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Assay Method Information

Assay Name:  Protease-Free PPIase Assay
Description:  The protease-free PPIase assay measures the rate of cis to trans conversion of a peptide substrate catalyzed by the enzyme cyclophilin A. Addition of a cyclophilin A inhibitor (e.g., a test compound) slows the catalyzed rate and a Ki value is obtained. A Ki value of less than 10 nM demonstrates that the test compound is a potent inhibitor of cyclophilin A.MaterialsAssay Buffer:35 mM HEPES pH 7.8, filtered through a 0.2 μm filter. 50 μM DTT was added prior to use each day and then the buffer was stored on ice.Enzyme:Human recombinant cyclophilin A (Cyp A) (Sigma C3805) enzyme was diluted to 1 μM with enzyme dilution buffer (20 mM HEPES pH 7.8, 40% glycerol, 50 μM DTT and 1 μM BSA) and stored at −20 C.Substrate:Succinimide-Ala-Ala-Pro-Phe-p-nitroanilide (SUC-AAPF-pNA) (from Bachem AG, L-1400), 20 mg/ml prepared in 0.5 M LiCl in trifluoroethanol.MethodAll readings were taken with an Agilent 8453 Spectrophotometer which includes of a cuvette holder, stirrer and chiller to maintain a stirred cuvette temperature of 10.00.1 C. The temperature is monitored by the use of a temperature probe. To prevent UV degradation of test compounds, the light below 290 nm was blocked using a glass slide in the light path. 1.5 ml of the assay buffer was put into a 3 ml quartz cuvette and cooled to 10.00.1 C. while stirring (vigorous but not so fast as to produce cavitation). The inhibitor was diluted in 100% DMSO, and then added to the assay to a maximum final concentration of 0.5% DMSO in the assay. A blank spectrum was obtained, then 3 μL of enzyme was added (2 nM final concentration) and then 3 μL substrate (60 μM final concentration) added. The absorbance was measured at 330 nm for 300 s or 500 s for blank runs (NOTE: the substrate must be added in one quick injection and the measurements started immediately to minimize mixing errors).A first order rate equation was fitted to the absorbance data, for each concentration of inhibitor, to obtain the rate constant (the first 10 to 15 seconds were excluded as mixing causes errors in this portion of curve). The catalytic rate was calculated from the enzymatic rate constant minus the background rate constant. An exponential curve was generated using the catalytic rate constants versus the inhibitor concentration to obtain the Ki value for the inhibitor. The Ki value is indicative of the binding affinity between the test compound and cyclophilin A.
Affinity data for this assay
 

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Last update November 1, 2007
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