A photoaffinity probe covalently modifies the catalytic site of the cGMP-binding cGMP-specific phosphodiesterase (PDE-5)

The cGMP-binding cGMP-specific phosphodiesterase (PDE-5) contains distinct catalytic and allosteric binding sites, and each is cGMP-specific. Cyclic nucleotide phosphodiesterase inhibitors, such as 3-isobutyl-1-methylxanthine (IBMX), are believed to compete with cyclic nucleotides at the catalytic sites of these enzymes, but the portion of PDE-5 that accounts for interaction of either of these inhibitors of the substrates themselves with the catalytic domain of the enzymes has not been identified. IBMX was derivatized to yield the photoaffinity probe 8([3-125I,-4-azido]-benzyl)-IBMX, which is referred to as 8(125IAB)-IBMX. This probe was incubated with partially purified recombinant bovine PDE-5. After UV irradiation and SDS-PAGE, a single radiolabeled band that coincided with the position of PDE-5 was visualized on the gel, and the photoaffinity labeling of PDE-5 was linear with increasing concentration of the 8(125IAB)-IBMX. Prominent Coomassie blue-stained bands other than PDE-5 were not labeled significantly. The photoaffinity labeling was progressively blocked by cGMP at concentrations higher than 10 microM, whereas cAMP or 5'-GMP exhibited only weak inhibitory effects. Other compounds that are believed to interact with the PDE-5 catalytic site, including IBMX, cIMP, and beta-phenyl-1,N2-etheno-cGMP (PET-cGMP), also inhibited the photoaffinity labeling in a concentration-dependent manner. The IC50 of PET-cGMP for inhibition of photoaffinity labeling was 10 microM, which compared favorably with an IC50 of 5 microM for inhibition of PDE-5 catalytic activity by this compound. It is concluded that the interaction of this photoaffinity probe with PDE-5 is highly specific for the catalytic site over the allosteric binding sites of PDE-5 and could prove useful in studies to map the catalytic site of PDE-5.