Interactions of sodium pentobarbital with d-glucose and l-sorbose transport in human red cells

Abstract

Pentobarbital acts as a mixed inhibitor of net d-glucose exit, as monitored photometrically from human red cells. At 30°C the K i of pentobarbital for inhibition of V max of zero-trans net glucose exit is 2.16±0.14 mM; the affinity of the external site of the transporter for d-glucose is also reduced to 50% of control by 1.66±0.06 mM pentobarbital. Pentobarbital reduces the temperature coefficient of d-glucose binding to the external site. Pentobarbital (4 mM) reduces the enthalpy of d-glucose interaction from 49.3±9.6 to 16.24±5.50 kJ/mol ( P<0.05). Pentobarbital (8 mM) increases the activation energy of glucose exit from control 54.7±2.5 kJ/mol to 114±13 kJ/mol ( P<0.01). Pentobarbital reduces the rate of l-sorbose exit from human red cells, in the temperature range 45°C–30°C ( P<0.001). On cooling from 45°C to 30°C, in the presence of pentobarbital (4 mM), the K i (sorbose, glucose) decreases from 30.6±7.8 mM to 14±1.9 mM; whereas in control cells, K i (sorbose, glucose) increases from 6.8±1.3 mM at 45°C to 23.4±4.5 mM at 30°C ( P<0.002). Thus, the glucose inhibition of sorbose exit is changed from an endothermic process (enthalpy change=+60.6±14.7 kJ/mol) to an exothermic process (enthalpy change=−43±6.2 7 kJ/mol) by pentobarbital (4 mM) ( P<0.005). These findings indicate that pentobarbital acts by preventing glucose-induced conformational changes in glucose transporters by binding to ‘non-catalytic’ sites in the transporter.