Specific interaction between bovine cyclophilin A and synthetic analogues of cyclolinopeptide A

Like cyclosporin A, cyclolinopeptide A binds specifically bovine cyclophilin A, inhibiting its peptidyl-prolyl cis-trans isomerase activity. We describe here the protein interaction with several synthetic analogues of cyclolinopeptide A, which are either homodetic or disulphide bridged heterodetic cyclopeptides characterized by different ring dimensions, in terms of dissociation and inhibition constants evaluated by fluorescence and inhibition of the enzyme activity, respectively. Dissociation constants from fluorescence experiments are practically identical and about 20-fold lower than for cyclosporin A. On the other hand, inhibition constants differ from compound to compound and are higher than for cyclosporin A. This result is therefore difficult to rationalize, but we would suggest decoupling between binding and inhibitory ability of cyclopeptides. The Pro1 residue of cyclolinopeptide A seems to play a fundamental role in determining the inhibition of the rotamase activity of cyclophilin A, as the homodetic analogue lacking this residue does not show any inhibitory ability. Similarly, heterodetic analogues with a ring size smaller than 7 residues do not display inhibition. We presume that the sequence -Pro-Pro-Phe-Phe- and a ring size of 8 residues for homodetic cyclic peptides could be used as starting points in the targeted synthesis of cyclopeptides able to bind both cyclosporin A and calcineurin. The only peptide showing similar values of the dissociation and inhibition constant is cyclolinopeptide A. This compound can be considered a novel model for the molecular design of immunosuppressant drugs.