Compartmental behavior of ornithine in Neurospora crassa

In Neurospora cells grown on minimal medium, most of the large ornithine pool is found in osmotically sensitive organelles, the "vesicles." In this paper kinetic studies on the compartmental behavior of ornithine and its derivatives are reported. Analysis of the metabolism of a 10(-7) M pulse of uniformly labeled L-[14C] ornithine supports the following conclusions: (a) Over 98% of the cellular ornithine is in the vesicles. (b) The amount of ornithine normally in the cytosol is about 0.3% of the cellular ornithine, as shown by the kinetics of incorporation of 14C into putrescine via the cytosolic enzyme, ornithine decarboxylase (EC 4.1.1.17). (c) Mitochondria, the site of ornithine synthesis, contain about 1% of the cellular ornithine, as demonstrated by the kinetics of incorporation of 14C into citrulline via the mitochondrial enzyme, ornithine transcarbamylase (EC 2.1.3.3). (d) Considerable ornithine exchange, and a net efflux of ornithine, takes place across the mitochondrial membrane. (e) Ornithine aminotransferase (EC 2.6.1.13), a catabolic enzyme, may have a special relation to the cell membrane in cells grown in minimal medium. This enzyme uses ornithine efficiently while it enters from the medium, but very poorly after all the [14C] ornithine is within the cell. (f) Citrulline and proline are not compartmented with respect to the enzymes using them. (g) In contrast, arginine is distributed such that over 99% is in vesicles. We suggest that the vesicles; with their ability to sequester ornithine and arginine, are potentially significant in regulation.