Transport of sugars in chick-embryo fibroblasts. Evidence for a low-affinity system and a high-affinity system for glucose transport.
The rate of D-glucose uptake by cells that had been deprived of sugar for 18-24h was consistently observed to be 15-20 times higher than that in control cells maintained for the same length of time in medium containing glucose. This increased rate of glucose transport by sugar-starved cells was due to a 3-5-fold increase in the Vmax. value of a low-affinity system (Km 1 mM) combined with an increase in the Vmax of a separate high-affinity system (Km 0.05-0.2 mM). The high-affinity system, which was most characteristic of starved cells, was particularly sensitive to low concentrations of the thiol reagent N-ethylmaleimide; 50% inhibition of uptake occurred at approx. 0.01 mM-N-ethylmaleimide. In contrast with the high-affinity system, the low-affinity system of either the fed cells or the starved cells was unaffected by N-ethylmaleimide. In addition to the increases in the rate of D-glucose transport, cells deprived of sugar had increased rates of transport of 3-O-methyl-D-glucose and 2-deoxy-D-glucose. No measurable high-affinity transport system could be demonstrated for the transport of 3-O-methylgucose, and N-ethylmaleimide did not alter the initial rate. Thus the transport of 3-O-methyglucose by both fed and starved cells was exclusively by the N-ethylmaleimide-insensitive low-affinity system. The low-affinity system also appeared to be the primary means for the transport of 2-deoxyglucose by fed and starved cells. However, some of the transport of 2-deoxyglucose by starved cells was inhibited by N-ethylmaleimide, suggesting that 2-deoxyglucose may also be transported by a high-affinity system. The results of experiments that measured transport kinetics strongly suggest that glucose can be transported by a least two separate systems, and 3-O-methylglucose and 2-deoxyglucose by one. Support for these interpretations comes from the analysis of the effects of N-ethylmaleimide and cycloheximide as well as from the results of competition experiments. The uptake of glucose is quite different from that of 2-deoxyglucose and 3-O-methylglucose. The net result of sugar starvation serves to emphasize these differences. The apparent de-repression of the transport systems studied presents an interesting basis for further studies of the regulation of transport in a variety of cells.
D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002478:Cells, Cultured; D002642:Chick Embryo; D002854:Chromatography, Paper; D003470:Culture Media; D003513:Cycloheximide / Q000494:pharmacology; D003837:Deoxy Sugars / Q000378:metabolism*; D003847:Deoxyglucose / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism*; D006593:Hexokinase / Q000378:metabolism; D007460:Iodoacetamide / Q000494:pharmacology; D007700:Kinetics; D008757:Methylglucosides / Q000378:metabolism*; D008759:Methylglycosides / Q000378:metabolism*; D013217:Starvation; D013997:Time Factors
D003470:Culture Media; D003837:Deoxy Sugars; D008757:Methylglucosides; D008759:Methylglycosides; D002731:4-Chloromercuribenzenesulfonate; D003513:Cycloheximide; D003847:Deoxyglucose; D006593:Hexokinase; D005947:Glucose; D005033:Ethylmaleimide; D007460:Iodoacetamide