Fig 1: Inhibition of PDE1 enzyme activity by BTTQ. The compound was pre-incubated with different recombinant PDE1 isoforms for 30 min before the addition of the substrate. After incubating at room temperature for 60 min, reactions were stopped by the addition of SPA beads. cAMP level was measured after 12 h. IC50 values of PDE1A (A), PDE1B (B), and PDE1C (C) were calculated by plotting the normalized data vs. log [compound] and fitting the data using a four-parameter logistic equation.