Calcium binding to plasma membranes from normal and SV40 transformed hamster lymphocytes

The calcium binding properties of isolated plasma membranes from normal and SV40 transformed hamster lymphocytes were compared over the Ca2+ concentration range of 10(-5)M to 5 x 10(-3)M and at physiological ionic strength. At all Ca2+ concentrations, normal membranes bound more Ca2+ than tumor membranes; at blood Ca2+ levels (1-2 mM) plasma membranes of normal cells bind twice as much as membranes from tumor cells. Normal plasma membranes demonstrated positive cooperative Ca2+ binding whereas tumor membranes displayed non-interacting Ca2+ binding sites. Ca2+ binding to both membranes was insensitive to Mg2+ (0.1 to 2.5 mM). A pH shift from 7 to 6 resulted in a 70% decrease of normal membrane-bound Ca2+ compared to a 40% decrease observed with tumor membranes. Extracellular surface Ca2+ binding to intact cells was also studied after a 72-hour equilibration of cells with 45 Ca2+ and with ethyleneglycol-bis-(beta-amino-ethyl ether) N,N'-tetraacetate chelation as marker for surface Ca2+. Tumor cell surface Ca2+ binding was only 10% of that observed with quiescent lymphocytes. Normal lymphocytes stimulated to divide with phytohemagglutinin also showed a decreased level of surface Ca2+ (50%). However, plasma membranes isolated from non-dividing and phytohemagglutinin-stimulated lymphocytes exhibited equivalent Ca2+ binding.