Bile acid acyl adenylate: a possible intermediate to produce a protein-bound bile acid.

The non-enzymatic production of a protein-bound adduct by the action of the acyl adenylate of bile acids is described. On incubation of deoxycholyl adenylate with substance P in phosphate buffer, peptides covalently bound with one or two molecules of the bile acid were detected. The modified peptides were structurally characterized by time-of-flight mass spectrometry with matrix-assisted laser desorption/ionization (MALDI-TOFMS) in the post-source decay mode, and by liquid chromatography/electrospray ionization MS/MS. The deoxycholic acid was bound on substance P through the amino group at Arg-1 and/or Lys-3. The adenylate of cholic acid also produced the protein-bound bile acid on incubation with lysozyme, and the binding sites of the cholic acid appeared to be the lysine residues at 1, 33, 97 and 116. The results clearly suggest that bile acid adenylates in vivo may act as active intermediates to produce covalently bound bile acid adducts with peptides and proteins by nucleophilic displacement of the 5'-adenylic acid through the free amino groups.