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Identification of N-ß-L-Aspartyl-L-Phenylalanine as a Normal Constituent of Human Plasma and Urine

Earl G. Burton, Grant L. Schoenhard, Julie A. Hill, R. Eric Schmidt, Jeremy D. Hribar, Frank N. Kotsonis^* and James A. Oppermann

Department of Drug Metabolism Department of Physical Methodology, G. D. Searle & Co., Research and Development Division, Skokie, IL 60077 ^* Preclinical and Clinical Research, The NutraSweet Company, Deerfield, IL 60015

A new ß-aspartyl dipeptide, N-ß-L-aspartyl-L-phenylalanine (ß-AP), has been isolated and identified in urine and plasma from normal human volunteers. ß-AP was isolated from urine samples by high performance liquid chromatography (HPLC). Its identity and stereochemistry were demonstrated by HPLC and gas chromatography/mass spectrometry (GC-MS). The mean urinary ß-AP concentration in the subjects was 0.63 ± 0.14 µg/mg creatinine when averaged over two consecutive days of urine collection. Daily ß-AP excretion, determined from two 24-h urine samples collected from five individuals, was 801 ± 117 µg/d (2.7 µmol/d). No diurnal rhythm was evident within the 24-h collection periods. ß-AP was also isolated from human plasma by HPLC and identified by GC-MS. Plasma from subjects contained approximately 5 ng ß-AP/ml. Furthermore, ß-AP was formed when asparagine and phenylalanine were incubated with an enzyme extract from human kidney. Thus, at least some of the ß-AP present in humans, and presumably other ß-aspartyl dipeptides as well, appears to be synthesized endogenously.

KEY WORDS: • ß-aspartyl dipeptides • human urine • human plasma • endogenous synthesis

Manuscript received 10 January 1989. Revision accepted 10 February 1989.