Unlike the OGDH-encoded 2-oxoglutarate dehydrogenase (OGDH), which is an essential enzyme present in all animal tissues, expression of the DHTKD1-encoded isoenzyme, 2-oxoadipate dehydrogenase (OADH), depends on a number of factors, and mutant DHTKD1 phenotypes are rarely manifested. Physiological significance of OADH is also obscured by the fact that both isoenzymes transform 2-oxoglutarate and 2-oxoadipate. By analogy with other members of the 2-oxo acid dehydrogenases family, OADH is assumed to be a component of the multienzyme complex that catalyzes oxidative decarboxylation of 2-oxoadipate. This study aims at molecular characterization of OADH from animal tissues. Phylogenetic analysis of 2-oxo acid dehydrogenases reveals OADH only in animals and Dictyostelium discoideum slime mold, within a common branch with bacterial OGDH. Examination of partially purified animal OADH by immunoblotting and mass spectrometry identifies two OADH isoforms with molecular weights of about 130 and 70 kDa. These isoforms are not observed upon the expression of human DHTKD1 protein in either bacterial or yeast system, where the synthesized OADH is of expected molecular weight (about 100 kDa). Thus, the OADH isoforms present in animal tissues, may result from the animal-specific regulation of the DHTKD1 expression and/or posttranslational modifications of the encoded protein. Mapping of the peptides identified in the OADH preparations, onto the protein structure suggests that the 70-kDa isoform is truncated at the N-terminus, but retains the active site. Since the N-terminal domain of OGDH is required for the formation of the multienzyme complex, it is possible that the 70-kDa isoform catalyzes non-oxidative transformation of dicarboxylic 2-oxo acids that does not require the multienzyme structure. In this case, the ratio of the OADH isoforms in animal tissues may correspond to the ratio between the oxidative and non-oxidative decarboxylation of 2-oxoadipate.

中文翻译：

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在动物组织中鉴定的 DHTKD1 编码的 2-氧代己二酸脱氢酶的同种型在细菌或酵母系统中的人类 DHTKD1 表达中未观察到

与 OGDH 编码的 2-氧代戊二酸脱氢酶 (OGDH)（存在于所有动物组织中的一种必需酶）不同，DHTKD1 编码的同工酶 2-氧代己二酸脱氢酶 (OADH) 的表达取决于多种因素，而突变型 DHTKD1表型很少表现出来。OADH 的生理意义也被这两种同工酶转化为 2-氧代戊二酸和 2-氧代己二酸的事实所掩盖。通过与 2-氧代酸脱氢酶家族的其他成员类比，OADH 被认为是催化 2-氧代己二酸氧化脱羧的多酶复合物的组成部分。本研究旨在对来自动物组织的 OADH 进行分子表征。2-氧代酸脱氢酶的系统发育分析表明，OADH 仅存在于动物和盘基网柄菌粘菌中，与细菌 OGDH 处于同一分支内。通过免疫印迹和质谱法检测部分纯化的动物 OADH，鉴定出两种 OADH 亚型，其分子量约为 130 和 70 kDa。在细菌或酵母系统中表达人 DHTKD1 蛋白时未观察到这些同种型，其中合成的 OADH 具有预期的分子量（约 100 kDa）。因此，动物组织中存在的 OADH 亚型可能是由动物特异性调节 DHTKD1 表达和/或编码蛋白质的翻译后修饰引起的。将 OADH 制剂中鉴定的肽映射到蛋白质结构上表明 70-kDa 同种型在 N 端被截断，但保留了活性位点。由于形成多酶复合物需要 OGDH 的 N 端结构域，70-kDa 异构体可能催化不需要多酶结构的二羧酸 2-氧代酸的非氧化转化。在这种情况下，动物组织中 OADH 异构体的比例可能对应于 2-氧代己二酸的氧化和非氧化脱羧之间的比例。