Abstract

Cyclic 3',5'-AMP phosphodiesterase and cyclic 3',5'-GMP phosphodiesterase of rat lung were found predominantly in the 100,000 x g supernatant fraction. Both enzymes had biphasic Hofstee plots, with apparent K_m values of 0.3 µM and 115 µM for cyclic AMP, and 1.5 µM and 19 µM for cyclic GMP. Electrophoretic analysis of the soluble supernatant fraction of lung on a preparative polyacrylamide gel column showed two stable peaks of phosphodiesterase activity. The first peak, which had relatively little phosphodiesterase activity, hydrolyzed only cyclic AMP, whereas the other peak hydrolyzed both cyclic AMP and cyclic GMP. Both peaks of cyclic AMP phosphodiesterase activity of lung had biphasic Hofstee plots. The first had apparent K_m values of 2.9 µM and 200 µM, and the second had K_m values of 1.9 µM and 190 µM; the cyclic GMP peak had a single K_m of 7 µM. These peaks of phosphodiesterase were markedly different in their response to drugs; the first peak was highly resistant to pharmacological inhibition while the second was sensitive to several phosphodiesterase inhibitors. For example, the inhibitor constant (K_i) of trifluoperazine was approximately 1000 µM for the first peak but only about 50 µM for the second peak. In this second peak, both cyclic AMP and cyclic GMP phosphodiesterase activities were inhibited to a similar extent by a variety of inhibitors. Neither the cyclic AMP nor cyclic GMP phosphodiesterase of crude or electrophoretically purified fractions was significantly activated by a heat-stable factor derived from rat cerebrum on rat lung. However, the lung does contain an activator of phosphodiesterase which is capable of stimulating brain phosphodiesterase. The differential effects of inhibitors on the multiple forms of phosphodiesterase suggest that the concentration of cyclic nucleotides in lung may be selectively altered by appropriate pharmacological agents.