Institute of Biochemistry, Charité - University Medicine Berlin, Corporate member of Free University Berlin, Humboldt University Berlin and Berlin Institute of Health, Charitéplatz 1, D-10117 Berlin, Germany.
Key Laboratory of Animal Models and Human Disease Mechanisms of the Chinese Academy of Sciences & Yunnan Province, Kunming Institute of Zoology, Kunming, Yunnan 650223, China.
Biochim Biophys Acta Mol Cell Biol Lipids. 2020 Feb;1865(2):158550. doi: 10.1016/j.bbalip.2019.158550. Epub 2019 Oct 29.
The tree shrew (Tupaia belangeri) is a rat-sized mammal, which is more closely related to humans than mice and rats. However, the use of tree shrew to explore the patho-mechanisms of human inflammatory disorders has been limited since nothing is known about eicosanoid metabolism in this mammalian species. Eicosanoids are important lipid mediators exhibiting pro- and anti-inflammatory activities, which are biosynthesized via lipoxygenase and cyclooxygenase pathways. When we searched the tree shrew genome for the presence of cyclooxygenase and lipoxygenase isoforms we found copies of functional COX1, COX2 and LOX genes. Interestingly, we identified four copies of ALOX15 genes, which encode for four structurally distinct ALOX15 orthologs (tupALOX15a-d). To explore the catalytic properties of these enzymes we expressed tupALOX15a and tupALOX15c as catalytically active proteins and characterized their enzymatic properties. As predicted by the Evolutionary Hypothesis of ALOX15 specificity we found that the two enzymes converted arachidonic acid predominantly to 12S-HETE and they also exhibited membrane oxygenase activities. However, their reaction kinetic properties (K for arachidonic acid and oxygen, T- and pH-dependence) and their substrate specificities were remarkably different. In contrast to mice and humans, tree shrew ALOX15 isoforms are highly expressed in the brain suggesting a role of these enzymes in cerebral function. The genomic multiplicity and the tissue expression patterns of tree shrew ALOX15 isoforms need to be considered when the results of in vivo inflammation studies obtained in this animal are translated into the human situation.
树鼩（Tupaia belangeri）是一种体型与老鼠相当的哺乳动物，与人类的亲缘关系比老鼠和大鼠更为密切。然而，由于我们对这种哺乳动物的类二十烷酸代谢一无所知，因此树鼩在探索人类炎症性疾病发病机制方面的应用受到了限制。类二十烷酸是具有促炎和抗炎活性的重要脂质介质，可通过脂氧合酶和环氧化酶途径生物合成。当我们在树鼩基因组中搜索环氧化酶和脂氧合酶同工酶的存在时，发现了功能性 COX1、COX2 和 LOX 基因的拷贝。有趣的是，我们鉴定了四个 ALOX15 基因的拷贝，这些基因编码了四个结构上不同的 ALOX15 直系同源物（tupALOX15a-d）。为了研究这些酶的催化特性，我们表达了具有催化活性的 tupALOX15a 和 tupALOX15c，并对其酶学特性进行了表征。根据 ALOX15 特异性的进化假说，我们发现这两种酶主要将花生四烯酸转化为 12S-HETE，它们还表现出膜氧化酶活性。然而，它们的反应动力学特性（对花生四烯酸和氧的 K 值、T 值和 pH 依赖性）和底物特异性有显著差异。与小鼠和人类不同，树鼩 ALOX15 同工酶在大脑中高度表达，表明这些酶在大脑功能中发挥作用。当将在这种动物体内获得的体内炎症研究结果转化为人类情况时，需要考虑树鼩 ALOX15 同工酶的基因组多样性和组织表达模式。
