Absorption Kinetics of Some Purines, Pyrimidines, and Nucleosides in Taenia Crassiceps Larvae

In vitro kinetic studies revealed that Taenia crassiceps larvae absorb purine bases (adenine and hypoxanthine) and nucleosides (adenosine and uridine) by mediated transport. Pyrimidines (thymine, uracil, and cytosine) are absorbed primarily by diffusion. The presence of hypoxanthine stimulated adenosine uptake, an effect which was enhanced by preloading the worms with hypoxanthine. Furthermore, of the compounds tested, hypoxanthine alone displayed sigmoid absorption kinetics, suggesting an allosteric mechanism. This mechanism was repressed by preloading the worms with hypoxanthine, however, as well as by decreasing the incubation time from 2 minutes to 30 seconds. Under the latter experimental conditions hypoxanthine uptake appeared to occur by diffusion alone. The results suggest that the allosteric effects of hypoxanthine in this organism may involve an effect of hypoxanthine on metabolism and/or the adsorption of hypoxanthine and adenosine on cytoplasmic binding sites.