THE DIFFERENTIAL IN-VITRO STIMULATION OF 3',5'-CYCLIC NUCLEOTIDE PHOSPHODIESTERASE BY CALCIUM-BINDING PROTEINS

A eterogeneous class of proteins exhibit within their sequence a parti cular structure, named EF-hand, able to bind calcium with high affinit y. These calcium binding proteins have been described in most cells an d tissues and are suggested to work as calcium buffers, thereby partic ipating in the regulation of calcium-dependent cellular activity. Rece nt circumstantial evidences suggest that calcium binding proteins may serve other functions as well, possibly as enzyme modulators. Since 3' ,5'-cyclic nucleotide phosphodiesterase is a well-known calmodulin-mod ulated enzyme, in this work we studied the effect in vitro of differen t purified calcium binding proteins on the activity of this enzyme. Am ong the proteins tested, calmodulin and recombinant rat brain parvalbu min could stimulate the 3',5'-cyclic nucleotide phosphodiesterase acti vity in vitro, whereas rabbit muscle parvalbumin, rat renal and brain calbindin D-28K, and bovine brain S-100B were ineffective. Immunopreci pitation with the specific antiserum completely abolished either calmo dulin or recombinant brain parvalbumin activation of 3',5'-cyclic nucl eotide phosphodiesterase. Moreover, while the presence of calcium in t he incubation mixture was critical in the calmodulin-mediated stimulat ion of the enzyme, it did not modify the effect of the recombinant bra in parvalbumin. We suggest that, in addition to calmodulin, parvalbumi n may be a regulator of 3',5'-cyclic nucleotide phosphodiesterase, and possibly of other yet to be identified enzymes in certain tissues.