Membrane transport of L-triiodthyronine by human red cell ghosts.

L-T₃ transport has been investigated in human red cell ghosts. Determination of initial T₃ uptake revealed two separate saturable uptake systems, one with a K_m of 1.6 × 10^?8M, the other with a K_m of 3.3 × 10^?6M. Binding experiments resulted in two dissociation constants, 1.4 × 10^?7M.and 2.6 × 10^?6M. Uptake was dependent on the ghost volume, indicating an intravesicular location of T₃. The T₃ was concentrated 6 times by the ghosts. Ouabain reduced the uptake by the low K_m system, but was without effect on the high K_m system. Thus evidence is provided both of binding of T₃ to the ghost membrane and of its uphill transport across the membrane.