Figure 3.

CatalyCEST MRI of uPA activity. (A) 25 mM of the agent was incubated with 50 units of urokinase Plasminogen activator (uPA) enzyme and 9.7 pmol DTT for 12 hours at 37°C, resulting in a 3.7-fold decrease in the CEST signal amplitude at 5.0 ppm relative to 9.5 ppm. (B) The same experiment without DTT showed almost no change in CEST signals, indicating that DTT was necessary for uPA activity. (C) The same experiment without enzyme showed almost no change in CEST signals, indicating that the agent was stable in a reducing environment with DTT. CEST spectra were acquired by applying selective saturation at 4 μT for 6 s before acquiring a FISP MR image, at 85 selective frequencies spanning 15 to −15 ppm. Experimental CEST spectra are shown as blue circles, the fitting of the experimental spectra are shown as blue lines, and the CEST lineshape spectra from these fittings are shown as red lines. Results for the substrate before adding enzyme are shown as dark lines and labeled with a “s”, while results for the product after adding enzyme are shown as light lines and labeled with a “p”, respectively.