Recent investigations have shown that multiple d-amino acids are present in mammals and these compounds have distinctive physiological functions. Free d-glutamate is present in various mammalian tissues and cells and in particular, it is presumably correlated with cardiac function, and much interest is growing in its unique metabolic pathways. Recently, we first identified d-glutamate cyclase as its degradative enzyme in mammals, whereas its biosynthetic pathway in mammals is unclear. Glutamate racemase is a most probable candidate, which catalyzes interconversion between d-glutamate and l-glutamate. Here, we identified the cDNA encoding l-serine dehydratase-like (SDHL) as the first mammalian clone with glutamate racemase activity. This rat SDHL had been deposited in mammalian databases as a protein of unknown function and its amino acid sequence shares ∼60% identity with that of l-serine dehydratase. Rat SDHL was expressed in Escherichia coli, and the enzymatic properties of the recombinant were characterized. The results indicated that rat SDHL is a multifunctional enzyme with glutamate racemase activity in addition to l-serine/l-threonine dehydratase activity. This clone is hence abbreviated as STDHgr. Further experiments using cultured mammalian cells confirmed that d-glutamate was synthesized and l-serine and l-threonine were decomposed. It was also found that SDHL (STDHgr) contributes to the homeostasis of several other amino acids.

中文翻译：

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在哺乳动物中鉴定具有谷氨酸消旋酶活性的 l-丝氨酸/l-苏氨酸脱水酶

最近的研究表明，哺乳动物中存在多种 d-氨基酸，这些化合物具有独特的生理功能。游离 d-谷氨酸存在于各种哺乳动物组织和细胞中，特别是，它可能与心脏功能有关，并且对其独特的代谢途径越来越感兴趣。最近，我们首先将 d-谷氨酸环化酶鉴定为其在哺乳动物中的降解酶，而其在哺乳动物中的生物合成途径尚不清楚。谷氨酸消旋酶是最有可能的候选物，它催化 d-谷氨酸和 l-谷氨酸之间的相互转化。在这里，我们将编码 l-丝氨酸脱水酶样 (SDHL) 的 cDNA 鉴定为第一个具有谷氨酸消旋酶活性的哺乳动物克隆。这种大鼠 SDHL 已作为未知功能的蛋白质存放在哺乳动物数据库中，其氨基酸序列与 l-丝氨酸脱水酶的氨基酸序列具有约 60% 的同一性。大鼠 SDHL 在大肠杆菌中表达，并表征了重组体的酶学特性。结果表明，大鼠SDHL是一种多功能酶，除具有l-丝氨酸/l-苏氨酸脱水酶活性外，还具有谷氨酸消旋酶活性。该克隆因此缩写为 STDHgr。使用培养的哺乳动物细胞进行的进一步实验证实，合成了 d-谷氨酸并分解了 l-丝氨酸和 l-苏氨酸。还发现 SDHL (STDHgr) 有助于其他几种氨基酸的稳态。并对重组体的酶学性质进行了表征。结果表明，大鼠SDHL是一种多功能酶，除具有l-丝氨酸/l-苏氨酸脱水酶活性外，还具有谷氨酸消旋酶活性。该克隆因此缩写为 STDHgr。使用培养的哺乳动物细胞进行的进一步实验证实，合成了 d-谷氨酸并分解了 l-丝氨酸和 l-苏氨酸。还发现 SDHL (STDHgr) 有助于其他几种氨基酸的稳态。并对重组体的酶学性质进行了表征。结果表明，大鼠SDHL是一种多功能酶，除具有l-丝氨酸/l-苏氨酸脱水酶活性外，还具有谷氨酸消旋酶活性。该克隆因此缩写为 STDHgr。使用培养的哺乳动物细胞进行的进一步实验证实，合成了 d-谷氨酸并分解了 l-丝氨酸和 l-苏氨酸。还发现 SDHL (STDHgr) 有助于其他几种氨基酸的稳态。使用培养的哺乳动物细胞进行的进一步实验证实，合成了 d-谷氨酸并分解了 l-丝氨酸和 l-苏氨酸。还发现 SDHL (STDHgr) 有助于其他几种氨基酸的稳态。使用培养的哺乳动物细胞进行的进一步实验证实，合成了 d-谷氨酸并分解了 l-丝氨酸和 l-苏氨酸。还发现 SDHL (STDHgr) 有助于其他几种氨基酸的稳态。