Study of novel biosynthetic pathway of BH_4 which concern human aldo-keto reductases

四氢生物蝶呤（BH_4）是芳香氨基酸羟化酶和一氧化氮合酶的辅助因子。该生物合成包括两个还原步骤，由七叶皂苷还原酶催化。PPH_4还原为6-（1'-氧-2'-羟丙基）-四氢蝶呤（1'- oxph_4）或6-（1'-羟基-2'-氧丙基）-四氢蝶呤（2'- oxph_4），进一步转化为BH_4。2001年，在肌张力障碍样疾病中报道了sepapterin还原酶（SPR）缺乏症。然而。SPR缺乏症患者尿中蝶呤排泄正常，无高苯丙氨酸血症，提示存在另一种与人类SPR无关的BH_4生物合成途径。我们报道了一个与spr无关的PPH_4形成BH_4的新途径，其中人类醛酮还原酶（AKR1C3和AKR1B1）协同工作。AIM家族酶还原PPH_4时，2′-OXPH_4由3α-羟基类固醇脱氢酶2型（AKR1C3）生成，1′-OXPH_4由醛糖还原酶euk生成。R1B1),体面地。Blau等人提出了一种BH_4生物合成回收途径，该途径包含1'-OXPH_4到sepapterin的非酶转化。本研究利用人醛酮还原酶（AKR1B1）和人单体羰基还原酶，考察了1′-OXPH_4非酶转化为七叶黄素的可能性。由于1'-OXPH_4被认为可以非酶转化为sepapterin，在含有PPH_4和AKR1B1的孵育混合物中，sepapterin被确定为生物蝶呤（biopterin）。将含有800 pmol 1′-OXPH_4的培养液在37℃、黑暗条件下孵育2 h后，未检测到sepapterin。这一结果表明，1′-OXPH_4非酶促生成七叶蝶呤的速率很低。本文的研究结果表明，1'-OXPH_4非酶转化为sepapterin的途径可能很难或不太可能在人类中进行。

Tetrahydrobiopterin (BH_4) is a cofactor for aromatic amino acid hydroxylases and nitric oxide synthase. The biosynthesis includes two reduction steps catalyzed by sepiapterin reductase.PPH_4 is reduced to 6-(1'-oxo-2'-hydroxypropyl)-tetrahydropterin (1'-OXPH_4) or 6-(1'-hydroxy-2'-oxopropyl)-tetrahydropterin (2'-OXPH_4), which is further converted to BH_4.In 2001, sepiapterin reductase (SPR) deficiency was reported in dystonia like disease. However. patients with SPR deficiency show normal urinary excretion of pterins without hyperphenylalaninemia, suggesting the existence of another BH_4 biosynthetic pathway, which is not concerned with SPR in humans.Recently. we reported a new SPR-unrelated BH_4 formation route from PPH_4, in which human aldo-keto reductase (AKR1C3 and AKR1B1) work in concert.. In the reduction of PPH_4 by AIM family enzymes, 2'-OXPH_4 was formed by 3α-hydroxysteroid dehydrogenase type 2(AKR1C3), whereas 1'-OXPH_4 was produced by aldose reductaseUK.R1B1), respectably.Blau et al. proposed a BH_4 biosynthetic salvage pathway containing nonenzymatic conversion of 1'-OXPH_4 to sepiapterin. In this study, the possibility of the nonenzymatic conversion of 1'-OXPH_4 to sepiapterin was examined by using human aldo-keto reductase(AKR1B1) and human monomeric carbonyl reductase. Since 1'-OXPH_4 has been suggested to be nonenzymatically converted into sepiapterin, in an incubation mixture containing PPH_4 and AKR1B1, sepiapterin was determined by its derivative as biopterin. No sepiapterin was detected in the incubation mixture when the mixture containing 800 pmol of 1'-OXPH_4 was further incubated at 37℃ for 2 h in darkness. This result suggests that the rate of the nonenzymatic formation of sepiapterin from 1'-OXPH_4 is quite low. The findings obtained here indicate that the proposed pathway in which a nonenzymatic conversion of 1'-OXPH_4 to sepiapterin occurs may be difficult or unlikely to proceed in humans.