Comparative study of the thiourea carrier in erythrocytes

A densimeter technique was used to measure the rate of exit of thiourea from erythrocytes of various species of mammals. The cells were first equilibrated with a 200 mM thiourea solution in 1% NaCl. An aliquot of these cells was added to 1% NaCl containing 4.6-23.1 mM thiourea. Facilitated diffusion was demonstrated in each case. Using exit times or initial rates, calculations of half-saturation constants (phi) in mM and maximum transport rates (K) in isotones per min were made by three different methods. The following values were obtained: human-phi=60, 42, 35; K=1.2, 2.9, 0.9; rabbit-phi=46, 33, 32; K=0.8, 2.1, 0.8; mouse-phi=46, 40, 30; K=3.4, 8.5, 3.2; rat-phi=65, 42, 23; K=6.1, 15.3, 3.7; ox-phi=107, 63, 88; K=0.6, 1.4, 0.4; sheep-phi=56, 38, 56; K=0.9, 2.2, 0.6; and pig-phi=110, 64, 49; K=1.6, 3.6, 1.1.