Equilibrium studies of a fluorescent tacrolimus binding to surfactant protein A

Tacrolimus (FK506) is a hydrophobic immunosuppressive agent used in kidney, liver, and lung transplantation. The objective of this study was to characterize the binding of FK506 to surfactant protein A (SP-A), an abundant lipoprotein found in the alveolar fluid that functions as part of the innate immune system in the lung. We have synthesized a novel derivative of FK506 in which a dansyl moiety was covalently bound via cadaverine to the C22 position of the FK506 molecule (DNS-FK). Using the fluorescence and anisotropy properties of DNS-FK. we demonstrated that tacrolimus avidly binds to SP-A with an apparent equilibrium association constant (K-app) of 10(7) M-1 and a Gibbs binding free energy of -40 kJ mol(-1) K-1. Derivatization of FK506 at the C22 position did not block FK506 binding to the cytosolic immunophilin FK506-binding protein (FK-BP) or human serum albumin (HSA). both used as controls of tacrolimus-binding proteins. K-app for FK-BP/DNS-FK and HSA/DNS-FK complexes were 1.5 x 10(7) and 10(7) M-1, respectively. The high sensitivity of this analytical technique Makes it suitable for binding analysis of FK506 to proteins. (c) 2005 Elsevier Inc. All rights reserved.