Distinction in vitro between rat liver phosphatidate phosphatase and phospholipase C

Hepatocellular membranes (1000 x g) were incubated with sn-[1,3-14C] glycerol-3-P, ATP, Ca2+, NaF and palmitate to form labeled, membrane-associated phosphatidate(PA). Membranes incubated with 2mM oleate or 5mM bromobenzene showed rapid (5-10 min) and significant (2-6 fold) increases in the dephosphorylation of PA. However, oleate and bromobenzene activated the dephosphorylation of PA by phosphatidate phosphatase (PAP) and phospholipase C (PLC), respectively. This conclusion is supported by the observation that the PAP stimulated by oleate is: 1) Mg2+-dependent; 2) inhibited by Ca2+ and NaF; 3) specific for PA; 4) associated with a rise in liver cell triacylglycerol (TG) formation. Bromobenzene, however, activated a PLC that is: 1) stimulated by various metals; 2) enhanced by NaF; 3) is associated with a rise in the degradation of membrane phospholipids and liver cell injury. These results suggest that under the appropriate conditions in vitro the dephosphorylation of PA can be used to assess chemical-dependent changes in PAP and/or PLC activity