Sera from AIH (
autoimmune hepatitis) type 2 patients contain an
autoantibody against the UGT1A subtype, called anti-LKM3. Previously, we reported that sera in AIH type 1 patients contained
autoantibodies against
drug-metabolizing
enzymes (Shinoda et al. (2004) Autoimmunity, 37, 473). In this report, we showed that AIH-1 sera did not react with some
peptides in the C-terminal half of the UGT1A subtype but reacted with a
peptide P1(33-42) among several common
peptides in the N-terminal half of the UGT1A subtype. This result suggests that the P1 site (33-42) presents on the outside of the UGT1A molecule to be recognized by lymphocytes of the immune system to produce an
autoantibody. To detect a key recognition site on
peptide P1(33-42), we studied the reactivity of two
peptides, M1(28-37) and M2(38-47), containing the N-terminal and C-terminal half of
peptide P1.
Peptide M2 did not react with AIH-1 serum but
peptide M1 did. Thus, the common
peptide sequence 33-37 in the positive
peptide M1(28-37) and P1(33-42) is a key recognition sequence. Next, we studied the reactivity of some other synthetic
peptides, in which some
amino acids in the sequence 33-37 in
peptide M1 changed to Ala. The
peptides changing to Ala (PQ33-34AA) or (DGS35-37AAA) did not react with AIH-1 sera. Meanwhile, these AIH sera did not inhibit the glucuronidation of
p-nitrophenol by UGT1A6, suggesting that the key sequence 33-37 might not be contained in active sites of glucuronidation by UGT1A6. In conclusion, sera from AIH-1 patients reacted with the
amino acids in the sequence 33-37 (PQDGS) of the N-terminal of UGT1A6.