The oxidation of 6-methyltetrahydropterin and tetrahydrobiopterin coupled to the formation of tyrosine by phenylalanine hydroxylase generates a precursor species to the quinonoid product that is tenatively identified as a 4a-hydroxy adduct based on its spectral similarity to the 4a-hydroxy-6-methyl-5-deazatetrahydropterin. The rate of appearance of this intermediate and that of tyrosine are equal and hydroxylase catalyzed in accord with the completion of the hydroxylation event. This observation, which confirms and extends an earlier one by Kaufman [Kaufman, S. (1975) in Chemistry and Biology of Pteridines (Pfleiderer, W., Ed.) p 291, Walter de Gruyter, Berlin], serves to link the reaction courses followed by pterin and pyrimidine cofactor analogues and supports the hypothesis that the 4a position is a site of 02 attachment. Thus, as expected, no prereduction of the enzyme was observed in anaerobic experiments utilizing stoichiometric amounts of enzyme and tetrahydropterin in the presence or absence of 1 mM phenylalanine. Activation of the hydroxylase by 1 mM lysolecithin leads to oxidation of the tetrahydropterin in the absence of phenylalanine. A ring-opened pyrimidine analogue of the tetrahydropterin, 2,5-diamino-4- [(meso-1 -methyl-2-aminopropyl)amino]-6-hydroxypyrimidine, was studied to examine the possibility of tetrahydropterin ring opening in the enzymatic reaction prior to 4a-hydroxy adduct formation. However, no hydroxylasecatalyzed ring closure was observed.
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