mature enzyme of rat liver competed effectively with the putative precursor for interaction with the antibody. Digestion of the putative precursor by S. aureus protease gave a pattern of peptide fragments similar to that of the mature enzyme. A rat liver mitochondrial preparation converted the putative precursor to a polypeptide which comigrated with the mature subunit on sodium dodecyl sulfate/polyacrylamide gels. The "processed" product was recovered in sedimented mitochondria and was no longer susceptible to externally added proteases. These results indicate that the enzyme is synthesized as a larger precursor which may be imported into mitochondria in association with post-translational proteolytic processing to the mature form of the enzyme.