J. Biol. Chem., Vol. 263, Issue 33, 17312-17316, Nov, 1988
The tyrosine-dependent oxidation of tetrahydropterins by lysolecithin- activated rat liver phenylalanine hydroxylase
MD Davis and S Kaufman
Laboratory of Neurochemistry, National Institute of Mental Health, Bethesda, Maryland 20205.
In the presence of tyrosine, phenylalanine hydroxylase, which has been
activated with lysolecithin, catalyzes the oxidation of tetrahydrobiopterin
at a rate 10-20% that of the parallel reaction with phenylalanine. Unlike
the reaction with phenylalanine, there is no net concomitant hydroxylation
of tyrosine, although the amino acid is still a necessary component.
Tyrosine appears to form an abortive complex with the activated enzyme, the
pterin cofactor and molecular oxygen. The Km for tetrahydrobiopterin is
identical for the reactions with phenylalanine and tyrosine, whereas the Km
for tyrosine is approximately 3 1/2 times greater than the Km for
phenylalanine. The tyrosine-dependent oxidation of tetrahydrobiopterin
proceeds at both pH 6.8 and 8.2 and shows a similar dependence on the pH as
that of the physiological reaction. Tetrahydrobiopterin can be replaced by
the artificial cofactor, 6-methyltetrahydropterin, in the tyrosine-
dependent oxidation at both pH 6.8 and 8.2. As in the parallel reaction
with phenylalanine, both the Km for the cofactor and the Km for the
aromatic amino acid increase with this substitution.
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