The formation of tyrosine from phenylalanine catalyzed by rat liver phenylalanine hydroxylase is coupled to the generation of a 4a-hydroxy adduct from the requisite tetrahydropterin cofactor. As indicated by its circular dichroism (CD) spectrum, the optical activity of the adduct generated from racemic 6-methyltetrahydropterin requires stereoselectivity of the oxygenation. The absolute configuration of this new stereocenter is 4a(S)-hydroxy-6(RS)-methyltetrahydropterin by analogy to the CD spectrum of one of the four stereoisomers of 5-deaza-4a-hydroxy-6-methyltetrahydropterin. The source of the 4a-hydroxy oxygen is 02, as demonstrated by the observation of a 18O-induced 13C shift in the 13C NMR spectrum of the adduct when generated from [4a-13C]-6-methyltetrahydropterin and 1802.
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