Six of the substrates currently used for the detection and quantitation of phosphodiesterase I (PDE I) were examined. Michaelis-Menten kinetic analysis of the hydrolysis of each of these by venom and bovine intestinal PDE I was performed to allow a comparison of the sensitivity of detection for each substrate. The results confirm that phenolic esters of phenylphosphonate are hydrolyzed 2- to 4-fold more rapidly than are the same esters of thymidine 5'-phosphate when both are present in saturating concentrations. The identification of the bovine intestinal enzyme as a PDE I is verified by its extensive similarity to the paradigmatical PDE I from rattlesnake venom. A new definition for PDE I activity is proposed which takes into account its ability to hydrolyze several types of substrates.

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