Mitochondrial aspartate aminotransferase: a third kynurenate-producing enzyme in the mammalian brain
Mitochondrial aspartate aminotransferase: a third kynurenate-producing enzyme in the mammalian brain
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DOI：
10.1111/j.1471-4159.2007.04556.x
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发表时间：
2007-07-01
影响因子：
4.7
通讯作者：
Schwarcz, Robert
中科院分区：
文献类型：
--
作者：
Guidetti, Paolo;Amori, Laura;Schwarcz, Robert
色氨酸代谢物犬尿氨酸（KYNA）是通过L-犬尿氨酸的不可逆转氨作用酶促产生的，是α 7烟碱和NMDA受体的拮抗剂，因此可调节胆碱能和多巴胺能神经传递。两种犬尿氨酸氨基转移酶（KAT I和II）目前被认为是脑中KYNA的主要生物合成酶。在这项研究中，我们报告存在的第三种酶显示KAT活性在哺乳动物的大脑。新型KAT的最适pH为8.0，转氨谷氨酰胺或α-氨基己二酸（分别为KAT I和KAT II的经典底物）的能力较低。该酶被天冬氨酸，谷氨酸和使君子酸抑制，但对谷氨酰胺或抗KAT II抗体的阻断不敏感。纯化后的蛋白质进行测序和酶被确定为线粒体天冬氨酸转氨酶（mitAAT）。最后，在小鼠、大鼠和人脑中，在生理pH下，使用抗mitAAT抗体测定KAT I、KAT II和mitAAT对总KAT活性的相对贡献。KAT II在大鼠和人脑中含量最高，而mitAAT在小鼠脑中起主要作用。在体内生理和/或病理条件下，mitAAT是否参与脑KYNA合成仍有待观察。
The tryptophan metabolite kynurenic acid (KYNA), which is produced enzymatically by the irreversible transamination of L-kynurenine, is an antagonist of alpha 7 nicotinic and NMDA receptors and may thus modulate cholinergic and glutamatergic neurotransmission. Two kynurenine aminotransferases (KAT I and II) are currently considered the major biosynthetic enzymes of KYNA in the brain. In this study, we report the existence of a third enzyme displaying KAT activity in the mammalian brain. The novel KAT had a pH optimum of 8.0 and a low capacity to transaminate glutamine or alpha-aminoadipate (the classic substrates of KAT I and KAT II, respectively). The enzyme was inhibited by aspartate, glutamate, and quisqualate but was insensitive to blockade by glutamine or anti-KAT II antibodies. After purification to homogeneity, the protein was sequenced and the enzyme was identified as mitochondrial aspartate aminotransferase (mitAAT). Finally, the relative contributions of KAT I, KAT II, and mitAAT to total KAT activity were determined in mouse, rat, and human brain at physiological pH using anti-mitAAT antibodies. KAT II was most abundant in rat and human brain, while mitAAT played the major role in mouse brain. It remains to be seen if mitAAT participates in cerebral KYNA synthesis under physiological and/or pathological conditions in vivo.