Hydrolysis of the cyclic 3':5'-monophosphates of adenosine and guanosine by rat pancreas.

1 Crude rat pancreas homogenates had two apparent Km values for hydrolysis of adenosine 3′:5′-monophosphate (cyclic AMP) of 1.9 and 32 μM with corresponding apparent V values of 147 and 1033 pmol × min−1× mg protein−1 at 30°C. The same homogenates exhibited two apparent Km for hydrolysis of guanosine 3′:5′-monophosphate (cyclic GMP) of 4 and 40 μM with apparent V values of 70 and 300 pmol × min−1× mg protein−1, respectively. In comparison with liver and parotids the affinity of low-Km phosphodiesterases for cyclic nucleotides was low but the velocity of hydrolysis was higher. Cyclic GMP interfered somewhat with cyclic AMP hydrolysis but it was not possible to define Ki values. Aminophylline was a weak non-competititive inhibitor of both forms of cyclic AMP phosphodiesterases. 2 A major portion (65%) of both cyclic AMP phosphodiesterase activities and of the enzyme with a 40-μM Km for cyclic GMP was confined to the cytosol while the remnant was partitioned among particulate fractions. The subcellular distribution of the enzyme, presenting a 4-μM Km for cyclic GMP, differred since as much as 82% of its activity was recovered in the cytosol. 3 A secretory cycle induced in vivo by pilocarpine (30 mg/kg) did not influence phosphodiesterase activities for 3 h after the intraperitoneal injection. A 10-day adaptation to protein malnutritution tended to increase the activity of the low-Km cyclic AMP phosphodiesterase. 4 It was concluded that the rat pancreas was able to hydrolyse cyclic nucleotides at physiological concentrations and that more than one enzyme was apparently involved.