Fig 3. Anabolic and catabolic reactions from ¹³C₆ ¹⁵N₄-arginine (Arg-10) leading to formation of “heavy” ammonium and “heavy” bicarbonate, and resynthesis of ¹³C₅ ¹⁵N₂-arginine (Arg-7).

Two S. pombe arginases, Car1 and Aru1, catalyze the conversion of Arg-10 into ¹³C₅ ¹⁵N₂-ornithine (“heavy” ornithine; “heavy” atoms are shown in red). This also produces “heavy” urea, which can be hydrolyzed by urease Ure1 to produce “heavy” ammonia and “heavy” carbon dioxide (shown here as ammonium and bicarbonate ions, respectively), which can subsequently be incorporated into additional amino acids. Ornithine can be converted back into arginine through citrulline and arginino-succinate intermediates, ultimately leading to formation of Arg-7.