Cytochalasin B inhibition and temperature dependence of 3-O-methylglucose transport in fat cells

The transport of $\text{3-O-}\text{methylglucose}$ in white fat cells was measured under equilibrium exchange conditions at $\text{3-O-}\text{methylglucose}$ concentrations up to 50 mM with a previously described method (Vinten, J., Gliemann, J. and Østerlind, K. (1976) J. Biol. Chem. 251, 794–800). Under these conditions the main part of the transport was inhibitable by cytochalasin B. The inhibition was found to be of competitive type with an inhibition constant of about 2.5 · 10^?7 M, both in the absence and in the presence of insulin (1μM). The cytochalasin B-insensitive part of the $\text{3-O-}\text{methylglucose}$ permeability was about 2 · 10^?9 cm · s^?1, and was not affected by insulin. As calculated from the maximum transport capacity, the half saturation constant and the volume/ surface ratio, the maximum permeability of the fat cell membrane to $\text{3-O-}\text{methylglucose}$ at 37°C and in the presence of insulin was 4.3 · 10^?6 cm · s^?1. From the temperature dependence of the maximum transport capacity in the interval 18–37°C and in the presence of insulin, an Arrhenius activation energy of $\text{14.8 ± 0.44}\text{kcal/mol}$ was found. The corresponding value was $\text{13.9 ± 0.89}$ in the absence of insulin. The half saturating concentration of $\text{3-O-}\text{methylglucose}$ was about 6 mM in the temperature interval used, and it was not affected by insulin, although this hormone increased the maximum transport capacity about ten-fold to 1.7 mmol · s^?1 per 1 intracellular water at 37°C.