On the probable involvement of arginine residues in the bile-salt-binding site of human pancreatic carboxylic ester hydrolase.

Modification of arginine residues with 2,3-butanedione inhibits the carboxylic-ester hydrolase activity on soluble and emulsified substrates when assayed with bile salts. The alpha-dicarbonyl reagent modifies seven of the nineteen arginine residues present per enzyme molecule. Nevertheless the inactivation with butanedione is greatly diminished when the protein is in the presence of negatively charged micellar bile salt. In these conditions we observe the protection of one arginine residue by sodium taurodeoxycholate and of two arginine residues by sodium cholate. This suggests that the carboxylic-ester hydrolase from human pancreatic juice contains at least two arginine residues essential for the activation by bile salts. All our data confirm the presence of two bile-salt-binding sites on the enzyme in which one arginine per site is involved and plays the general role of an anionic binding site. This study provides evidence that arginine residues may play an essential role in the interaction between bile salts and protein.