Ornithine transcarbamylase (
ornithine carbamoyltransferase, EC 2.1.3.3), the second
enzyme of
urea synthesis, is localized in the matrix of liver mitochondria of ureotelic animals. The
enzyme is encoded by a nuclear gene, synthesized outside the mitochondria, and must then be transported into the organelle. The rat liver
enzyme is initially synthesized on membrane-free polysomes in the form of a larger precursor with an amino-terminal extension of 3 400-4 000 daltons. In rat liver slices and isolated rat hepatocytes, the pulse-labeled precursor is first released into the cytosol and is then transported with a half life of 1-2 min into the mitochondria where it is proteolytically processed to the mature form of the
enzyme. The precursor synthesized in vitro exists in a highly aggregated form and has a conformation different from that of the mature
enzyme. The precursor has an isoelectric point (pI = 7.9) higher than that of the mature
enzyme (pI = 7.2). The precursor synthesized in vitro can be taken up and processed to the mature
enzyme by isolated rat liver mitochondria. The mitochondrial transport and processing system requires membrane potential and a high integrity of the mitochondria. The transport and processing activities are conserved between mammals and birds or amphibians and is presumably common to more than one precursor.
Potassium ion,
magnesium ion, and probably a cytosolic
protein(s), in addition to the transcarbamylase precursor and the mitochondria, are required for the maximal transport and processing of the precursor. A mitochondrial matrix
protease which converts the precursor to a product intermediate in size between the precursor and the mature subunit has been highly purified. The
protease has an estimated molecular weight of 108 000 and an optimal pH of 7.5-8.0, and appears to be a
metal protease. The
protease does not cleave several of the
protein and
peptide substrates tested. The role of this
protease in the precursor processing remains to be elucidated. Rats subjected to different levels of
protein intake and to fasting show significant changes in the level of
enzyme protein and activity of
ornithine transcarbamylase. The dietary-dependent changes in the
enzyme level are due mainly to an altered level of functional
mRNA for the
enzyme. In contrast, during fasting, the increase in the
enzyme level is associated with a decreased level of translatable
mRNA for the
enzyme. Pathological aspects of
ornithine transcarbamylase including the
enzyme deficiency and reduced activities of the
enzyme in
Reye's syndrome are also described. A possibility that impaired transport of the
enzyme precursor into the mitochondria leads to a reduced
enzyme activity, is proposed.