Effects of 8-substituted adenosine 3',5'-monophosphate derivatives on high Km phosphodiesterase activity

Most of twenty-one 8-substitued adenosine 3',5'-monophosphate derivatives were found to inhibit competitively the hydrolysis of adenosine 3'5'-monophosphate by partially purified high Km (Michaelis-Menten constant) phosphodiesterase from hog brain cortex, which had one active site at high concentration of adenosine 3',5'-monophosphate (0.3 to 4.0 mM). The Ki value for the 8-substituted alkylaminoadenosine 3'5'-monophosphate derivative was found to decrease with increasing unbranched carbon chain of the substituent, and a minimum value was obtained in the case of 8-octylaminoadenosine 3',5'-monophosphate. The Ki value, however, increased gradually as the substituent of derivative became longer than that of 8-octylminoadenosine 3'5'-monophosphate. The similar phenomenon was observed in the 8-substituted alkylthioadenosine 3',5'-monophosphate. The standard affinity for adenosine 3,5'-monophosphate of the high Km phosphodiesterase was 5.0 kcal/mol, which was calculated from Km. The standard affinity for 8-hexylthioadenosine 3',5'-monophosphate, which inhibited most strongly the enzyme activity, was 7.2 kcal/mol. The difference (2.2 kcal/736) between the standard affinity for adenosine 3',5'-monphosphate and that for 8-hexylthioadenosine 3',5'-monophosphate seems to be based on the partial affinity for the substituent (hexylthio group) of the active site on the enzyme or its neighborhood. A characteristic similar interrelation between substituent length of derivatives and their inhibitory effect on the enzyme activity was observed similarly in two different series of derivatives, 8-substituted alkylaminoadenosine 3',5'-monophosphate and alkylthioadenosine 3',5'-monophosphate. The results may indicate the characteristic structure of the active site of the enzyme or its neighborhood.