An improved protein binding assay for guanosine-3',5'-monophosphate using a binding protein from the pupa of the silkmoth, Bombyx mori L.

A modification of the protein binding assay for cyclic guanosine-3',5'-monophosphate (cyclic GMP) is described that is more sensitive and less subject to interference by cyclic AMP than are previously published protein binding methods. The assay employs a purified binding protein from the fat body of the pupa of the common silkmoth, Bombyx mori. The dissociation constant of the binding protein for cyclic GMP is 4.3 nM. A protein kinase modulator protein isolated from the same species increases the binding affinity and capacity of the cyclic GMP binding protein and can be used to advantage in the assay for cyclic GMP. As little as 0.1 pmoles of cyclic GMP can be detected by this procedure. Changes in the level of cyclic GMP in the frog heart during the cardiac cycle were determined by means of the new assay.