We reported that one of the isoquinolinesulfonamide derivatives, KN-62, is a 
potent and specific inhibitor of Ca2+/calmodulin-dependent protein kinase II 
(CaMKII) (Tokumitsu, H., Chijiwa, T., Hagiwara, M., Mizutani, A., Terasawa, M. 
and Hidaka, H. (1990) J. Biol. Chem. 265, 4315-4320). We have now investigated 
the inhibitory property of a newly synthesized methoxybenzenesulfonamide, KN-93, 
on CaMKII activity in situ and in vitro. KN-93 elicited potent inhibitory 
effects on CaMKII phosphorylating activity with an inhibition constant of 0.37 
microM but this compound had no significant effects on the catalytic activity of 
cAMP-dependent protein kinase, Ca2+/phospholipid dependent protein kinase, 
myosin light chain kinase and Ca(2+)-phosphodiesterase. KN-93 also inhibited the 
autophosphorylation of both the alpha- and beta-subunits of CaMKII. Kinetic 
analysis indicated that KN-93 inhibits CaMKII, in a competitive fashion against 
calmodulin. To evaluate the regulatory role of CaMKII on catecholamine 
metabolism, we examined the effect of KN-93 on dopamine (DA) levels in PC12h 
cells. The DA levels decreased in the presence of KN-93. Further, the tyrosine 
hydroxylase (TH) phosphorylation induced by KCl or acetylcholine was 
significantly suppressed by KN-93 in PC12h cells while events induced by 
forskolin or 8-Br-cAMP were not affected. These results suggest that KN-93 
inhibits DA formation by modulating the reaction rate of TH to reduce the 
Ca(2+)-mediated phosphorylation levels of the TH molecule.