CALCIUM-INDEPENDENT AND CALMODULIN-INDEPENDENT MODULATION OF CALMODULIN-SENSITIVE HYPOTHALAMIC CYCLIC-NUCLEOTIDE PHOSPHODIESTERASE ACTIVITYBY THE (11-19)-FRAGMENT OF THYMOSIN BETA-4

A fragment (11-19) of thymosin beta 4 was found to stimulate phosphodi esterase activity even in the absence of calcium and calmodulin. Half- maximal enzyme activation occurred with 10 nM peptide, and was further increased by phospholipids such as phosphatidylserine. The mechanism of stimulation is an increase in the V-max of cAMP degradation without a substantial change in the K-m for the substrate. In the presence of calcium ions and calmodulin the peptide was also stimulatory.