Figure 3. Cancerous cell lines excrete less ¹⁵N-depleted arginine (Arg).

The role of Arg metabolism (urea cycle) is further demonstrated by the differences in ratios of endometabolites since Arg appears to be more retained relative to urea in cancerous cell lines (a). The extracellular-to-intracellular relative difference (D) in Arg was found to (i) be the best D value that anti-correlates to δ¹⁵N (b) and (ii) have the largest average D value across cell lines (c). In all cases, the δ¹⁵N value of the influx (substrate consumption) was close to 0‰ while the efflux (excreted N) was ¹⁵N-depleted and Arg was even more ¹⁵N-depleted in cancerous cell lines by up to ≈7‰ (d). Therefore, the relative isotopic efflux represented by Arg excretion (i.e., the quotient of Arg isoefflux to total isoefflux) was large in ZR75-1 and MCF10A and low in other cell lines (e). Asterisks: significant differences between cell lines (*P < 0.05; **P < 0.01); dashed line: δ¹⁵N of source Gln (d); lower case letters stand for statistical classes (P < 0.05) (e). EA, ethanolamine, 2HG, 2-hydroxyglutarate; PGA, phosphoglycerate; PHB, parahydroxybenzoate.