The optical activity of plasma membranes and its modification by lysolecithin, phospholipase A and phospholipase C

Abstract

1. 1. The circular dichoroism and optical rotatory dispersion of the plasma membranes of human erythrocytes and of Ehrlich ascites carcinoma cells for the spectral range 190–250 mμ are reported. 2. 2. The spectra indicate the presence of some peptide in an α-helical conformation but also suggest that the helical π°- π − transitions of membrane proteins are of a lower rotational strength and are located at a longer wavelength than the corresponding bands in synthetic polypeptides. This view is supported by the analysis of the spectra by curve-fitting using the Gauss nonlinear least-squares method. 3. 3. The helical n- π − transitions of these membranes have the same spectral position as those of synthetic α-helical polypeptides but are of low intensity and of a arge bandwidth, of which the latter accounts for the red displacement of the optical rotatory dispersion through observed in many membranes. 4. 4. Except for the width of the n- α − bands, the optical activity parameters of these membranes differ from those of synthetic polyptide standards in the same manner, although more so, as do certain globular proteins of which the structure is known from X-ray analysis. These deviations are attributed to the presence of short helices, distorted helices and location of some helical peptide chromophores in a highly polarizable apolar environment. 5. 5. We present the effects upon the membrane optical activity of phospholipase A. phospholipase C, lysolecithin and digitonin and conclude that the protein architecture of these membranes depends upon lipid-protein interactions and/or protein-protein interactions which are sensitive to lipid.