FLUORESCENT BILE-ACID DERIVATIVES - RELATIONSHIP BETWEEN CHEMICAL-STRUCTURE AND HEPATIC AND INTESTINAL TRANSPORT IN THE RAT

Studies were performed to characterize hepatic and intestinal transpor t, as well as biotransformation during transport, of a spectrum of flu orescent bile acids containing a fluorophore attached to the side chai n, The following two classes of compounds were studied: 1) aminofluore scein (amF) coupled directly to the carboxylic group of a bile acid wh ich was cholic, ursodeoxycholic, or cholylglycine; and 2) nitrobenzoxa diazolyl (NBD) coupled to the epsilon-amino group of a lysine conjugat ed bile acid, which was cholic or ursodeoxycholic. Fluorescein, a chol ephilic organic anion, was studied as a control, Fluorescent bile acid s were synthesized and their structures confirmed by nuclear magnetic resonance and mass spectrometry, Using the biliary fistula rat, hepati c transport, biotransformation, and choleretic activity were defined; intestinal absorption was assessed by jejunal or ileal perfusion studi es, All fluorescent bile acids had hepatic transport maxima about one- sixth that reported for cholyltaurine, but derivatives of cholylglycin e were transported best. Bile acids underwent little (<5%) biotransfor mation during hepatocyte transport. Only the amF conjugate of cholylgl ycine had normal choleretic activity; other compounds were hypocholere tic or cholestatic. In contrast, fluorescein was well transported, was partly glucuronidated, and had normal choleretic activity, NBD-tagged , but not amF-tagged, bile acids were actively transported by the inte stine (ileum > jejunum), and no fluorescent bile acid had passive inte stinal permeability; NBD-tagged bile acids were biotransformed during intestinal transport (jejunum > ileum), We conclude that the structure of the fluorophore as well as that of the bile acid influences transp ort by the hepatocyte and enterocyte. These fluorescent bile acids dif fer from fluorescein in being impermeable to cell membranes and underg oing little biotransformation during hepatocyte transport, Of these fl uorescent bile acids, cholylglycylamF has hepatocyte transport and cho leretic properties most closely resembling those of a natural bile aci d.