Radiometric assay of S-adenosylmethionine:calmodulin(lysine)N-methyltransferase by calcium-dependent hydrophobic...

A radiometric assay for S-adenosylmethionine:calmodulin(lysine)N-methyltransferase in tissue extracts has been developed. This assay utilizes the non-N-methylated calmodulin from Dictyostelium discoideum as a substrate and calcium-dependent hydrophobic interaction chromatography to isolate the reaction product, N-methylated calmodulin. The assay measures the transfer of 3H-labeled methyl groups from [methyl-3H]S-adenosylmethionine to calmodulin. Methylated calmodulin is eluted from phenyl-Sepharose columns with EDTA and protein carboxylmethyl esters are removed by heat treatment prior to liquid scintillation counting of [methyl-3H]calmodulin. This assay is more specific, more sensitive, and more precise than the acid-precipitation assay previously employed and lends itself more readily to the assay of large numbers of samples.