Use of photosensitive hydrophobic probes to label the membrane of the human erythrocyte.

Two photosensitive hydrophobic probes, azido [3H]benzene and 1-azido-4-iodo[3H]-benzene, have been compared for their effectiveness in labelling, selectively, the intramembranous domains of lipid and proteins. Both partition preferentially into the lipid bilayer and, upon irradiation, covalently attach to both phospholipids and membrane proteins; the more extrinsic polypeptides have a significantly lower specific radioactivity than that of the intrinsic species. Proteolytic experiments also reveal higher labelling of intramembranous regions of the proteins. Consistently, the iodinated form of the probe showed the greater preference for the non-polar phase and a higher degree of selectivity for labelling hydrophobic regions. The results also suggest that penetration through the annulus of tightly bound lipid surrounding integral proteins occurs readily.