Properties of a cyclic nucleotide phosphodiesterase of amphibian oocytes that is activated by calmodulin and calcium, by tryptic proteolysis, and by phospholipids

Trypsin induces Ca(2+)-activated Cl- currents in X. laevis oocytes

The protease trypsin induces Ca(2+)-activated Cl- currents when applied in concentrations as low as 0.1 mg/ml to defolliculated, voltage clamped X. laevis oocytes. The response is dose-dependent and specific, as other proteases (chymotrypsin, Lys-C and Arg-C), or trypsin pretreated with soybean trypsin inhibitor, did not induce currents. Intracellular trypsin injection did not induce responses. The current does not appear to result from proteolytic activation of the endogenous receptor for lysophosphatidic acid, the only known Ca(2+)-mobilizing receptor consistently present in oocytes. These results suggest the presence on the oocyte membrane of a specific receptor for trypsin.

Characterization of cyclic nucleotide phosphodiesterases in Xenopus laevis ovary

A cGMP-stimulated cyclic nucleotide phosphodiesterase present in cytosol of Xenopus laevis ovary has been purified and characterized. A cAMP-specific phosphodiesterase which is not activated by either cGMP or calmodulin, has also been characterized. Brief exposure of intact oocytes to 10 micro M progesterone results in an increase in activity of the cAMP-specific enzyme. The cGMP-stimulated and the calmodulin-activated phosphodiesterases are not altered. Changes in cyclic nucleotide levels during progesterone-induced maturation of oocytes may be modulated by these isoenzymes.

A protein inhibitor of calmodulin-regulated cyclic nucleotide phosphodiesterase in amphibian ovaries

The cytosol fraction of an extract of Xenopus laevis ovaries contains a protein inhibitor that can specifically block the activation of calmodulin-sensitive cyclic nucleotide phosphodiesterase (PDE I) found in that tissue. This inhibitor was purified by DEAE-cellulose chromatography, gel filtration on Sephacryl S-200, and affinity chromatography on calmodulin-Sepharose. It has a molecular weight of approximately 90,000, and is heat-labile and susceptible to inactivation by chymotrypsin. The inhibitor blocks calmodulin activation of cyclic nucleotide phosphodiesterases from amphibian ovary and bovine brain and of the myosin light chain kinase from rabbit smooth muscle, but does not affect the activity of a calmodulin-insensitive cyclic nucleotide phosphodiesterase. The inhibitor not only affects the activation of Xenopus PDE I and of the bovine brain phosphodiesterase by calmodulin, but also inhibits the stimulation of these enzymes by lysophosphatidylcholine. The inhibitor also acts on PDE I activated by partial tryptic proteolysis, but the enzyme fully activated by trypsin is only slightly susceptible to inhibition by this protein. The inhibition of PDE I activation caused by this ovarian factor can be reversed by adding excess amounts of calmodulin or lysophosphatidylcholine. The presence of this inhibitor provides a possible explanation for the previously observed inactivity of PDE I in vivo.