Implication of cysteine residues in the selection of oxorhenium inhibitors of cyclophilin hCyp18.

The dynamic combinatorial assembly of libraries of modular cyclophilin hCyp18 oxorhenium inhibitors of general formula [A˙ReO˙B] was accelerated by addition of increasing concentrations of hCyp18 ('Cyclophilin Enhancing Effect', CEE). This result suggested that modules assembly might proceed through an in situ coordination chemistry process. However, we observed that the CEE was not strictly related to the affinity of the complexes for hCyp18. The CEE was not altered by cyclosporine A, a potent competitive inhibitor of hCyp18. The use of a non-degassed buffer caused a fall in complexation yields that could be reversed upon addition of hCyp18. All these data suggested that the CEE results from a partial protection of exogenous thiols against reoxidation. As anticipated, carbamido-methylation of cyclophilin Cys52 and Cys62 residues with iodoacetamide annihilated the CEE. All these results highlight the role of hCyp18 in maintaining chemical modules in a reduced state.