Transport properties of photolabile sugar analogues

Abstract

The transport properties of photolabile substituted sugars have been determined in human erythrocytes. Half-saturation constants ( K i values ) for these compounds have been estimated by measuring the inhibition of the uptake of 100 μM d-galactose. 4-Azido-4-deoxy- d-galactose has a K i of 7.9 mM. 4-(2′-Diazo-3′,3′,3′-trifluoropropionyl)- d-glucose has a K i of 5.9 mM. 3,7-Anhydro-2,2-azo-1,2-dideoxy- d-glycerol- l-mannooctitol has negligible affinity. 1′-Methoxy-3′,3′-azobutyl β- d-glucopyranoside has low affinity outside the cell but shows a K i of 2.3 mM when equilibrated with the cells. d-Galactose competitively reduces the inhibition produced by this glucoside. The 4,4-azo-4-deoxy and the 6,6-azo-6-deoxy derivatives of d-glucose both have good affinity. The K i values are 31.6 mM and 7.3 mM, respectively. The 4,4-azo and 6,6-azo derivatives are particularly important sugars because they contain nonbulky photolabile substituents. All of the above analogues have been tested to determine whether they can induce counterflow of d-galactose. The 4,4-azo and the 6,6-azo derivatives induce large transient accumulations of d-galactose, indicating that these sugars are capable of forming a mobile sugar-carrier complex. The other sugars do not show counterflow. Thus the 4,4-azo and the 6,6-azo derivatives could be extremely useful analogues which may label parts of the transport system that side-specific nontransported sugars cannot reach.