Stereoselective effects in the interactions of pterin cofactors with rat-liver phenylalanine 4-monooxygenase

The (6R) and (6S) epimers of l-erythro-tetrahydrobiopterin (BH4) and some of its structural analogs, were tested as cofactors and non-covalent effectors in the phenylalanine 4-monooxygenase (phenylalanine hydroxylase, EC 1.14.16.1) reaction. The oxidation-reduction potentials (Em,7) of the free (not enzyme-bound) form of the (6R) and (6S) epimers were rather similar (range 174-184 mV) for the oxidation of tetrahydropterins to quinonoid dihydropterins. Rapid-mixing kinetic experiments were performed at 20 degrees C under conditions which allow only a few turnover reactions of the enzyme. Three main oxidation products were identified spectroscopically at pH 6.8 for all three tetrahydropterins tested: the C(4a)-hydroxy derivatives, the quinonoid dihydropterins, and the stable 7,8-dihydropterins (in that sequence). The formation of the C(4a)-hydroxy forms closely paralleled that of tyrosine, and supports the proposal that this covalent adduct is formed as an immediate product on completion of the catalytic cycle. Assay of the initial rate of C(4a)-hydroxy derivative formation represents a new approach in kinetic studies of this enzyme, and the kinetic parameters obtained for the phenylalanine-activated enzyme are presented. The affinity of binding of (6R)-BH4 and (6S)-BH4 to phenylalanine hydroxylase was also estimated on the basis of their quenching of the intrinsic tryptophan fluorescence of the enzyme. The apparent affinities were found to correspond well to the Km values estimated in kinetic studies of the hydroxylation reaction with the phenylalanine activated enzyme, i.e. higher for (6R)-BH4 than for (6S)-BH4. The lower V value observed for the native enzyme with the (6R) epimer in steady-state kinetics is explained by its higher potency as a negative effector, since the oxidation-reduction potentials of the two diastereomers were similar. Dihydrobiopterin (BH2) was found to inhibit the hydroxylation reaction and quenched the intrinsic tryptophan fluorescence of the enzyme with the same concentration dependence as that observed with (6S)-BH4.