A simple, sensitive protein-binding assay for guanosine 3':5'-monophosphate

A protein-binding assay for guanosine 3':5'-monophosphate, which employs an enzyme preparation from lobster tail muscle, is described. The binding constant for cyclic GMP was 2-10 nM for four different enzyme preparations. The method can be set up in several days and permits the detection of as little as 0.5-1 pmol of cyclic GMP. In some instances, it is possible to assay the nucleotide in urine or deproteinized extracts of tissue without purification. For other samples, a simple method for separating and purifying cyclic AMP and cyclic GMP is described.