Adenosine 5'-(beta, gamma-imino)triphosphate and guanosine 5'-O-(2-thiodiphosphate) do not necessarily provide non-phosphorylating conditions in adenylate cyclase studies.

Commercial preparations of adenosine 5'-(beta, gamma-imino)triphosphate (App(NH)p) were found to be contaminated with a GTP-like substance(s) as well as a phosphate donor(s) for GDP. Thus, when these preparations were used as substrate with no purification, GDP was as effective as GTP in promoting PGE1 stimulation of human platelet adenylate cyclase. With purified App(NH)p as substrate, the effect of PGE1 with GDP was reduced but still observable, while that with GTP was unaltered. PGE1 also caused a stimulation in the presence of guanosine 5'-o-(2-thiodiphosphate)(GDP beta S) with ATP as substrate. Both of the PGE1-stimulated activities observed with GDP and its analog were completely lost by the addition of UDP, thereby, inhibiting GTP formation catalyzed by membrane-associated nucleoside diphosphate kinase. The results demonstrate that the stimulatory effects of PGE1 observed with GDP and App(NH)p, and with GDP beta S and ATP were transphosphorylation dependent and, therefore, the analogs must be used with special caution in adenylate cyclase studies.