Ron,
This is my guess. I think phenylalanine when accumulated or in excess in the blood is toxic to body as seen in PKU patients. I suspect drug developers didnt want to take a chance with Phe and the next step in biosynthesis seemed good enough.
Quote:
Journal of Cerebral Blood Flow & Metabolism (1998) 18, 1184–1191; doi:10.1097/00004647-199811000-00004
Blood-Brain Barrier Phenylalanine Transport and Individual Vulnerability in Phenylketonuria
Harald E Möller, Josef Weglage*, Dirk Wiedermann† and Kurt Ullrich‡
1. Center for In Vivo Microscopy, Duke University Medical Center, Durham, North Carolina, U.S.A.
2. *Department of Pediatrics, University of Münster, Münster, Germany
3. †Department of Physical Chemistry, University of Münster, Münster, Germany
4. ‡Department of Pediatrics, University Hospital Eppendorf, Hamburg, Germany
Correspondence: Harald E Möller, Center for In Vivo Microscopy, Duke University Medical Center, P.O. Box 3302, Durham, NC 27710 U.S.A.
Phenylketonuria (PKU), caused by a deficiency of the liver enzyme phenylalanine hydroxylase, which converts phenylalanine (Phe) to tyrosine (Tyr), is the most frequent inborn error of amino acid metabolism with an incidence of 1:8,000 in whites.
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http://www.nature.com/jcbfm/journal/v18/n11/index.html
© 2009 International Society for Cerebral Blood Flow & Metabolism