Carbamoyl phosphate compartmentation in Neurospora: histochemical localization of aspartate and ornithine transcarbamoylases.

Carbamoyl phosphate is required for arginine and pyrimidine synthesis. In the arginine pathway, it is used in the ornithine transcarbamoylase (EC 2.1.2.1) reaction; in the pyrimidine pathway, it is used in the aspartate transcarbamoylase (EC 2.1.3.2) reaction. In Neurospora crassa, two pathway-specific enzymes catalyze the synthesis of carbamoyl phosphate, and two path-specific pools of carbamoyl phosphate are maintained. Histochemical studies show that ornithine transcarbamoylase is located in mitochondria, and, with less certainty, that aspartate transcarbamoylase is confined largely to nuclei. The enzymes that form carbamoyl phosphate are associated with the respective transcarbamoylases in the cell. Therefore, the segregation of carbamoyl phosphate pools could be accounted for by one or both organellar membranes, which demarcate two separate sites of carbamoyl phosphate metabolism in Neurospora. The alternative possibility that the enzyme complex that produces and consumes carbamoyl phosphate in the pyrimidine pathway could explain the channelling of carbamoyl phosphate, wholly or in part, is discussed.