Synthesis and biliary excretion of tyrosine-conjugated bile salts in Wistar rats.

Tyrosine-labelled free and glycine-conjugated bile acids were synthesized and radiolabelled with 125I to high purity. The synthetic method utilized excess tyrosine methyl ester hydrochloride (1.4 equiv.) and bile acid (one equiv.) via dicyclohexylcarbodiimide (1.4 equiv.) with yields of 90-93% for tyrosine bile acid conjugates and glycyltyrosine conjugates and 56-60% yields for the glycylglycyltyrosine conjugates. All of the eight iodinated tyrosine bile acids tested were rapidly excreted into bile following intravenous injection. In bile duct-cannulated rats with ligated renal pedicles under pentobarbital anaesthesia the percentages of injected dose recovered from bile within 20 min were as follows: cholylglycine ([14C]cholylGly), 81.2 +/- 1.3%; taurocholate ([14C]taurocholate), 94.3 +/- 1.0%; cholyltyrosine (125I-cholylTyr), 85.5 +/- 3.3%; deoxycholyltyrosine (125I-deoxycholylTyr), 87.9 +/- 6.3%; chenodeoxycholyltyrosine (125I-chenodeoxycholylTyr), 93.4 +/- 2.9; cholylglycyltyrosine (125I-cholylGlyTyr), 95.7 +/- 6.7%; deoxycholylglycyltyrosine (125I-deoxylcholylGlyTyr), 92.5 +/- 3.2%; chenodeoxycholylglycyltyrosine (125I-chenodeoxycholylGlyTyr), 94.1 +/- 3.1%; cholyldiglycyltyrosine (125I-cholylGlyGlyTyr), 85.2 +/- 3.6%, and deoxycholyldiglycyltyrosine (125I-deoxycholylGlyGlyTyr), 85.5 +/- 2.7%. Values are means +/- SD. Thus the biliary excretion of 125I-chenodeoxycholylGlyTyr, 125I-chenodeoxycholylTyr, 125I-deoxycholylGlyTyr and 125I-cholylGlyTyr was similar to that of [14C]taurocholate, the major naturally occurring bile acid in the rat, and the biliary excretion of all the tyrosine conjugates was similar to or exceeded that of [14C]cholylglycine. Conjugation with tyrosine enhanced the efficiency of plasma-to-bile transport of most naturally occurring bile acids. Comparison of glycyltyrosine conjugates with glycylglycyltyrosine conjugates suggests that any additional benefit derived by elongation of the side-chain is probably negated by obscuring the 12 alpha-hydroxyl function on the steroid nucleus in the bile acid glycylglycyltyrosine conjugates.