Import of carbamylphosphate synthetase and ornithine transcarbamylase into mitochondria of rat liver: detection of aggregates of enzyme in cytoplasm and mitochondria using immunoelectron microscopy with the protein A-gold method.

We investigated the distribution of the nuclear encoded mitochondrial enzymes, carbamylphosphate synthetase (CPS; EC 6.3.4.16) and ornithine transcarbamylase (OTC; EC 2.1.3.3) in liver by immunocytochemistry on ultrathin sections using the protein A-gold technique. Both enzymes were found to be present as aggregates in the cytoplasm of hepatocytes, in association with ER membranes adjacent to mitochondria. Clusters of the enzymes were also found inside the mitochondria. The aggregation of these enzymes was found only with antibodies to CPS and OTC and not with antibodies against albumin or with IgG from unimmunized serum, nor were aggregates found in cells other than hepatocytes. The results are suggestive of localized uptake of clusters of enzyme or co-translational uptake of enzyme at discrete localizations and that endoplasmic reticulum (ER) associations may be necessary for uptake of the percursor forms of CPS and OTC. The possible involvement is discussed of micropinosomes which are seen associated with inner membrane, intermembrane space and outer membrane in mitochondria obtained from a perinuclear pellet where ER and mitochondria are frequently found in close association.