3′:5′‐Cyclic Nucleotide Phosphodiesterases in Rat Tissues

The rates of enzymatic hydrolysis of cyclic AMP, cyclic GMP, cyclic IMP, cyclic UMP, cyclic CMP and cyclic dTMP (all 3′:5′‐phosphates) have been assayed in several rat tissues. Subcellular distribution studies indicated that, in all tissues except brain and heart, most of the enzymatic activity is confined to the soluble cell fraction. The purine cyclic nucleotides are the preferred substrates for soluble 3′:5′‐cyclic nucleotide phosphodiesterase in all tissues. Cyclic AMP is a slightly better substrate than cyclic GMP and cyclic IMP in most tissues. However, spleen, lung and erythrocytes hydrolyse cyclic AMP 2–3 times faster than the other purine cyclic nucleotides. Hydrolysis of the cyclic pyrimidine nucleotides was very low or not detected in most tissues. The exceptions were cyclic UMP and cyclic dTMP hydrolysis by brain, and cyclic dTMP hydrolysis by liver. Polyacrylamide gel electrophoresis resolved the soluble cyclic nucleotide phosphodiesterase activity into a number of components. Two components, designated B and C, arc present in all tissues except brain although there are quantitative differences between tisues. These two components appear to be specific for purine cyclic nucleotides. A third form, A, is present in spleen, lung and erythrocytes. It hydrolyses all the cyclic nucleotides except cyclic CMP, although cyclic AMP is the best substrate. However, this component also has 5′‐nucleotidase activity which may indicate that A is not specific for 3′:5′‐cyclic nucleotides. A fourth component D, detected only in liver, hydrolyses cyclic AMP and cyclic dTMP at comparable rates. The electrophoretic patterns obtained for brain phosphodiesterase differ from those of all other tissues investigated and suggest that the enzyme(s) probably has markedly different properties from the cyclic nucleotide phosphodiesterases of other tissues.