Cyclic AMP phosphodiesterase activity in mouse pancreatic islets. Effects of calmodulin and phospholipids.

The activity of cyclic AMP phosphodiesterase in mouse pancreatic islets was investigated. 85% of the total activity was found in a 27000 g supernatant fraction. The phosphodiesterase activity in the supernatant fraction, but not in the particulate fraction, was stimulated approximately 20% by Ca2+ (10(-5)M) and calmodulin (1 microM). The Km (cyclic AMP) of the unstimulated enzyme in the supernatant fraction was 20 microM, and the Vmax was 2 nmol/min X mg protein-1. The possible influence of a range of phospholipids was investigated. PI* and PS (150 micrograms/ml) inhibited the enzyme 20-30% both in the absence and presence of Ca2+/calmodulin, whereas PE, PC and PA did not affect the enzyme activity. ATP (1 mM) did not affect the particulate or supernatant fraction phosphodiesterase either in the absence or presence of Ca2+/calmodulin or Ca2+/phospholipid. It is concluded that, contrary to islet adenylate cyclase, islet cyclic AMP phosphodiesterase may be regulated by Ca2+/calmodulin.