The transport and metabolism of naturally occurring pyrimidine nucleosides by isolated rat jejunum.

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Abstract
1. Uridine perfused through the lumen of isolated loops of rat jejunum over a concentration range of 0.1-1.0 mM gave rise to higher serosal concentrations of uracil than the equivalent luminal concentration of uracil (P < 0.001). No serosal uridine could be detected. 2. Luminal thymidine over a concentration range of 0.1-0.5 mM gave rise to the same serosal concentration of thymine as the equivalent luminal concentration of thymine (P > 0.1). Low concentrations of serosal thymidine were detected. Both luminal thymidine and thymine gave rise to elevated levels of serosal uracil. 3. Luminal cytidine at concentrations of 0.1-0.5 mM was poorly transported and yielded low serosal concentrations of cytidine. No serosal cytosine was detected, although elevated levels of uracil were found in the serosal secretions. 4. Cytosine over a luminal concentration range of 0.1-0.5 mM gave rise to low concentrations of cytosine in the serosal secretions. These results were consistent with a passive diffusion model for cytosine transport. No increase in serosal uracil was detected. 5. The cleavage of uridine and thymidine to their respective pyrimidine bases occurred via a cytoplasmic nucleoside phosphorylase, which has a similar Michaelis constant (Km), (61.0 .+-. 4.4 and 97.1 .+-. 5.7 .mu.M for uridine and thymidine, respectively) but a maximal velocity (Vmax) for uridine cleavage (320 .+-. 32 nmol min-1 (mg protein)-1) 13 times that for thymidine cleavage (24.7 .+-. 1.4 nmol min-1 (mg protein)-1). 6. The differences between the three pyrimidine nucleosides are discussed with reference to the interactions between their epithelial transport and metabolism.