text: Catalytic-site affinities for cGMP, vardenafil, sildenafil, << tadalafil >>, or 3-isobutyl-1-methylxanthine (IBMX) were respectively weakened 14-, 123-, 30-, 51-, and 43-fold for Y612A; 63-, 511-, 43-, 95- and 61-fold for [[ Q817A ]]; and 59-, 448-, 71-, 137-, and 93-fold for F820A.