The kinetics of a novel cyclic reaction system and its use in the assay of polynucleotide phosphorylase or adenosine diphosphate plus adenosine triphosphate.

1. A sensitive new method is described for the rapid optical assay of polynucleotide phosphorylase in crude bacterial extracts of low specific activity. 2. The assay is non-linear with time and a quick method for determining the approximate activity by using a nomograph is given. 3. The dependence of the assay on Mg(2+) concentration was investigated and the conditions that gave first-order kinetics were determined. 4. The theoretical behaviour of the system was examined mathematically and shown to tend to a simple dynamic steady state, under certain conditions. 5. The potential uses of such cyclic systems are discussed.