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Cyclic Adenosine 3':5'-Monophosphate in Moss Protonema

A Comparison of Its Levels by Protein Kinase and Gilman Assays

^a Molecular Biology Unit, Tata Institute of Fundamental Research, Bombay 400 005, India

From the protonema of the moss Funaria hygrometrica (L.) Sibth, a factor indistinguishable from cyclic adenosine 3':5'-monophosphate (cAMP) has been isolated. The factor stimulated the activity of protein kinase from rabbit skeletal muscle and co-chromatographed with authentic cAMP in two solvent systems. Its ability to stimulate protein kinase activity was completely abolished by 3':5'-cyclic nucleotide phosphodiesterase, the rate of inactivation being similar to that of authentic cAMP. Based on these properties, this factor is identified as 3',5'-cAMP. Cyclic AMP could be readily removed from the cells and washing the cells with water reduced the endogenous level of cAMP by 2- to 3-fold. A comparison of cAMP levels by protein kinase and Gilman assays was made. The intracellular levels determined by protein kinase assay were about 7-fold lower than the values obtained by Gilman assay. This discrepancy was due to the presence of unidentified compounds which were completely degraded by 3':5'-cyclic nucleotide phosphodiesterase. Although these displaced labeled cAMP in the Gilman assay, they did not stimulate the protein kinase activity. The protonema may contain cyclic nucleotides other than cAMP; these will not be detected in the protein kinase assay due to the specificity of this reaction. The crude extracts were found to be unsuitable for assaying cAMP by either method.