In contrast, the structures at the active sites are highly conserved in pro- and eukaryotes. The reactivity of autoantibodies with evolutionary find more highly conserved enzymes and their interaction with enzyme function in vitro is a quite frequently observed phenomenon in autoimmune disorders in general[33,34]. The etiopathogenetic role of anti-M2/PDC-E2 antibodies for PBC is still a matter of debate. Coupling of the inner lipoyl domain with 2-octynoic acid instead of lipoic acid has been discussed to increase antigenicity of the enzyme PDC-E2, and common environmental, cosmetic and food additives containing this 2-octynoic acid have, therefore, be postulated to play a role in their induction by formation of an altered PDC-E2 in the sense of a neoantigen[15,35,36].

Our observation that anti-PDC-E2 antibodies react preferentially with the active site of the catalytic domain of PDC-E2 is strongly suggestive for a further mechanisms including molecular mimicry or defects in apoptosis[5,37,38]. Interestingly, similarity alignment (http://www.expasy.org) revealed that a sequence consisting of the five aa TFTIS within peptide 29 containing the active site S480 shares an identity of 80% with a five-aa-peptide within an envelope protein of human ��-retrovirus previously cloned from patients with PBC[39]. The relevance of this observation is, of course, still rather speculative. Another peptide strongly recognized by PBC sera (up to 58%) was the peptide 25. However, 18% of healthy controls also reacted with this peptide indicating that probably natural autoantibodies towards this region exist.

Also the fact that antibody reactivity to some peptides within the catalytic domain did not differ significantly between healthy individuals and PBC-patients (i.e. peptide 22, 30, 31) or were even lower in the PBC sera than in sera of the controls (peptides 15, 21 and 24) may point towards the existence of natural autoantibodies to several PDC-E2 epitopes as also outlined by others[5,13]. It is well known that natural autoantibodies, which play an important role in the ��first line defense�� react preferentially with archaic enzymes, and this would fit with our observation. Interestingly, we also observed antibodies to the PDC-E2 complex in sera from healthy family members of PBC-patients, but these seem to recognize other epitopes than the patients�� sera (manuscript in preparation).

In general, most sera recognized several epitopes on the entire PDC-E2 molecule, and diversity of IgM antibodies was even higher than that of IgG antibodies. Using overlapping octameric peptides representing AV-951 the inner lipoyl domain of PDC-E2 similar observations were made by Mackay et al[5], and they also noted that normal human sera reacted with multiple peptides, although levels of reactivity were generally lower than those observed using PBC sera[5,40].