Transport of Hexoses across the Liver‐Cell Membrane

The uptake of radioactively labelled hexoses into the cellular space of isolated liver cells has been studied using silicone layer filtering centrifugation. From the hexoses tested, D-glucose is transported most rapidly followed by D-galactose and D-fructose. The rate of L-glucose transport is only 5% of that of D-glucose. This transport enables the concentration of free aldohexoses in the cellular space to reach the external concentration. For D-glucose the rate of transport into the cell largely exceeds the rate of metabolic conversion. This is different with D-fructose where the rate of transport is considerably lower but the rate of metabolism higher. Here the transport may even be a limiting step. The transport of D-glucose and D-galactose shows a saturation characteristic, whereas D-fructose appears not to be saturatable. The Km for D-glucose is found to be 30 mM. All these hexoses compete with each other for transportation. The temperature dependency of D-glucose reveals an activation energy of 22 kcal (92 kJ)/mol (4-18 degrees C) and 7 kcal (29 kJ)/mol (18-37 degrees C). The transport of D-glucose, D-galactose and D-fructose in inhibited by cytochalasin B, phloretin and phlorizin. It is not dependent on Na+ ions. Preliminary results showed no stimulation of the transport by insulin. It is concluded that D-glucose and other hexoses are transported by carrier-mediated diffusion across the plasma membrane of liver cells. This transport shows a large resemblance to the transport of D-glucose into human erythrocytes.