title: Identification of cyclophilin as the erythrocyte ciclosporin-binding protein.
abstract: 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 microgram/ml in whole blood) greater than 90% of the erythrocyte associated ciclosporin was found in the cytosol. The cytosolic binding capacity was approximately (2-2.5).10(5) 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).10(5) copies of this binding protein within each erythrocyte is responsible for the ciclosporin found associated with erythrocytes.
authors.affiliation: Sandoz Ltd., Basle, Switzerland.
authors.forename: B M
authors.initials: BM
authors.lastname: Foxwell
authors.forename: G
authors.initials: G
authors.lastname: Frazer
authors.forename: M
authors.initials: M
authors.lastname: Winters
authors.forename: P
authors.initials: P
authors.lastname: Hiestand
authors.forename: R
authors.initials: R
authors.lastname: Wenger
authors.forename: B
authors.initials: B
authors.lastname: Ryffel
mesh_terms: D001667:Binding, Competitive; D001798:Blood Proteins / Q000378:metabolism; D002352:Carrier Proteins / Q000097:blood*; D002846:Chromatography, Affinity; D003524:Cyclosporins / Q000097:blood; D003600:Cytosol / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004910:Erythrocyte Membrane / Q000378:metabolism; D004912:Erythrocytes / Q000032:analysis / Q000378:metabolism* / Q000648:ultrastructure; D006454:Hemoglobins / Q000032:analysis; D006801:Humans; D007118:Immunoassay; D019696:Peptidylprolyl Isomerase
chemical_list: D001798:Blood Proteins; D002352:Carrier Proteins; D003524:Cyclosporins; D006454:Hemoglobins; D019696:Peptidylprolyl Isomerase