Demonstration of bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes by monoclonal antibodies

Calmodulin-dependent cyclic nucleotide phosphodiesterase from bovine brain is found to be composed of two distinct subunits, 60,000- and 63,000-dalton polypeptides. Peptide mapping of the subunits by partial proteolysis demonstrated that the 60-kDa polypeptide is not derived from the 63-kDa species. The interaction of the enzyme with three monoclonal antibodies, A6, C1, and A2, and the analysis of immunocomplexes by sucrose density gradient centrifugation revealed that calmodulin-dependent cyclic nucleotide phosphodiesterase exists in three different forms, i.e. (a) homodimer of 60-kDa, (b) heterodimer of 60- and 63-kDa, and (c) homodimer of 63-kDa. A6 antibody reacts with both 60- and 63-kDa polypeptides indicating that they are immunologically related. C1 and A2 antibodies react with only 60-kDa polypeptide species. By using C1 Sepharose 4B affinity column chromatography, the 63-kDa homodimer which did not bind to the column (Fraction I) was separated from the 60-kDa polypeptide containing isozymes (the heterodimer and the 60-kDa homodimer) which were retained on the column and later eluted as a mixture (Fraction II). Fraction I, the 63-kDa homodimer enzyme, has higher V(max) toward cGMP as substrate than cAMP whereas the opposite was found with Fraction II. The specific activity of Fraction II enzyme toward cAMP was approximately 500 μmol/min/mg, the highest value ever reported for brain calmodulin-dependent cyclic nucleotide phosphodiesterase preparations.