The simple model of adipocyte hexose transport. Kinetic features, effect of insulin, and network thermodynamic...

Kinetic studies of the rat adipocyte hexose transport system were performed using the integrated rate approach and these compared to the simple carrier model of transport. Equilibrium exchange 3-O-methylglucose entry and exit studies showed directional symmetry with Km = overall dissociation constants = 8-10 mM. Comparison of zero-trans and equilibrium exchange entry also revealed similar Km and Vmax values. Insulin pretreatment increased the maximal rate of transport at 20 mM 3-O-methylglucose about 5- to 6-fold with each procedure. Studies of glucose-induced steady state 3-O-methylglucose countertransport provided evidence that carrier permeability and not carrier-substrate dissociation was rate limiting for overall transport. These data, therefore, indicate equal mobility of the loaded and unloaded carriers. Network thermodynamic computer simulations of the simple carrier model using kinetic parameters derived from zero-trans experiments provided good fits of actual data. The effect of insulin was best represented by an increase in total number of carrier units. It is concluded that the adipocyte hexose carrier displays bidirectional symmetry, limitation of transport by carrier movement rather than substrate-carrier interaction, equal rates of movement of loaded and unloaded carriers, and adherence to a simple carrier model in which insulin increases the total number of carrier units.