Phospholipid alterations in canine cardiac sarcoplasmic reticulum induced by an acid-active phospholipase C.

Phospholipid alterations and phospholipase activities were studied in a preparation of canine cardiac sarcoplasmic reticulum (SR) known to contain lysosomes. Incubation of SR at pH 5.0 (37°C) resulted in a loss in total lipid phosphorus which was maximal (10%) by 30 min. whereas a modest increase in lipid phosphorus occurred during incubation at pH 7.0. The content of phospholipid phosphorus was decreased (8.6–19.0%) in sphingomyelin, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine when SR was incubated at pH 5.0; however phosphatidic acid increased 14% relative to control. Lysophospholipids were not detected. Liposomes of 1-acyl 2-[1 14C]-linoleoyl-sn-glycero-3-phosphorylethanolamine (14C-PE) were hydrolyzed by SR at pH 5.0 to yield radiolabelled diglyceride (18.1 nmol/hr mg protein) and free fatty acid (8.3 nmol/hr mg protein). SR-mediated diglyceride production at pH 5.0, but not free fatty acid release, was markedly potentiated when 14C-PE was peroxidized by preincubation at pH 5.0 for 3-24 hours at 37°C. After 24 hours of preincubation at pH 5.0 the specific activity of SR-mediated diglyceride production was 188% of control. A comparable increase in diglyceride production occurred when 14C-PE was peroxidized at pH 7.0 and hydrolyzed by SR at pH 5.0. No increase in enzymatic activity occurred when liposomes were both preincubated and assayed at pH 7.0. When 14C-PE was exposed to air at pH 5.0 or 7.0 more polar radiolabelled derivativtes were formed as determined by thin layer chromatography. Up to twice as much polar lipid was formed at pH 5.0 compared to pH 7.0. The time dependent generation of thiobarbituric acid reactive substances indicated the formation of peroxidation products during incubation of PE liposomes. The concentration of peroxidation products in solution was greater at pH 7.0 than at pH 5.0. These results demonstrate that peroxidation of PE increases susceptibility to hydrolysis by a nonspecific, acid-active, phospholipase C associated with SR. This enzyme, probably of lysosomal origin, may be important in cardiac membrane dysfunction during ischaemia in which H+ and oxygen radicals are known to participate.