Nitrogen Isotope Effects in Urea Metabolism: From Biochemistry to ¹⁵N Natural Abundance in Cancer

The urea cycle (UC) is usually described as the hepatic metabolic pathway responsible for ammonia detoxification, but its role extends far beyond nitrogen (N) elimination to include cellular biosynthesis and metabolic signalling. In cancer cells, the UC is reconfigured/reorchestrated to support high anabolic demand, often involving the dysregulation of key enzymes such as ASS1, ASL, OTC and CPS1. While these changes support biomass production and stress resistance, they also generate measurable biochemical signatures through kinetic and thermodynamic isotope effects (¹⁴N/¹⁵N). In this review, we summarise UC biochemistry and recall key enzymatic mechanisms. Together, these elements provide a mechanistic framework to interpret changes in ¹⁵N abundance observed in tumour tissues and cells. We discuss how the redirection of N flux toward nucleotide and polyamine synthesis, coupled with partial excretion of ¹⁵N-depleted urea, may shape the isotopic composition of cancer cells. By integrating molecular oncology with stable isotope analysis, this review highlights the potential of natural isotope abundance as a functional readout of tumour metabolism and supports further investigation of its translational relevance in cancer phenotyping and monitoring.