Assay of the concentration and 13C-labeling pattern of phenylacetylglutamine by nuclear magnetic resonance.

Phenylacetate, derived from phenylalanine, is converted in human and primate liver to phenylacetylglutamine. The latter, which is excreted in urine, has been used to probe noninvasively the labeling pattern of liver citric acid cycle intermediates. We present nuclear magnetic resonance assays for the urinary concentration of phenylacetylglutamine and for the 13C-labeling pattern of its glutamine moiety. The concentration of phenylacetylglutamine is calculated from the natural 13C signals of all carbons of its benzene ring and C-2 of its acetyl moiety. The limit of detection is 13 mumol of unlabeled phenylacetylglutamine. The minimum amount of phenylacetylglutamine needed to determine a 1% enrichment of one of its carbons is 26 mumol. The technique was tested by analyzing phenylacetylglutamine in the urine from monkeys infused with various 13C tracers. The labeling patterns obtained agreed with theoretical calculations and patterns reported in phenylacetylglutamine and glutamine labeled from 14C and 13C tracers, respectively.