Characterization of apocytochrome C binding to human erythrocytes

The binding of 125 I‐labeled apocytochrome c to human erythrocytes was determined for free apocytochrome c concentrations at 10 −10 ‐10 −6 M. At about 2 × 10 −9 M, maximum cell 22°C have three classes of apocytochrome c binding sites: one high‐affinity noncooperative site (n 1 = 728 per cell, K d1 = 1.5 × 10 −9 M) and two positively cooperative sites (n 2 = 3.7 × 10 4 per cell, K d2 = 1.2 × 10 −7 M, α 2 = 2.0, and n 3 = 2.5 × 10 5 per cell, K d3 = 7.1 × 10 −7 M, α 3 = 12). Erythrocytes at 37°C, and erythrocyte ghosts at 22°C, also have three classes of apocytochrome c binding sites, and most sites are positively cooperative. © 1994 Wiley‐Liss, Inc.