Mechanisms of renal excretion of urobilinogen.

Abstract

Several arabinosyl-, xylosyl-, lyxosyl, 59-deoxy-, acyclo-, 2,29-anhydro-, 2,39-anhydro-29-deoxy-, 2,59-anhydro-, 6,59-cyclo-, and carbocyclic analogues of uridine with various 5-substitutions (fluoro, methyl, bromo, ethyl, benzyl, or benzyloxybenzyl) have been tested and compared with their corresponding ribo- and 29-deoxyribosides for their potency to inhibit uridine phosphorylase (UrdPase) from both mouse and human livers. The effect of the alpha- and beta-configurations of the glycosidic bond was also tested. Xylo-, lyxo-, 2,39-anhydro-29-deoxy-, 6,59-cyclo-, and carbocyclic uridines did not bind to the enzyme. Ribosides bound better than the corresponding 29-deoxyribosides, which were better than the 59-deoxyribosides. 29-alpha-Deoxyribosides bound to the enzyme, albeit less tightly than the corresponding beta-anomers. The acyclo- and 2,29-anhydrouridines were all potent inhibitors with the 2,29-anhydro- derivatives being the most potent. 2,59-Anhydrouridine bound to UrdPase less effectively than 2,29-anhydrouridine and acyclouridine. Arabinosyl uracil was at best a very poor inhibitor but binds better if a benzyl group is added at the 5-position of the pyrimidine ring. This binding was enhanced further by adding a 5-benzyloxybenzyl group. A similar enhancement of the binding with increased hydrophobicity at the 5-position of the pyrimidine ring was observed with ribosides, alpha- and beta-anomers of the 29-deoxyribosides, acyclonucleosides, and 2,29-anhydronucleosides. The inhibitory potencies of these compounds with UrdPase from human liver roughly parallel those obtained with UrdPase from mouse liver. It is concluded that the presence of a N-glycosidic bond as well as a properly oriented 39-hydroxyl group are prerequisites for a nucleoside ligand to bind to UrdPase. On the other hand, the presence of a 29- or 59-hydroxyl group or an N-glycosidic bond in the beta-configuration enhances but is not essential for binding. Furthermore, the potency of the binding of 2,29-anhydrouridines (fixed syn-isomers) in contrast to the complete lack of binding of the 6,59-cyclouridines (fixed anti-isomers) to UrdPase indicates that the binding of ligands to this enzyme is in the syn-conformation around the N-glycosidic bond.