Phospholipase activity of rat tissues and its modification by trypsin1,2

Phospholipase and lysophospholipase activity of rat eosinophil leukocytes

Previous studies have shown the high lysophospholipase activity of rat eosinophilic leukocytes and used this enzyme to measure the rise in eosinophilic population of peripheral tissues caused by parasitic infections. This report details the methods and results of an investigation showing the presence in the same cells of high phospholipase (PLA) activity. Unfractionated and metrizamide-purified peritoneal eosinophil preparations were assayed using a mixed micelle substrate (6/15 mM lecithin/Triton X-100) at experimentally determined pH (6.4) and ionic strength (I=0.2) optima: the attendant reaction products included free fatty acids and organic P in a 2/1 molar proportion with a correspondent loss in the initial phospholipid concentration. The organic P fragment was further characterized as GPC (glycerylphosphorylcholine) by quantitative precipitation and acid hydrolysis. Estimates of PLA activity averaged 5 micromol/h/10(6) unfractionated eosinophils and metrizamide-purified eosinophil preparations. Paired tests for PLA and LysoPLA on unfractionated and enriched cell preparations, cytosolic extracts, and chromatographic fractions yielded similar activity ratios, supporting the inference of a close association of the two activities which could also be confirmed for the major tissues of eosinophil production and distribution.

Regulatory aspects of mitochondrial phospholipase A2 from rat liver: effects of proteins, phospholipids and calcium ions

This paper deals with the search for specific inhibitors or activators of the mitochondrial phospholipase A2. Convincing evidence for the existence of proteins in the mitochondrial or cytosolic fraction that function as specific regulators of this enzyme was not obtained. The enzymatic activity appeared to be inhibited at low substrate concentrations by lipocortin isolated from human monocytes. However, at higher substrate concentrations, the inhibition disappeared, suggesting either that lipocortin sequestered the phospholipid substrate or that the putative inactive complex of enzyme and lipocortin dissociated in the presence of excess phospholipids. The hydrolysis of the neutral phospholipid phosphatidylethanolamine was stimulated by the presence of cardiolipin and phosphatidylglycerol. It is unlikely that this is caused merely by the negative charge of these phospholipids, since other negatively charged phospholipids did not show this effect. Using a phospholipid extract from mitochondria as substrate, the enzymatic activity as a function of the Ca2+ concentration was determined. Only one enzyme activity plateau was observed. The calculated KCa2+ value of 0.05 mM suggests that the mitochondrial phospholipase A2 could be regulated strictly by the modulation of the free Ca2+ concentration in vivo. The two activity plateaus observed previously upon variation of the Ca2+ concentration using phosphatidylethanolamine as substrate could be explained by a Ca2+-induced transition of the phospholipid structure.

Light and electron microscopic demonstration of phospholipase B activity in the mouse eosinophil

Phospholipase B (EC 3.1.1.5) which hydrolyzes phospholipids in the alpha and beta positions was demonstrated in murine leukocytes using light and electron microscopic histochemical techniques. Leukocytes (neutrophils, lymphocytes, macrophages, eosinophils) were harvested from peritoneal exudates of mice. Cells were fixed in 4% calcium-formol fixative for 10 min at 4 degrees C for light microscopy and 30 min at room temperature for electron microscopy, after which they were incubated at 37 degrees C in medium at pH 6.6 containing 2 microM lysolecithin and CaCl2. The fatty acids released during the hydrolytic reaction were trapped as a calcium precipitate and were converted to a cobalt precipitate for light microscopy by treatment with cobalt acetate or to a lead precipitate for electron microscopy by treatment with lead nitrate. The reaction products were observed to be present in eosinophils and absent in neutrophils, lymphocytes and macrophages. It is concluded that the eosinophilic leukocyte is the carrier cell for phospholipase B in inflammatory reactions.

Characterization of phospholipase A2 from rabbit lung microsomes

A phospholipase A2 activity associated with the microsomal fraction of rabbit lung homogenates was studied. The enzyme showed specificity for the sn-2 ester bond of phosphatidylcholine, had an alkaline pH optimum and required Ca2+ for activity. Other divalent cations were unable to support hydrolysis. In the absence of detergents, exogenous phosphatidylethanolamine was deacylated at a rate sevenfold higher than phosphatidylcholine. The activity toward both substrates could be enhanced by sodium deoxycholate or, more effectively, by sodium taurodeoxycholate. Phosphatidylethanolamine required higher detergent/phospholipid molar ratios than phosphatidylcholine. Under these conditions, the preference for the former substrate over the latter was nearly abolished. The zwitterionic detergent 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS) and the nonionic detergent Triton X-100 were either ineffective (phosphatidylcholine) or inhibitory (phosphatidylethanolamine). Addition of KCl produced opposite effects on the activity depending on the bile salt used to disperse the substrate. The phospholipase A2 activity was inhibited by p-bromophenacyl bromide but remained unaffected after treatment with diisopropylfluorophosphate or NaF. N-Ethylmaleimide, but not other thiol reagents, partially inhibited the activity.

Intestinal phospholipase, a novel enzyme

We evaluated phospholipase activity in the intestine of rats and other species. Phospholipase activity was assayed by a surface barostat technique or an egg yolk titration system. Mucosal activity was found only by the surface barostat technique with phosphatidylglycerol as substrate; it was not found with phosphatidylcholine as substrate in assays by either technique. In gut luminal fluid activity was found when both phosphatidylcholine and phosphatidylglycerol were used as substrate in assays by the surface barostat technique, and phosphatidylcholine as substrate yielded activity in egg yolk titration. In rats in which pancreatic juice had been diverted, mucosal and gut luminal phospholipase activity was greater than in controls, thus demonstrating that enzyme activity was not due to pancreatic phospholipase. Bacterial origin of phospholipase activity was excluded in that phospholipase activity was found in germ-free rats; gastric and salivary gland origins were excluded in that continued phospholipase activity was found in rats with gastric fistula. The physiological importance of the enzyme was established by the finding that rats with pancreatic fistula absorbed 111 mumol of phosphatidylcholine and that controls absorbed 119 mumol of a 135-mumol load. Activity was found to be three times greater in the distal than in the proximal intestine; in cryptal cells it was 10 times greater than in villus tip cells. 65% of the activity in the gut lumen was tightly bound to particulate matter. We propose that intestinal phospholipase may be important in gut bacterial control, in the digestion of vegetable matter (phosphatidylglycerol is a major phospholipid in both plants and bacteria), and in the digestion of phospholipids in the gut lumen.

Phospholipase and prophospholipase activities in bronchoalveolar lavage fluid in severe acute pulmonary disease with or without ARDS

Bronchoalveolar lavages were performed in 21 patients undergoing mechanical ventilation: Group I: coma due to sedative overdose (11 cases), without pulmonary impairment, serving as control group; Group II: severe acute pulmonary disease without ARDS (5 cases); Group III: patients with ARDS (5 cases). In the recovered fluid we measured: total proteins (P) and phospholipids (PL), phospholipasic (PLase) and prophospholipasic (PPLase) activities. In ARDS group, considerable increase of P, (p less than 0.001), and a doubling of PL (p less than 0.02) was found. Total PLase activity was present in all three groups, with a higher mean level in Group III (p less than 0.01). PPLase activities were low or undetectable in four patients of this group. PLase/PL ratio was increased in acute respiratory insufficiency, with or without ARDS, suggesting an increase of surfactant catabolism. The decrease of this ratio when pulmonary function improved, or its increase after deterioration suggest that it is related to changes of surfactant.

[Activation of blood phospholipases by proteins present in crude trypsin extracts]

Blood phospholipase activity in increased by crude trypsin (Sigma). This activity is not induced by trypsin itself but by some proteins contained in the extract and isolated from trypsin by G 75 Sephadex filtration. Purified trypsin or pure trypsin (Fluka, Merck, Boehringer) do not increase phospholipase activity.

Altered lysophospholipase B responsiveness in lactating mice infected with intestinal nematode parasites

Lactating and nulliparous outbred Swiss (CF-1 strain) mice were infected at 12-16 weeks of age with Nippostrongylus brasiliensis or Trichinella spiralis. Lysophospholipase B levels in the intestinal tissue and faecal pellets were greatly in infected lactating mice in contrast to infected nulliparous mice. Correlated with these depressions in lysophospholipase levels were markedly reduced numbers of bone-marrow eosinophils in infected lactating mice. Although the peak levels of lysophospholipase in the intestinal tissues occurred in both nulliparous and lactating mice by days 9 and 14 after infection with N. brasiliensis and T. spiralis, respectively, lactating mice had significantly lower lysophospholipase peak levels than nulliparous mice. The peak of luminal levels of the enzyme coincided with peak levels of the enzyme in the intestinal tissues and with the expulsion of the majority of the worms from the small intestines of nulliparous mice. However, lactation delayed the temporal relation between the peak of lysophospholipase levels in the intestinal lumen and worm expulsion. These results suggest that lactation depressed the levels of the enzyme and interfered with its release into the intestinal lumen.

Studies on the phospholipases of rat intestinal mucosa

1. Subcellular distribution and characteristics of different phospholipases of rat intestinal mucosa were studied. 2. The presence of free fatty acid was necessary for the maximal hydrolysis of lecithin (phosphatidylcholine), but there was no accumulation of lysolecithin (1 or 2-acylglycerophosphorylcholine);lysolecithin accumulated when the reaction was carried out in the presence of sodium deoxycholate and at or above pH8.0. 3. The fatty acid-activated phospholipase B as well as lysolecithinase showed optimum activity at pH6.5, whereas for the phospholipase A it was about pH8.6. 4. The bulk of the phospholipase A was present in the microsomal fraction, whereas the phospholipase B and lysolecithinase activities were distributed between the microsomal and soluble fractions of the mucosal homogenate. 5. Phospholipase A was equally distributed between the brush border and brush-border-free particulate fraction, with the brush border having highest specific activity, whereas the other two activities were distributed between the brush-border-free particulate and soluble fractions. 6. Various treatments showed marked differences between the phospholipase A and phospholipase B activities, but not between phospholipase B and lysolecithinase activities. 7. By using (beta[1-(14)C]-oleoyl) lecithin it was shown that the mucosal phospholipase A was specific for the beta-ester linkage of the lecithin molecule.