Calmodulin-activated cyclic nucleotide phosphodiesterase from brain. Relationship of subunit structure to activity assessed by radiation inactivation.

The apparent target sizes of the basal and calmodulin-dependent activities of calmodulin-activated phosphodiesterase from bovine brain were estimated using target theory analysis of data from radiation inactivation experiments. Whether crude or highly purified samples were irradiated, the following results were obtained. Low doses of radiation caused a 10 to 15% increase in basal activity, which, with further irradiation, decayed with an apparent target size of approximately 60,000 daltons. Calmodulin-dependent activity decayed with an apparent target size of approximately 105,000 daltons. The percentage stimulation of enzyme activity by calmodulin decreased markedly as a function of radiation dosage. These observations are consistent with results predicted by computer-assisted modeling based on the assumptions that: 1) the calmodulin-activated phosphodiesterase exists as a mixture of monomers which are fully active in the absence of calmodulin and dimers which are inactive in the absence of calmodulin; 2) in the presence of calmodulin, a dimer exhibits activity equal to that of two monomers; 3) on radiations destruction of a dimer, an active monomer is generated. This monomer-dimer hypothesis provides a plausible explanation for and definition of basal and calmodulin-dependent phosphodiesterase activity.