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1. The asymmetries of affinities of two non-transportable competitive inhibitors of hexose transfer across fetal and new-born guinea-pig erythrocytes have been studied. 2. At 16 degrees C 4,6-O-ethylidene-alpha-D-glucopyranose (ethylidene glucose) inhibited 3-O-methyl glucose exchange at 20 mM with a K1 oc ca. 52 mM when present inside the cells and with a K1 oc ca. 10 mM when outside. This fivefold asymmetry is qualitatively similar to but smaller than the tenfold asymmetry of human erythrocytes (Baker, Basketter & Widdas, 1978). 3. Methyl-2,3-di-O-methyl-alpha-D-glucopyranoside (trimethyl glucoside) had K1 values of ca. 120 mM and ca. 160mM for inside and outside inhibition respectively. This is also qualitatively similar to the inhibition in human erythrocytes. 4. The inhibition produced by phlorizin, phloretin and Cytochalasin B was also studied in the erythrocytes of new-born guinea-pigs. The results were qualitatively similar to those for human erythrocytes but the inhibitory affinities were different. Thus while phlorizin and phloretin had higher affinities for the inhibition of exchange in new-born guinea-pig cells than human cells, the affinity of Cytochalasin B was less for new-born guinea-pig cells than for human cells. 5. It is concluded that the hexose transfer system in fetal and new-born guinea-pig red cells has asymmetric affinities similar to the system in human red cells but with different values of the inhibitory constants. The differences may represent species variations in a structural protein serving identical functions in the two species. 6. The possibility that fetal red cells with their facilitated transfer system play a role in sugar transport is discussed.

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