Effect of Ca2+ ions on liver cell plasma membrane polypeptides.

Plasma membrane fractions, microsomes, and mitochondrial membrane were prepared from rat liver. The effects of incubation of these membranes in various ionic strengths (1-12 mM) of NaHCO3 (pH 7.4) on the polypeptide patterns were studied in the absence or the presence of various concentrations of CaCl2 (1-12 mM). Many polypeptides were extracted from each of the membranes in the low ionic strength buffers (1 mM); these polypeptides were essentially similar to those remaining in the membrane after incubation. The extraction of these polypeptides continued when concentration of NaHCO3 increased. However, the presence of CaCl2 at a concentration of 4 mM or higher stabilized the membrane proteins and reduced the continuous loss of the polypeptides, and only low molecular weight polypeptides were affected. These data suggest that the organization of proteins in liver cell membranes may depend on the nature of their interaction with other membrane components (i.e., phospholipids), which could be affected by Ca2+ ions.