Phenylacetyl-L-glutamine is formed in the liver of primates and humans by condensation of glutamine with phenylacetyl-CoA and can be detected by HPLC or GC-MS in the urine. Furthermore, it is a component of 3-phenylacylamino-2,6-piperidinone (PAP), a naturally occurring peptide derivative used for inhibiting growth of cancer tissues, and can be used as reference compound in the enantioseparation of PAP.