Polyion-sensitive membrane electrodes for detecting phosphate-rich biological polyanions

Appropriately formulated polymeric membrane electrodes are shown to exhibit analytically useful nonequilibrium potentiometric responses to biologically important polyanions with high phosphate content, including polyphosphates and nucleic acids, in the mu g/mL concentration range. This response is dependent on the charge density, lipophilicity, and molecular weight of the polyanion species. The application of these electrodes for monitoring enzymatic activities is also demonstrated, using acid phosphatase/polyphosphate as a model system. The detection limit of this assay was found to be 0.9 +/- 0.4 mU/mL with a linear range of 5-100 mU/mL. A similar electrochemical assay scheme can be utilized to monitor ribonuclease activity, using a RNA homopolymer as the substrate.