The present technology provides 13C- and 15N-labeled probes for imaging one or more mammalian cells using magnetic resonance. Thus, 13C- and 15N-labeled arginine (compound of formula I), xanthine (compounds of formula II and formula III), urea (compounds of formula IV), and glutamine (compounds of formula V), stereoisomers, tautomers, and pharmaceutically acceptable salts thereof are provided. Further methods of making the labelled probes and methods of using the probes to detect arginase, xanthine oxidase, and glutaminase metabolites and activity are provided.
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