Measurement of 5′-adenylic acid by stimulation of phosphorylase a

A method for measuring 5′-adenylic acid is described which is based on its capacity to stimulate the activity of phosphorylase a in the presence of low levels of glycogen and inorganic P. The method is useful with very low concentrations of AMP (0.02 to 0.5 μ m) and is unaffected by much higher levels of ADP and ATP. Other common mononucleotides do not interfere at concentrations likely to be encountered. Yields of final product (NADPH) as high as 500 mol/mol of AMP can be obtained. Protocols are given for measuring as much as 5 × 10 −10 mol at one extreme and as little as 10 −15 mol at the other.