Laboratory of Biochemistry, Department of Chemistry and Biotechnology, Faculty of Science and Technology, National University Corporation Kochi University, Japan.
Arterial Inflammation and Redox Biology Group, Heart Research Institute, Newtown, NSW, Australia.
FEBS J. 2021 Aug;288(16):4892-4904. doi: 10.1111/febs.15806. Epub 2021 Mar 26.
Indoleamine 2,3-dioxygenases (IDOs) catalyze the oxidative cleavage of L-tryptophan (Trp) to N-formylkynurenine. Two IDOs, IDO1 and IDO2, are present in vertebrates. IDO1 is a high-affinity Trp-degrading enzyme involved in several physiological processes. By comparison, IDO2 generally has been reported to have low affinity (high K -value) for Trp, and the enzyme's in vivo function remains unclear. Using IDOs from different species, we show that compared with ferrous-oxy (Fe -O ) IDO1, Fe -O IDO2 is substantially more stable and engages in multiple turnovers of the reaction in the absence of a reductant. Without reductant, Fe -O IDO2 showed K -values in the range of 80-356 μM, that is, values substantially lower than reported previously and close to the physiological concentrations of Trp. Methylene blue and ascorbate (Asc), used commonly as the reducing system for IDO activity determination, significantly affected the enzymatic activity of IDO2: In combination, the two reductants increased the apparent K - and k -values 8- to 117-fold and 2-fold, respectively. Asc alone both activated and inhibited IDO2 by acting as a source of electrons and as a weak competitive inhibitor, respectively. In addition, ferric (Fe ) IDO1 and IDO2 exhibited weak dioxygenase activity, similar to tryptophan 2,3-dioxygenase. Our results shed new light in the enzymatic activity of IDO2, and they support the view that this isoform of IDO also participates in the metabolism of Trp in vivo.
色氨酸 2,3-双加氧酶（IDO）催化 L-色氨酸（Trp）的氧化裂解生成 N-甲酰犬尿氨酸。脊椎动物中存在两种 IDO，即 IDO1 和 IDO2。IDO1 是一种高亲和力的 Trp 降解酶，参与多种生理过程。相比之下，IDO2 通常被报道对 Trp 的亲和力较低（高 K 值），其在体内的功能尚不清楚。我们使用来自不同物种的 IDO 表明，与亚铁氧（Fe-O）IDO1 相比，Fe-O IDO2 更稳定，在没有还原剂的情况下，该酶可以进行多次反应循环。在没有还原剂的情况下，Fe-O IDO2 的 K 值在 80-356 μM 范围内，即值明显低于先前报道的值，接近 Trp 的生理浓度。亚甲基蓝和抗坏血酸（Asc）通常被用作 IDO 活性测定的还原系统，它们显著影响 IDO2 的酶活性：联合使用这两种还原剂分别将表观 K 值和 k 值提高了 8-117 倍和 2 倍。Asc 本身既是电子源，又是弱竞争性抑制剂，分别激活和抑制 IDO2。此外，Fe 3+ IDO1 和 IDO2 表现出较弱的双加氧酶活性，类似于色氨酸 2,3-双加氧酶。我们的结果为 IDO2 的酶活性提供了新的见解，并支持了这种同工酶也参与体内 Trp 代谢的观点。
