Measurement of anionic sites on the surfaces of baby hamster kidney cells using radiolabeled polycationic ferritin

A method is described to determine relative numbers of anionic sites on the surfaces of cells under physiological conditions by binding studies with radiolabeled polycationic ferritin. Labeling of cells by polycationic ferritin occurred very rapidly even at 2°C and was essentially complete within 1 min. At 22°C, a rapid initial phase of labeling was followed by a second, slower binding phase. The interaction of rapidly labeled cell surface anionic sites with polycationic ferritin had a binding constant of 3.6 × 10⁶m^-1 (measured at 2°C) and there were about 4 × 10⁶ of these sites per cell.