Carboxylmethylation of phosphodiesterase attenuates its activation by Ca²⁺-calmodulin

Carboxylmethylation of several preparations of cAMP phosphodiesterase by the enzyme protein O-carboxylmethyltransferase and S-adenosylmethionine reduces the extent to which the enzyme was activated by native calmodulin. In contrast, carboxylmethylation of calmodulin produced only a slight reduction in the ability of calmodulin to activate cAMP phosphodiesterase. The effect of carboxylmethylation of calmodulin was most prominent at subsaturating calmodulin concentrations, whereas the reduction in the activation of carboxylmethylated cAMP phosphodiesterase was independent of calmodulin concentration. Kinetics and stoichiometric analysis of calmodulin carboxylmethylation indicated that less than 5% of calmodulin was methylated and that the K(m), of protein O-carboxylmethyltransferase for calmodulin was approximately 350 μM. The extent of calmodulin carboxylmethylation was not affected by either EGTA or Ca²⁺. When homogeneous bovine brain phosphodiesterase was carboxylmethylated, a rapid decrease in calmodulin-induced stimulation was noted, occurring within 30 s of incubation. Acidic sodium dodecyl sulfate-gel electrophoresis of bovine brain phosphodiesterase revealed a major band of 60,000 daltons which contained radioactivity after carboxylmethylation. Stoichiometric analysis revealed that approximately 20% of the phosphodiesterase was carboxylmethylated. Thus, although calmodulin can serve as a substrate for carboxylmethylation, it appears that carboxylmethylation, has a greater effect on calmodulin-dependent phosphodiesterase activity when the target enzyme, rather than calmodulin, is carboxylmethylated.