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Phenylacetate metabolism is of clinical importance because its conjugation with glutamine to form phenylacetylglutamine, which can be excreted in the urine, provides an alternative pathway for nitrogen excretion in patients with urea cycle defects (James et al. 1972; Batshaw et al. 2001; Brusilow and Horwich 2001). This conjugation proceeds in two steps. First, phenylacetate and ATP react with coenzyme A to form phenylacetyl CoA, AMP, and pyrophosphate (Vessey et al. 1999). Two human CoA ligases have been characterized that catalyze this reaction efficiently in vitro: acyl-CoA synthetase medium-chain family member 1 (BUCS1) (Fujino et al. 2001) and xenobiotic/medium-chain fatty acid:CoA ligase (Vessey et al. 2003). Their relative contributions to phenylacetate metabolism in vivo are unknown. Second, phenylacetyl CoA and glutamine react to form phenyacetyl glutamine and Coenzyme A. The enzyme that catalyzes this reaction has been purified from human liver mitochondria and shown to be a distinct polypeptide species from glycine-N-acyltransferase (Webster et al. 1976). This human glutamine-N-acyltransferase activity has not been characterized by sequence analysis at the protein or DNA level, however, and thus cannot be associated with a known human protein in the annotation of phenylacetate conjugation.
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