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Xenobiotic Incorporation into Pyruvate Dehydrogenase Complex Can Occur Via the Exogenous Lipoylation Pathway

Lookup NU author(s): Dr Hannah Walden, Professor John Kirby, Professor Steve Yeaman, Dr Joseph Gray, Professor David Jones, Dr Jeremy Palmer

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Abstract

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. Conclusion: 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 dear mechanism for the generation of an autoimmunogenic neo-antigen of relevance for the pathogenesis of primary biliary cirrhosis.


Publication metadata

Author(s): Walden HR, Kirby JA, Yeaman SJ, Gray J, Jones DE, Palmer JM

Publication type: Article

Publication status: Published

Journal: Hepatology

Year: 2008

Volume: 48

Issue: 6

Pages: 1874-1884

ISSN (print): 0270-9139

ISSN (electronic): 1527-3350

Publisher: John Wiley & Sons, Inc.

URL: http://dx.doi.org/10.1002/hep.22540

DOI: 10.1002/hep.22540


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