Bile acids in tissues: Binding of lithocholic acid to protein

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Abstract
Human liver contains two forms of lithocholic acid. One form is readily extractable by 95% ethanol/0.1% ammonia (soluble lithocholate, SL), while the other remains firmly bound to the residue (tissue‐bound lithocholate, TBL). TBL could be hydrolytically released using clostridial cholanoylamino acid hydrolase, suggesting a peptide link between lithocholate and protein. With bovine serum albumin (BSA), lithocholic acid showed spontaneous amino group‐modifying activity. When small molecular weight lysine (α‐t‐BOC‐1‐lysyl‐β‐naphthylamide) and arginine peptides (α‐CBZ‐di‐arginyl‐β‐naphthylamide) were used in place of BSA, lithocholate bound specifically to the lysine peptide. The unusual affinity for lysine suggested that this amino acid might be involved as a residue in TBL. Synthesis of lithocholyl lysines and comparison with products of acid hydrolysis of TBL established ε‐lithocholyl lysine as the predominant form in which lithocholic acid is found in tissue bound form.