Phosphatidylinositol-cleaving activity in smooth muscle from rat vas deferens

Abstract

1. 1. Phosphatidylinositol-cleaving activity was studied in subcellular fractions from smooth muscle of rat vas deferens. 2. 2. In the presence of calcium ions and deoxycholate most of the endogenous phosphatidylinositol was broken-down in 60 min, whilst the other phospholipids were stable. 3. 3. The enzymatic activity responsible for this breakdown catalyses a phospholipase C-type cleavage of the glycerol-phosphate bond, the water soluble products from exogenous [ 32P]-labelled phosphatidylinositol being d-myoinositol 1:2-cyclic phosphate (702-80%) and d- myoinositol 1-phosphate (202-30%). 4. 4. Activity was abolished by 1 mM ethanedioxybis(ethylamine)tetra-acetate (EGTA) and in the presence of deoxycholate both the soluble and total particulate fractions showed maximum activity at pH 6.52-6.8. The soluble fraction showed a second peak of activity at pH 5.52-5.8 that was independent of deoxycholate; this was not observed in the particulate fraction. 5. 5. About two-thirds of the activity was soluble. The remaining activity was particulate, with a preferential concentration in the microsomal fraction.