Walden Hannah R, Kirby John A, Yeaman Stephen J, Gray Joe, Jones David E, Palmer Jeremy M
Liver Research Group, Institute of Cellular Medicine, Newcastle University, Newcastle-upon-Tyne, UK.
Hepatology. 2008 Dec;48(6):1874-84. doi: 10.1002/hep.22540.
Lipoylated enzymes such as the E2 component of pyruvate dehydrogenase complex (PDC-E2) are targets for autoreactive immune responses in primary biliary cirrhosis, with lipoic acid itself forming a component of the dominant auto-epitopes. A candidate mechanism for the initiation of tolerance breakdown in this disease is immune recognition of neo-antigens formed by xenobiotic substitution of normal proteins. Importantly, sensitization with proteins artificially substituted with the lipoic acid analogue xenobiotic 6-bromohexanoic acid (6BH) can induce an immune response that cross-reacts with PDC-E2. This study investigated the potential of recombinant lipoylation enzymes lipoate activating enzyme and lipoyl-AMP(GMP):N-lysine lipoyl transferase to aberrantly incorporate xenobiotics into PDC-E2. It was found that these enzymes could incorporate lipoic acid analogues including octanoic and hexanoic acids and the xenobiotic 6BH into PDC-E2. The efficiency of incorporation of these analogues showed a variable dependence on activation by adenosine triphosphate (ATP) or guanosine triphosphate (GTP), with ATP favoring the incorporation of hexanoic acid and 6BH whereas GTP enhanced substitution by octanoic acid. Importantly, competition studies showed that the relative incorporation of both 6BH and lipoic acid could be regulated by the balance between ATP and GTP, with the formation of 6BH-substituted PDC-E2 predominating in an ATP-rich environment.
Using a well-defined system in vitro we have shown that an important xenobiotic can be incorporated into PDC in place of lipoic acid by the exogenous lipoylation system; the relative levels of lipoic acid and xenobiotic incorporation may be determined by the balance between ATP and GTP. These observations suggest a clear mechanism for the generation of an auto-immunogenic neo-antigen of relevance for the pathogenesis of primary biliary cirrhosis.
脂酰化酶，如丙酮酸脱氢酶复合体的E2组分（PDC-E2），是原发性胆汁性肝硬化自身反应性免疫应答的靶点，硫辛酸自身构成主要自身抗原表位的一个组分。该疾病中耐受性破坏起始的一种候选机制是对正常蛋白质经外源性取代形成的新抗原的免疫识别。重要的是，用硫辛酸类似物外源性物质6-溴己酸（6BH）人工取代的蛋白质致敏可诱导与PDC-E2发生交叉反应的免疫应答。本研究调查了重组脂酰化酶硫辛酸激活酶和脂酰-AMP（GMP）：N-赖氨酸脂酰转移酶将外源性物质异常掺入PDC-E2的可能性。发现这些酶可将包括辛酸和己酸在内的硫辛酸类似物以及外源性物质6BH掺入PDC-E2。这些类似物的掺入效率显示出对三磷酸腺苷（ATP）或三磷酸鸟苷（GTP）激活的可变依赖性，ATP有利于己酸和6BH的掺入，而GTP增强辛酸的取代。重要的是，竞争研究表明，6BH和硫辛酸的相对掺入可通过ATP和GTP之间的平衡来调节，在富含ATP的环境中，6BH取代的PDC-E2的形成占主导。
我们在体外使用一个明确的系统表明，外源性脂酰化系统可将一种重要的外源性物质掺入PDC以取代硫辛酸；硫辛酸和外源性物质掺入的相对水平可能由ATP和GTP之间的平衡决定。这些观察结果提示了一种明确的机制，用于产生与原发性胆汁性肝硬化发病机制相关的自身免疫原性新抗原。
