Nitrogen isotope effects in urea metabolism: from biochemistry to 15 N 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 signaling.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 ( 14 N/ 15 N).In this review, we summarize UC biochemistry and recall key enzymatic mechanisms.It allows us to describe specific isotope fractionation factors associated with UC enzymatic reactions and hence explain changes in 15 N 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 15 N-depleted urea, shapes the isotopic composition of cancer cells.By integrating molecular oncology with stable isotope analysis, this review provides a mechanistic framework for understanding nitrogen partitioning in the tumour microenvironment and proposes the use of natural isotope abundance as a functional, non-invasive biomarker for cancer phenotyping and monitoring.