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Department of Human Physiology, Royal Holloway and Bedford New College, Egham, Surrey.

1. Equilibrium exchanges in the range of 2-40 mM-3-O-methyl glucose at 16 degrees C suggested that the half-saturation concentration for exchange was 22 mM and that the maximum velocity (Vmax) was ca. 149 mmol l-1 min-1. 2. Initial rates of exchange influx from 1, 2, 4 and 8 mM into 76 mM solution gave a half-saturation value of 3.6 mM and a Vmax of 122 mmol-1 min-1. 3. The non-transportable inhibitor 4,6-O-ethylidene-alpha-D-glucopyranose (ethylidene glucose) acting on the outside of the cells inhibited 3-O-methyl glucose exchanges at 16 degrees C with an inhibition constant (KI) of ca. 11 mM. 4. Sen-Widdas exit experiments gave the half-saturation for 3-O-methyl glucose at 16 degrees C as only ca. 2 mM and the KI for ethylidene glucose as ca. 4 mM. 5. Efflux inhibitions by ethylidene glucose are satisfactorily predicted by the asymmetric carrier kinetics of Regen & Tarpley (1974) when using the parameters derived from the exchange experiments but not with parameters from Sen-Widdas exits. 6. Uphill transfer by counterflow experiments and Sen-Widdas exits cannot be fitted by the Regen and Tarpley kinetics (using the same parameters) unless the kinetics are modified to provide for an extra exchange element which replaces some of the net exit component in the equations. 7. At present the modification to the kinetics is only possible in computer simulations and data handling, but with it the fit to experimental results is good. The nature of the modification is described and in the light of it a revised interpretation of the significance of the Km derived from Sen-Widdas exits is discussed.

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