Properties of microsomal phospholipases in rat liver and hepatoma

Enhanced expression of group II phospholipase A2 in human hepatocellular carcinoma

Enzyme activity, protein contents, and mRNA contents of group II phospholipase A2 (PLA2) in hepatocellular carcinoma (HCC) surgically obtained from 8 patients were compared with those in either its neighboring liver tissues or control liver tissues. The PLA2 specific activity towards the mixed micelles of 1-palmitoyl-2-oleoyl-phosphatidylglycerol and cholate was significantly greater in the tumor tissues (6.62 +/- 1.46 nmol/min/mg) than those in the surrounding liver tissues (1.33 +/- 0.22 nmol/min/mg) and controls (0.43 +/- 0.04 nmol/min/mg). The results of immunoblot analysis using a specific anti-human group II PLA2 antibody and of Northern blot analysis using a human group II PLA2 cDNA as a probe demonstrated that group II PLA2 was responsible for the increased enzyme activity. The contents of immunoreactive group II PLA2 in the tumor tissues (8.81 +/- 1.24 ng/mg) were significantly higher than those in the surrounding liver tissues (1.74 +/- 0.27 ng/mg); those in the control tissues were below the analytical range of the method used. The group II PLA2 mRNA was also significantly increased in the tumor tissues, compared with that in the surrounding liver tissues, whereas it was not detectable in the controls. This indicates that group II PLA2 in HCC is induced at the pretranslational level.

Accumulation of polyunsaturated free fatty acids coincident with the fusion of rough endoplasmic reticulum membranes

The accumulation of polyunsaturated free fatty acids (PUFAs) was observed coincident with GTP-dependent fusion of liver rough microsomes. Whereas 0.5 mM NADPH led to a parallel reduction (greater than 50%) in membrane fusion and PUFA accumulation, indomethacin (50 microM) either had little effect or slightly augmented both processes. CTP was observed to stimulate accumulation of PUFAs and diacylglycerol (DAG). Therefore PUFAs may be relevant for GTP-dependent membrane fusion and together with DAG may play a role in fusion stimulated in the presence of CTP.

Ca(2+)-independent phospholipase A2 activity associated with secretory granular membranes in rat parotid gland

Phospholipase A2 activity was detected in a secretory granular fraction (SG) purified by Percoll gradient centrifugation from rat parotid gland using [3H]phosphatidylcholine (PC) as a substrate. High activity of this enzyme was observed at neutral pH. The enzyme was activated by Triton X-100 and did not require Ca2+ for its activity. In the absence of Ca2+, its apparent Km for exogenous PC was 28 microM while it was slightly increased by adding 5 mM CaCl2 (73 microM). Furthermore, the enzyme was located essentially in a granular membrane fraction separated from granular lysate. The deacylation activities were also detected in other subcellular fractions, which showed a different detergent-susceptibility or pH-dependency from that in SG. These results suggest that secretory granules have membrane-bound phospholipase A2 which has properties different from that found in other organelles.

Hydrolysis of exogenous substrates by mitochondrial phospholipase A2

Evidence is provided in this paper to indicate that hydrolysis of exogenously added phosphatidylethanolamine and phosphatidylcholine by the membrane-bound phospholipase A2 from rat-liver mitochondria is preceded by association of the substrates with the membranes. Hydrolysis of phosphatidylethanolamine after preincubation of mitochondria and substrate is nearly independent of incubation volume, indicating that substrate and mitochondria are not independently diluted. The association is greatly enhanced in the presence of Ca2+, especially for phosphatidylethanolamine. Association can be measured after sucrose-gradient centrifugation of mitochondria preincubated with phosphatidylethanolamine and can be visualized by freeze-fracture electronmicroscopy, showing substrate clusters fused with mitochondria. The association provides an explanation for the hydrolysis of exogenous substrates by a membrane-associated phospholipase A2 as well as for the high preference for phosphatidylethanolamine degradation often observed in studies on membrane-bound phospholipases A. This preference is likely to result in part from the tendency of unsaturated phosphatidylethanolamines to adopt non-bilayer lipid phases allowing a more extensive association with biomembranes in the presence of Ca2+, and does not reflect enzyme specificity per se. This phenomenon should be kept in mind when determining the substrate specificity of membrane-bound phospholipases A by the use of exogenous substrates.

Identification of a calcium-independent phospholipase A2 in rat lung cytosol and differentiation from acetylhydrolase for 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF-acether)

The 100 000 X g supernatant of total rat lung homogenate was found to contain at least three phospholipase A2-type activities. Gel filtration separated a low molecular weight and Ca2+-requiring phospholipase A2 from Ca2+-independent acylhydrolase peak with an apparent higher molecular weight. Upon DEAE-cellulose chromatography this fraction was separated into a Ca2+-independent acylhydrolase and a Ca2+-independent platelet-activating factor-acetylhydrolase with no apparent overlap in acyl chain length specificity. The long-chain acylhydrolase was shown to exhibit specificity for the ester bond at the sn-2-position. Ca2+-independent phospholipase A2 activity was inhibited by p-bromophenacylbromide and was resistant to diisopropylfluorophosphate. In contrast, the Ca2+-independent acetylhydrolase activity was inhibited by diisopropylfluorophosphate but was unaffected by p-bromophenacylbromide.

Rapid accumulation of diacyl lipid in rat liver microsomes by selective acylation of 2-acyl-sn-glycero-3-phosphorylserine

2-Acyl-sn-glycero-3-phosphoryl[U-14 C]serine (2-acylGPS) was prepared by digesting 1,2-dioleoyl-sn-glycero-3-phosphoryl[U-14C]serine, mixed with pig brain phosphatidylserine (PS), with Rhizopus arrhizus lipase. The labeled lysolipid was shown to be actively acylated by rat liver microsomes in the presence of acyl-CoA thioesters, with the formation of diacylPS. In experiments with unlabeled lysolipid and [14C]acyl-CoA thioesters, digestion of the products with pancreatic phospholipase A2 showed that the acyl groups had been incorporated almost exclusively at position-1. The acylation reactions were dependent on pH of the incubation medium, time of incubation, concentration of microsomal protein and of lysolipid and thioester substrates. With [14C]lysolipid there was a clear preference for long-chain saturated over unsaturated acyl group donors and highest acylation rates were recorded with stearoyl-CoA. Since stearate is the major fatty acid at position-1 of tissue PS, these findings suggest that deacylation-reacylation reactions may be of importance in regulating the fatty acid profile of this lipid. The observed rates of acylation of 2-acylGPS in vitro were high in relation to the endogenous level of PS in the microsomal membranes, and short-term incubations led to net synthesis of PS with a many-fold increase in its concentration. The question is raised whether this is a latent activity expressed under optimal assay conditions in vitro, conditions that may not obtain in a cell, or whether the high acylating potential is indicative of an important function for the addition, or turnover, of acyl groups at position-1 of the serine lipid.

Mechanism of release of integral proteins from rat liver microsomal membranes

The release of three integral enzymatic activities (NADH- and NADPH-cytochrome c reductase and 5'-nucleotidase) and total protein from washed rat liver microsomal membranes, upon simple incubation at 37 degrees C in aqueous media, was investigated. Release does not depend on contaminating proteases and is enhanced by alkaline pH. Total protein and enzyme release is consistent with a loss of phospholipids which are not recovered in the soluble phase. Following incubation at pH 9.0 large amounts of free fatty acids were recovered in the soluble phase, accounting for a ratio of 1/1 (w/w) with released protein. This evidence, together with the data available about densities (1.07-1.08 g/ml) and molecular weights (1 700 000-700 000) of the released enzymes, suggests that they are solubilized from microsomal membranes in the form of mixed micelles mostly formed by free fatty acids and integral proteins, probably owing to the activity of endogenous phospholipases on membrane lipids. Release of total protein and enzymatic activities is decreased by Ca2+, whose possible role in the phenomenon is discussed.

In vitro lipid metabolism in the rat pancreas. II. Effects of secretagogues on fatty acid metabolism, net lipolysis and ATP levels

1. The concentration of carbamylcholine, bombesin, pancreozymin, pentagastrin and secretin evoking a similar 4--5-fold maximal increase in amylase secretion from rat pancreatic fragments were 3.10(-6), 10(-7), 10(-8), 3.10(-6), and 3.10(-6) M, respectively. The maximal concentration of vasoactive intestinal peptide tested (3.10(-6) M) increased amylase secretion by 250%. The six secretagogues could be separated into two groups according to their effects on lipid metabolism and ATP levels. 2. When used at their optimal concentrations, carbamylcholine, bombesin, pancreozymin, and pentagastrin lowered pancreatic ATP levels by 18-26% and increased net release of free fatty acids by 68-105%. 3. The effects of 3.10(-6) M carbamylcholine and 10(-8) M pancreozymin on the metabolism of 3H2O, D-[U-14C]glucose and [1-14C]acetate were similar; the incorporation of radioactivity in the fatty acid moiety of glycerolipids decreased by 20--50% whereas the incorporation of 3H from 3H2O and of 14C from [U-14C]glucose increased by 20--35% in the glycerol moiety. In addition, the oxidation of [U-14C]glucose, [1-14C]acetate and [1-14C]palmitate to 14CO2 increased by 15--32% while the esterification of [1-14C]palmitate, [1-14C]-linoleate, and [1-14C]arachidonate was inhibited by 14--23%. The spectrum of fatty acids labeled with [1-14C]acetate indicated an inhibition of the malonic acid pathway whereas the elongation of polyenoic fatty acids was unaltered.