THE ROLES OF PHOSPHOLIPASE-D AND A GTP-BINDING PROTEIN IN GUANOSINE 5'-[GAMMA-THIO]TRIPHOSPHATE-STIMULATED HYDROLYSIS OF PHOSPHATIDYLCHOLINE IN RAT-LIVER PLASMA-MEMBRANES

1. Guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulated by 50 % the rate of release of [H-3]choline and [H-3]phosphorylcholine in rat liver plasma membranes labelled with [H-3]choline. About 70 % of the radioactivity released in the presence of GTP[S] was [H-3]choline and 30 % was [H-3]phosphorylcholine. 2. The hydrolysis of phosphorylcholine to choline and the conversion of choline to phosphorylcholine did not contribute to the formation of [H-3]choline and [H-3]phosphorylcholine respectively. 3. The release of [H-3]choline from membranes was inhibited by low concentrations of SDS or Triton X-100. Considerably higher concentrations of the detergents were required to inhibit the release of [H-3]phosphorycholine. 4. Guanosine 5'-[beta-gamma-imido]triphosphate and guanosine 5'-[alpha-beta-methylene]triphosphate, but not adenosine 5'-[gamma-thio]-triphosphate, stimulated [H-3]choline release to the same extent as did GTP[S]. The GTP[S]-stimulated [H-3]choline release was inhibited by guanosine 5'-[beta-thio]diphosphate, GDP and GTP but not by GMP. 5. It is concluded that, in rat liver plasma membranes, (a) GTP[S]-stimulated hydrolysis of phosphatidylcholine is catalysed predominantly by phospholipase D with some contribution from phospholipase C, and (b) the stimulation of phosphatidylcholine hydrolysis by GTP[s] occurs via a GTP-binding regulatory protein.