Giardia lamblia: uptake of pyrimidine nucleosides

The aerotolerant, anaerobic parasite Giardia lamblia, which depends solely upon salvage pathways for its pyrimidine requirements, was found to transport uridine, cytidine, and thymidine by a carrier mediated mechanism. Support for this conclusion comes from the facts that uptake of radiolabeled uridine, cytidine, and thymidine exhibited saturation kinetics, and uptake of these same radiolabeled nucleosides was inhibited by unlabeled homologs, certain pyrimidine analogs, iodoacetate, and N-ethylmaleimide. Uridine and cytidine (perhaps uracil and cytosine also) are postulated to be transported at a common site which is distinct from the site for thymidine transport. Thymidine does appear to bind nonproductively to the uridine/cytidine transport site, but the reverse of this does not appear to occur.