title: Allosteric control of pyrimidine transport in Hymenolepis diminuta: an unusual kinetic isotope effect.
abstract: A thymine-uracil transport system is present in the plasma membrane brush-border of the tapeworm, Hymenolepis diminuta. The relation between initial uptake and substrate concentration of either thymine or uracil was sigmoidal when 2-14C-labelled substrates were used. In contrast, absorption kinetics of methyl-14C- and 3H-labelled substrates were hyperbolic. Since there was no metabolism of the labelled substrates during the incubation period, these differences indicate that the sigmoidal kinetics of pyrimidine transport in this organism is an isotopic effect associated with the presence of the 14C label at the 2 position in the pyrimidine ring.
mesh_terms: D000818:Animals; D001692:Biological Transport; D002250:Carbon Radioisotopes; D002462:Cell Membrane / Q000378:metabolism; D006926:Hymenolepis / Q000378:metabolism*; D007700:Kinetics; D013941:Thymine / Q000378:metabolism*; D014498:Uracil / Q000378:metabolism*
chemical_list: D002250:Carbon Radioisotopes; D014498:Uracil; D013941:Thymine