Electron spin resonance investigation of the interaction of the anion and glucose transport inhibitor, p-azidobenzylphlorizin, with the human red cell membrane.

The membrane perturbations caused by the interaction of p-azidobenzylphlorizin (p-AzBPhz), a potential photoaffinity labeling agent of the anion and D-glucose transporters in the human erythrocyte, have been studied using electron spin resonance (ESR) spectrometry. Two lipid-specific spin labels have been employed; one of these agents, a hexadecyl-quarternary amine with the nitroxide reporter group covalently attached to the cationic nitrogen, (CAT-16), has been used to monitor changes in the physical state of the membrane's extracellular phospholipid/water interface. The other spin label, 5-doxylstearic acid (5-NS), is designed to examine the order and motion of the lipid bilayer near the cell surface. In separate experiments, intact human red cells labeled with these lipid-specific spin labels were exposed to small amounts of the phlorizin azide. A dose-dependent alteration in CAT-16 motion was observed, but the p-AzBPhz interaction with the membrane had no effect on the spectrum of 5-NS. The half-maximal effect of the phlorizin derivative on the CAT-16 spectrum occurred when about 2 million molecules were bound to each cell. This is also the combined amount of band 3 and band 4.5 present in the red cell membrane and represents the concentration necessary to inhibit both anion and glucose transport. Our results suggest that the first p-AzBPhz molecules binding to the red cell membrane interact with the anion and sugar transporters, and not with the bulk lipid bilayer.

Membrane action of phlorizinyl 5'-benzylazide induces volume and shape change of human erythrocytes and sickle cells.

In the absence of light, phlorizinyl benzylazide is a potent competitive inhibitor of sugar transport (app. Ki about 1.5 uM) and a reversible inhibitor of chloride exchange (app. Ki about 2 uM) in the normal red cell. It causes additional membrane alterations at about 10-fold higher concentrations which lead to a time-, pH-, temperature-, and cell storage time-dependent increase in cell volume in isosmotic phosphate-buffered saline. This drug effect may be related to its interaction with Bands 3 and 4.5 (both become covalently labeled when the azide is light activated) such that cation conductance is increased. Apparently, a channel is created or widened through which salt and water (but not sucrose) can enter causing cell swelling and eventually lysis. The membrane probe is capable of inhibiting deoxygenation-induced sickling of SS and SC cells; under conditions that cause normal cell bursting, sickle cells swell but do not lyse.