P1,P5-Di(Adenosine-5′)Pentaphosphate(Ap5A) as an Inhibitor of Adenylate Kinase in Studies of Fragmented Sarcoplasmic Reticulum from Bullfrog Skeletal Muscle1

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Abstract
We examined the effects of P1P5-di(adenosine-5′)pentaphosphate (Ap5A), a potent inhibitor of adenylate kinase, on fragmented sarcoplasmic reticulum (FSR) obtained from bullfrog skeletal muscle in view of the possible usefulness of the nucleotide in experiments with FSR to avoid complications due to contaminating adenylate kinase. Ap5A itself does not cause Ca uptake in the place of ATP. It inhibited adenylate kinase activity without affecting the Ca-ATPase or Ca uptake activity of FSR. The observed effect was a competitive inhibition of basic ATPase activity of the light fraction of FSR. Therefore, P1P5-di(adenosine-5′)-pentaphosphate represents an extremely useful tool in experiments with fragmented sarcoplasmic reticulum, such as studies of H+ movement accompanying Ca movement, ATP-ADP exchange reaction, and calorimetry of the Ca uptake process. A rather high concentration (50 μM or more) of Ap6A is required for complete inhibition of adenylate kinase. Further, we detected 1.3−2.8 nmol of (ATP+ADP), 2-4 nmol of P1, and unidentified metal(s) in 50 nmol of Ap5A, and Ap5A is more labile to acid and molybdate than ATP.