Direct Photo‐affinity Labeling of Cyclic Nucleotide Binding Proteins With Guanosine‐3‘,5’‐monophosphate

Radioactivity from [ 3 H]‐guanosine‐3′,5′‐monophosphate was shown to become stably linked to proteins in testis extracts during ultraviolet irradiation. Adenosine‐3′,5′‐monophosphate competitively inhibited incorporation into these proteins. Guanosine‐3′,5′‐monophosphate had a lower affinity for binding sites than did adenosine‐3′,5′‐monophosphate. Similar peptides were photo‐labeled with [ 3 H]‐guanosine‐3′,5′‐monophosphate and [3H]‐adenosine‐3′,5′‐monophosphate. Guanosine‐3′,5′‐monophosphate was more efficiently incorporated than was adenosine‐3′,5′‐monophosphate. Extracts from a number of tissues incorporated both cyclic nucleotides photochemically.