Inhibition of nucleoside transport in murine lymphoma L5178Y cells and human erythrocytes by the uridine phosphorylase inhibitors 5-benzylacyclouridine and 5-benzyloxybenzylacyclouridine.

The uridine phosphorylase inhibitors, 5-benzylacyclouridine (BAU) and 5-benzyloxybenzylacyclouridine (BBAU) (Biochem. Pharmacol., 31: 1857, 1982), inhibited uptake of uridine in L5178Y cells. By a rapid sampling technique, BAU and BBAU were shown to inhibit the transport (zero-trans influx) of uridine, thymidine, and adenosine in human erythrocytes as well as in murine L5178Y cells. In all cases, competitive inhibitions were observed. Km values for the transport of adenosine, uridine, and thymidine in erythrocytes were 2.2, 195, and 199 microM, while Vmax were 2.9, 118, and 96.5 pmol/min/10(6) cells, respectively. In L5178Y cells, Km values of 14.8 and 23.1 microM and Vmax of 389 and 176 pmol/min/10(6) cells were obtained for adenosine and uridine, respectively. For erythrocytes, the Ki values of BAU were 127, 124, and 198 microM using adenosine, uridine, and thymidine as the substrate; and those of BBAU, 14.1 and 19.2 microM for adenosine and uridine, respectively. In L5178Y, the Ki values of BAU were 202 and 234 microM, and those of BBAU, 39.8 and 27.9 microM for adenosine and uridine, respectively. These data indicate that, in two cell types, Ki values for BAU and BBAU did not vary regardless of the substrate used; that the values of Ki are different for the erythrocytes and L5178Y cells; and that BBAU is at least 5-fold more potent than BAU as an inhibitor of nucleoside transport. The inhibitory effects on the efflux of preloaded uridine indicate that BAU and BBAU are inhibitors, rather than permeants, of the nucleoside transport system.