title: Reassessment of the translocation hypothesis by kinetic studies on hexose transport in isolated rat adipocytes.
abstract: The effect of insulin and factors which have insulin-like activity on the kinetic parameters of 3-O-methyl-D-glucose (MeGlc) transport in rat adipocytes were assessed. Carrier-mediated uptake of MeGlc was estimated by the difference in the amounts of [14C]MeGlc and L-[3H]glucose taken up in cells under equilibrium exchange conditions at 37 degrees C. The Km and Vmax values in basal cells were 17.4 mM and 0.24 nmol/10(6) cells/s, respectively. Removal of endogenous adenosine by adenosine deaminase resulted in a 26% decrease in the basal rate due to a slight increase in the Km (19.6 mM) and a decrease in the Vmax value (0.20 nmol/10(6) cells/s). The maximum concentration (10 nM) of insulin decreased the Km to approximately one-half of the basal (7.1 mM) concomitant with an 8.5-fold increase in the Vmax value (2.04 nmol/10(6) cells/s). Submaximal concentrations (50 and 150 pM) of insulin, N6-phenylisopropyladenosine (1 microM), mechanical agitation of cells by centrifugal force (160 x g), low temperature (15 degrees C), 12-O-tetradecanoylphorbol-13-acetate (1 microM), and hydrogen peroxide (10 mM) all decreased the basal Km value to a range of 13.5-7.3 mM, concomitant with a 1.7-7.4-fold increase in the Vmax. A possible explanation for the alterations in the kinetic parameters may be that insulin and other factors cause the translocation of the mobile low-Km glucose transporters from an intracellular site to the cell surface, where the stationary high-Km transporters are located. Thus, when the Km and Vmax values of the hypothetical high-Km transporters were assumed to be 20 mM and 0.20 nmol/10(6) cells/s, respectively, and the Km of the low-Km transporters was assumed to be 7 mM, the theoretical Km decreased from 20 to 7.5 mM as the Vmax of the low-Km transporters increased from near 0 to 2.0 nmol/10(6) cells/s. The relation between empirical Km and Vmax values as affected by several agents and conditions followed closely the relation predicted by the above two-transporter model.
mesh_terms: D019325:3-O-Methylglucose; D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D002498:Centrifugation; D003080:Cold Temperature; D006601:Hexoses / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology; D007700:Kinetics; D008757:Methylglucosides / Q000378:metabolism; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D010660:Phenylisopropyladenosine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
chemical_list: D006601:Hexoses; D007328:Insulin; D008757:Methylglucosides; D009004:Monosaccharide Transport Proteins; D019325:3-O-Methylglucose; D010660:Phenylisopropyladenosine; D006861:Hydrogen Peroxide; D013755:Tetradecanoylphorbol Acetate