Photochemical labeling of the cytoplasmic surface of the membranes of intact human erythrocytes.

N-(4-azido-2-nitrophenyl)-2-aminoethyl sulfonate (NAP-taurine), a photolabile nitrene precursor, has been shown to permeate the human erythrocyte membrane at 37degrees but not at 0 degrees. Utilizing this differential permeability, we have loaded intact erythrocytes with NAP-[35S]taurine in the dark at 37degrees, cooled them to 0 degrees, washed them free of external NAP-[35S]taurine in the dark and cold, and photolyzed them, resulting in labeling of hemoglobin and of proteins on the cytoplasmic surface of the membrane. These experiments complement those previously reported on the labeling of the external surface of the membranes with this reagent (Staros, J. V., and Richards, F. M. (1974) Biochemistry 13, 2720-2726).