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Volume 271, Number 40, Issue of October 4, 1996 pp. 24534-24538
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Competition for Tetrahydrobiopterin between Phenylalanine Hydroxylase and Nitric Oxide Synthase in Rat Liver

(Received for publication, April 12, 1996, and in revised form, July 11, 1996)

Catherine M. Pastor , Debra Williams , Toshie Yoneyama , Kazuyuki Hatakeyama , Scott Singleton ^§ , Edwin Naylor ^§ and Timothy R. Billiar

From the Departments of  Surgery and ^§ Biochemical Genetics, University of Pittsburgh, Pittsburgh, Pennsylvania 15261

Tetrahydrobiopterin (BH4) is an important cofactor for two hepatic enzymes, inducible nitric oxide synthase (iNOS) and phenylalanine hydroxylase (PAH), and competition for BH4 between the two enzymes might limit hepatic iNOS or PAH activity. To test this hypothesis, we determined whether conversion of phenylalanine to tyrosine was modified by changes in NO synthase activity, and conversely whether NO synthesis was limited by the rate of phenylalanine conversion to tyrosine in rat hepatocytes and perfused livers. NO production was decreased only slightly, when flux through PAH was maximized in isolated perfused livers, and in isolated hepatocytes only when BH4 synthesis was inhibited. Increases in NO synthesis did not reduce tyrosine formation from phenylalanine. Phenylalanine markedly increased biopterin synthesis, whereas arginine had no effect. Thus, basal BH4 synthesis appears to be adequate to support iNOS activity, whereas BH4 synthesis is increased to support PAH activity.

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