Inhibition by diuretics of cyclic 3',5'-AMP-dependent protein kinase from toad bladder epithelium

1. A cAMP-dependent protein kinase enzyme has been isolated from toad bladder epithelium.2. This enzyme catalyses the phosphorylation of histones; a reaction stimulated by 0.1 muM cAMP.3. Activity of this enzyme in the presence of 0.1 muM cAMP was significantly inhibited by 100 muM mercuderamide, bumetanide and frusemide. A small, though statistically insignificant inhibition was seen with the same concentrations of hydrochlorothiazide and ethacrynic acid.4. The half maximal inhibition was achieved with mercuderamide, 5 muM, and with frusemide and bumetanide 600 muM.