Davis M D, Kaufman S
Laboratory of Neurochemistry, National Institute of Mental Health, Bethesda, Maryland 20892.
Arch Biochem Biophys. 1993 Jul;304(1):9-16. doi: 10.1006/abbi.1993.1315.
Phenylalanine hydroxylase, a tetrahydrobiopterin (BH4)-dependent oxygenase, catalyzes the conversion of phenylalanine to tyrosine. During this physiological reaction, the oxidation of BH4 is tightly coupled to the hydroxylation of the amino acid substrate with a stoichiometry of 1 mol of BH4 oxidized for every mole of tyrosine formed. In the presence of an activator and certain analogues of either the amino acid or the pterin coenzyme, the enzyme also catalyzes a partially uncoupled reaction in which the amount of tetrahydropterin oxidized exceeds the amount of amino acid hydroxylated. With tyrosine as the amino acid analogue, the reaction is completely uncoupled, i.e., the enzyme functions as a tetrahydropterin oxidase with no net hydroxylation of the amino acid. We have investigated whether the normal pterin intermediate, a 4a-hydroxytetrahydropterin, is formed during the completely uncoupled reaction and whether oxygen is reduced exclusively to the level of water, as in the tightly coupled reaction, or whether some reduction to hydrogen peroxide occurs. We present direct evidence that both the normal hydroxytetrahydropterin intermediate and hydrogen peroxide are formed. The amount of hydrogen peroxide formed, however, is not stoichiometric with the amount of oxygen reduced, an indication that there are at least two pathways for this reduction, one leading to the formation of water and one to hydrogen peroxide. We also postulate that hydrogen peroxide and the hydroxytetrahydropterin are both derived from alternate routes of breakdown of a common precursor, the corresponding 4a-hydroperoxytetrahydropterin. By contrast, in the normal tightly coupled reaction, we suggest that this peroxy compound participates in the hydroxylation of the amino acid substrate.
苯丙氨酸羟化酶是一种依赖四氢生物蝶呤（BH4）的加氧酶，催化苯丙氨酸向酪氨酸的转化。在这个生理反应过程中，BH4的氧化与氨基酸底物的羟基化紧密偶联，每形成1摩尔酪氨酸，就有1摩尔BH4被氧化，化学计量比为1:1。在激活剂以及氨基酸或蝶呤辅酶的某些类似物存在的情况下，该酶还催化一种部分解偶联反应，其中被氧化的四氢蝶呤的量超过被羟基化的氨基酸的量。以酪氨酸作为氨基酸类似物时，反应完全解偶联，即该酶作为一种四氢蝶呤氧化酶起作用，氨基酸没有净羟基化。我们研究了在完全解偶联反应过程中是否形成正常的蝶呤中间体4a - 羟基四氢蝶呤，以及是否像在紧密偶联反应中那样，氧气仅被还原到水的水平，还是会有一些被还原为过氧化氢。我们提供了直接证据，表明正常的羟基四氢蝶呤中间体和过氧化氢都形成了。然而，形成的过氧化氢的量与被还原的氧气的量不是化学计量关系，这表明这种还原至少有两条途径，一条导致水的形成，另一条导致过氧化氢的形成。我们还推测过氧化氢和羟基四氢蝶呤都源自共同前体相应的4a - 氢过氧化四氢蝶呤的交替分解途径。相比之下，在正常的紧密偶联反应中，我们认为这种过氧化合物参与氨基酸底物的羟基化反应。
