Arginine as a Precursor of Pyrimidines in Strain L-929 Fibroblasts Infected with Pleuropneumonia-like Organisms.

All metazoan cells in culture require either arginine or citrulline for growth (1, 2). The importance of arginine is emphasized by the fact that it is rapidly depleted from the medium in many primary and established cell strains (3). The L-929 fibroblasts maintained in this laboratory and shown to harbor a pleuropneumonia-like infection also demonstrate a.n elevated requirement for this amino acid. This information, plus the facts that ornithine failed to replace t,he arginine requirement and that arginine was converted to ornithine without equivalent urea formation, prompted us to evaluate the role of the urea cycle in the metabolism of these cells. The purpose of this investigation was to determine the metabolic fates of arginine, urea, and ornithine, and to assess the enzymatic activities of arginase, arginine deiminase, carbamyl phosphate synthetase, and ornithine transcarbamylase. Experiments reported here indicate that the guanidino carbon of arginine provides a C-2 carbon source for pyrimidines.