Previous studies on the distribution of circulating ciclosporin have shown that the majority of the drug is associated with erythrocytes. In order to investigate the nature of ciclosporin-erythrocyte binding, binding studies were performed on isolated erythrocytes. At therapeutic concentrations (approx. 0.5 μg/ml in whole blood) > 90% of the erythrocyte associated ciclosporin was found in the cytosol The cytosolic binding capacity was approximately (2-2.5) · 105 molecules of ciclosporin per cell. A lower affinity binding of the drug to the plasma membrane occurred only at higher ciclosporin concentrations. The ciclosporin-binding species was purified from erythrocyte cytosol using ciclosporin-Affigel affinity chromatography. This revealed a 16 kDa protein, similar in size to the ciclosporin-binding protein, cyclophilin, previously identified in lymphocyte cytosol. Immunochemical analysis using rabbit anti-bovine spleen cyclophilin antisera revealed that the erythrocyte ciclosporin-binding protein was either cyclophilin or a closely related protein. It is concluded that intracellular ciclosporin-binding within erythrocytes is mostly attributable to the presence of a single protein or protein family represented by cyclophilin. The presence of (2-2.5) · 105 copies of this binding protein within each erythrocyte is responsible for the ciclosporin found associated with erythrocytes. © 1988.
Foxwell, B. M. J., Frazer, G., Winters, M., Hiestand, P., Wenger, R., & Ryffel, B. (1988). Identification of cyclophilin as the erythrocyte ciclosporin-binding protein. BBA - Biomembranes, 938(3), 447–455. https://doi.org/10.1016/0005-2736(88)90142-3