Analysis of human plasma phosphatides by paper chromatography

Formation of Crystalline delta-Endotoxin or Poly-beta-Hydroxybutyric Acid Granules by Asporogenous Mutants of Bacillus thuringiensis

Parental strains and asporogenous mutants of Bacillus thuringiensis subspp. kurstaki and aizawai produced high yields of delta-endotoxin on M medium, which contained 330 mug of potassium per ml, but not on ST and ST-a media, each of which contained only 11 mug of potassium per ml. On ST and ST-a media, refractile granules were formed instead. These granules had no insecticidal activity against silkworms and were isolated and identified as poly-beta-hydroxybutyric acid. Supplementation of the potassium-deficient ST-a medium with 0.1% KH(2)PO(4) (3.7 mM) led to the formation of crystalline delta-endotoxin. The replacement of KH(2)PO(4) with equimolar amounts of KCl, KNO(3), and potassium acetate or an equivalent amount of K(2)SO(4) had a similar effect, whereas the addition of an equimolar amount of NaH(2)PO(4) or NH(4)H(2)PO(4) did not cause the endotoxin to form. An asporogenous mutant, B. thuringiensis subsp. kurstaki strain 290-1, produced delta-endotoxin on ST-a medium supplemented with 3 mM or more potassium but formed only poly-beta-hydroxybutyric acid granules on the media containing </=1 mM potassium. These results clearly indicate that a certain concentration of potassium is essential for the fermentative production of delta-endotoxin by these isolates of B. thuringiensis. Manganese could not be substituted for potassium. Phosphate ions stimulated poly-beta-hydroxybutyric acid formation by strain 290-1. The sporulation of B. thuringiensis and several other Bacillus strains was suppressed on the potassium-deficient ST medium. This suggests that potassium plays an essential role not only in Bacillus cell growth and delta-endotoxin formation but also in sporulation.

Dietary fatty acids and the plasma phospholipids of the lactating cow

Cows were given either a low-fat concentrate mixture or a concentrate mixture that contained 5 or 10% ‘stearic acid’ (85% pure) or 10% ‘palmitic acid’ (85% pure). The concentrate mixtures were given with a high roughage diet that supplied 4·4 kg of hay and 2·7 kg of sugar-beet pulp/day. Blood samples were taken from the cows on each dietary treatment and the plasma phospholipids were analysed.

Phosphatidyl choline accounted for 70% of the plasma phospholipids when the cows were given the low-fat diet and about 86% of the plasma phospholipids when the cows were given the diets supplemented with the fatty acids. The inclusion of the fatty acids in the concentrate mixtures decreased the relative proportions of the plasma phosphatidyl ethanolamine, sphingomyelin and lysophosphatidyl choline but the relative proportion of the plasma phosphatidyl serine remained unchanged.

When the diet was supplemented with stearic acid the concentrations of 18:0 and 16:0 in the phosphatidyl choline were unaltered but the concentration of 18:1 was increased and the concentration of 18:2 was decreased. When the diet was supplemented with palmitic acid the concentrations of 16:0 and 18:1 in the phosphatidyl choline were increased and the concentrations of 18:0 and 18:2 were decreased. In contrast, the inclusion of stearic acid in the diet increased the concentration of 18:0 in the phosphatidyl serine and decreased the concentration of 16:0; the concentrations of 18:1 and 18:2 were unchanged. The fatty acid composition of the plasma phosphatidyl ethanolamine was unaffected by dietary treatment. The effects of diet on the fatty acid compositions of the phosphatidyl choline and lysophosphatidyl choline were similar. Supplementation of the diet with stearic acid increased the concentrations of 18:0 and 18:1 in the plasma sphingomyelin and decreased the concentrations of 16:0 and 18:2. The addition of palmitic acid to the diet increased the concentration of 16:0 in the sphingomyelin but it decreased the concentrations of the other constituent fatty acids.

Presynaptically acting snake venom phospholipase A2 enzymes attack unique substrates

Synaptosomes were incubated with bovine serum albumin (BSA) to examine whether the presynaptic action of snake venom phospholipase A2 (PLA2) toxins is due either to the release of fatty acids resistant to extraction by BSA or to the liberation of a specific fatty acid type. In the presence of BSA (0.5% or 1.0%) two PLA2 enzymes from Naja naja atra and Naja naja kaouthia snake venoms that do not have a predominant presynaptic action at the neuromuscular junction (PS-) did not stimulate acetylcholine (ACh) release from synaptosomes. In contrast, two PLA2 enzymes (beta-bungarotoxin, scutoxin) that do have a predominant presynaptic action at the neuromuscular junction (PS+) did stimulate ACh release. BSA did not antagonize PS- enzymes by more efficiently extracting the fatty acids produced by these enzymes relative to PS+ enzymes. While absolute amounts of total and unsaturated fatty acid produced overlapped for the PS- and PS+ enzymes, the two PS+ enzymes produced a significantly greater absolute amount and relative percentage of palmitic acid (16:0) than did either of the PS- enzymes. However, the levels of free palmitic acid remaining in the synaptosomes where they would exert effects on ACh release were similar for the N. n. kaouthia PLA2 (PS-) and beta-bungarotoxin (PS+). Therefore, the total (supernatant plus synaptosomal) amount of palmitic acid produced per se did not account for stimulation of ACh release, since the greater amounts produced by the PS+ enzymes were removed from the synaptosomes by BSA. The production of higher levels of palmitic acid suggests either that PS+ enzymes gain access to sites containing phospholipid substrates unavailable to the PS- enzymes, or that they have a different substrate preference. These findings suggest new possibilities for the mechanism of PS+PLA2 action, including site-directed enzymatic activity and protein acylation.

Antibodies having markedly different effects on enzymatic activity and induction of acetylcholine release by two presynaptically-acting phospholipase A2 neurotoxins

The enzymatic and acetylcholine-releasing activities of two presynaptically-acting phospholipase A2 neurotoxins (pseudexin B and scutoxin) were studied in a synaptosomal fraction. Scutoxin (100 nM) induced greater [14C]acetylcholine release than did pseudexin B (100 nM). Both toxins caused fatty acid production in the synaptosomal fraction, although pseudexin B was more active than scutoxin. One monoclonal antibody raised against pseudexin B (#4) had no effect on the enzymatic activity of either pseudexin B or scutoxin. Two other monoclonal antibodies (#3 and #7), also raised against pseudexin B, antagonized the enzymatic activity of pseudexin B and scutoxin. Monoclonal antibody #3 was more effective than #7 in reducing the amount of acetylcholine released by the toxins, whereas #7 was more effective than #3 in reducing fatty acid production. Although antibody #3 caused complete inhibition of phospholipase A2 activity of pseudexin B on purified substrates, it only reduced phospholipase A2 activity by 35% in synaptosomes. These findings support the hypothesis that gross phospholipase A2 activity does not play a role in stimulation of acetylcholine release by the presynaptically-acting phospholipase A2 neurotoxins.

Exposure of phosphatidylcholine and phosphatidylinositol in plasma membranes from rat brain synaptosomes treated with phospholipase A2 toxins (beta-bungarotoxin, notexin) and enzymes (Naja nigricollis, Naja naja atra)

Phospholipase A2 (PLA2) toxins act presynaptically to block acetylcholine release and are much more potent and specific in their actions than PLA2 enzymes even though they have lower enzymatic activity. Since their mechanism of action is not completely understood, it was of interest to examine the toxins' effects on phospholipid asymmetry as changes in asymmetry are associated with changes in membrane functioning. Rat brain synaptosomes were treated with the PLA2 toxins beta-bungarotoxin (beta-BuTx) and notexin and with the PLA2 enzymes Naja nigricollis and Naja naja atra under relatively non-disruptive conditions as judged by leakage of lactate dehydrogenase (LDH) and levels of phospholipid hydrolysis. The exposure of phosphatidylcholine (PC) and phosphatidylinositol (PI) on the synaptosomal surface was investigated by means of a specific PC-exchange protein (PCEP) and a PI-specific phospholipase C (PI-PLC), respectively. Treatment of the synaptosomes with N. nigricollis PLA2, beta-BuTx and notexin did not affect the availability of PC to exchange by PCEP, but significantly increased the exposure of PI to hydrolysis by PI-PLC. In contrast, N. n. atra PLA2 slightly decreased the exposure of PC and did not affect that of PI. The differences between N. n. atra PLA2, on the one hand, and N. nigricollis PLA2, beta-BuTx and notexin, on the other hand, parallel differences in their pharmacological activities. Our earlier studies showed that PLA2 enzymes, and possibly PLA2 toxins, have a pharmacological site separate from the enzymatic site. Since in the present study the effect on PI was abolished by EDTA, the presence of an enzymatic site in addition to the pharmacological site may be required or alternatively divalent cations may be required for the effects on PI asymmetry independent of the inhibition of PLA2 by EDTA.

Phenytoin increases specific triacylglycerol fatty esters in skeletal muscle from horses with hyperkalemic periodic paralysis

Previous studies have demonstrated that phenytoin decreases the levels of triacylglycerols in several tissues other than skeletal muscle. Since phenytoin is clinically effective in several skeletal muscle disorders, triacylglycerol metabolism in skeletal muscle from four normal Quarter horses and four Quarter horses with hyperkalemic periodic paralysis was examined. The horses with hyperkalemic periodic paralysis had low levels of 18:3 in the phospholipids, low levels of 16:0, 16:1 and 18:3 in the free fatty acids and low levels of 20:4 in triacylglycerols. Triacylglycerol levels were increased in skeletal muscle from seven (three controls, four hyperkalemic periodic paralysis) of the eight horses on treatment with oral phenytoin for one week. Instead of an increase in all fatty ester types only 16:0, 16:1, 18:1 and 18:2 were significantly increased. Total lipid phosphorus and the distribution of phospholipid fatty esters and free fatty acids were not significantly altered by phenytoin treatment in most cases. An alteration in triacylglycerol metabolism by phenytoin was also observed in primary cultures of normal equine skeletal muscle radiolabeled with 18:1, but not in those radiolabeled with 18:2. These findings suggest that phenytoin does not just increase the levels of triacylglycerol in skeletal muscle, but alters the utilization and incorporation of fatty esters. These findings suggest a potential involvement of triacylglycerol metabolism in the clinical efficacy of phenytoin in hyperkalemic periodic paralysis.

The asymmetric distribution of phosphatidylcholine in rat brain synaptic plasma membranes

The distribution of phosphatidylcholine between inner and outer monolayers of rat brain synaptic plasma membrane was investigated by means of a phosphatidylcholine specific exchange protein. About 70% of the total membranal phosphatidylcholine was in the outer leaflet, 33% of which was exposed and readily exchanged in intact synaptosomes while the remainder was exchangeable following osmotic shock. Permeabilization of the synaptic plasma membranes by overnight incubation in buffer or by saponin (< 0.08%) exposed an additional 30% of phosphatidylcholine to exchange, presumably from the inner cytoplasmic leaflet. Phosphatidylcholine is therefore asymmetrically distributed in the synaptosomal plasma membrane, as it is in other plasma membranes.

An EDTA.Ca2+ complex inhibits the enzymatic activity but not the lethality of beta-bungarotoxin

An EDTA.Ca2+ complex inhibits the phospholipase A2 activity of the presynaptic neurotoxin beta-bungarotoxin without affecting its lethal potency. The EDTA.Ca2+ complex induces a conformational change in the enzymatic active site region of beta-BuTx, as indicated by the suppression of the 340 nm tryptophan fluorescence peak. Modification of the enzymatic site without loss of toxicity supports the presence of separate loci for the two activities.

Effects of snake venom phospholipase A2 toxins (beta-bungarotoxin, notexin) and enzymes (Naja naja atra, Naja nigricollis) on aminophospholipid asymmetry in rat cerebrocortical synaptosomes

The effects of snake venom phospholipase A2 (PLA2) toxins (beta-bungarotoxin, notexin) and PLA2 enzymes (Naja nigricollis, Naja naja atra) on aminophospholipid asymmetry in rat cerebrocortical synaptic plasma membranes (SPM) were examined. Incubation of intact synaptosomes with 2 mM 2,4,6-trinitrobenzene sulfonic acid (TNBS) for 40 min, under non-penetrating conditions, followed by SPM isolation, allowed us to calculate the percentage of phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the outer leaflet of the SPM, while incubation with disrupted synaptosomes provided total labeling values with the difference representing labeling of the inner leaflet. We found that 30% of the PE and 2% of the PS were in the outer leaflet, with 54% of the PE and 80% of the PS in the inner leaflet; 16% of the PE and 18% of the PS was inaccessible to TNBS. PLA2 toxins and enzymes increased in a concentration-dependent manner the percentage of PS and, to a lesser extent, the percentage of PE in the outer leaflet of the SPM, due to a redistribution from the inner to the outer leaflet. There was no correlation between the PLA2 enzymatic activities and the increased percentage of PS in the outer leaflet of the SPM induced by the PLA2 toxins and enzymes. Alteration of aminophospholipid asymmetry does not explain the greater presynaptic specificity and potencies of the PLA2 toxins as compared to the PLA2 enzymes, but may be associated with the increased acetylcholine release from synaptosomes induced by both the toxins and enzymes.

Phosphatidylcholine asymmetry in electroplax from the electric eel: use of a phosphatidylcholine exchange protein

Phosphatidylcholine asymmetry in the inner and outer leaflets of the plasma membrane bilayer of the innervated and noninnervated surfaces of the electroplax cell was determined, using a phosphatidylcholine exchange protein. The exchange protein from bovine liver catalyzed the exchange of phosphatidylcholine from small unilamellar vesicles to the outer monolayer of the plasma membrane bilayer. The exchange protein did not penetrate to the inner monolayer of the plasma membrane, did not modify the permeability of the electroplax, and did not alter the phospholipid or cholesterol content of the electroplax. In the innervated plasma membrane, 42% of the phosphatidylcholine is in the outer leaflet, 33% is in the inner leaflet, and 25% is inaccessible to the exchange protein. Corresponding values for the noninnervated plasma membrane are 56, 26, and 18%, respectively. These results are similar to phosphatidylcholine asymmetry in other biological membranes. This unique cell can be used as a model to test the effects on phospholipid asymmetry of compounds that act on the membrane.

Inhibitory effect of EDTA.Ca2+ on the hydrolysis of synaptosomal phospholipids by phospholipase A2 toxins and enzymes

Phospholipases A2 (PLA2) are Ca2(+)-dependent enzymes that are inhibited by EDTA; this inhibition would be expected to be reversed by restoring the Ca2+ concentration. By examining the hydrolysis of synaptosomal phospholipids by PLA2 enzymes, Naja naja atra and Naja nigricollis, and by toxins with PLA2 activity, beta-bungarotoxin (beta-BuTX) and notexin, we demonstrated a novel inhibitory action of EDTA manifested in the presence of excess Ca2+. We postulate the formation of an EDTA.Ca2+ complex which inhibits PLA2 activity in a concentration-dependent manner. Synaptosomes in which phospholipids are hydrolyzed by PLA2 have membranal damage expressed by increased acetylcholine (ACh) release and decreased osmotic activity. Addition of EDTA.Ca2+, which inhibits phospholipid hydrolysis, also reversed the PLA2 effect on ACh release, but not its effect on osmotic activity. The inhibition of PLA2 was observed on membranal phospholipids as well as on an artificial substrate of phospholipid-Triton mixed micelles. Moreover, we found that another enzyme, lactate dehydrogenase, was also inhibited. Our results indicate a non-specific inhibition exerted on the enzyme rather than on the substrate.

Phospholipid hydrolysis and loss of membrane integrity following treatment of rat brain synaptosomes with beta-bungarotoxin, notexin, and Naja naja atra and Naja nigricollis phospholipase A2

The effects of the phospholipase A2 (PLA2) toxins, beta-bungarotoxin and notexin, and the PLA2 enzymes from Naja naja atra and Naja nigricollis snake venoms on the plasma membrane integrity of synaptosomes were examined. Synaptosomes were isolated from rat brain cerebral cortex, corpus striatum and hippocampus. Osmotic activity, lactate dehydrogenase leakage, and leakage of 2-deoxy-D-(1-3H)-glucose-6-phosphate were monitored (37 degrees C, 10-120 min) following incubation with 0.5, 5 and 50 nM concentrations of toxins and enzymes. Damage to the synaptosomal plasma membrane was time and concentration but not tissue dependent. The potencies of the treatments were as follows: N. n. atra PLA2 greater than or equal to N. nigricollis PLA2 greater than notexin greater than beta-bungarotoxin. Chelation of Ca2+ with 5 mM EDTA completely inhibited plasma membrane disruption caused by beta-bungarotoxin and N. n. atra PLA2. One mg/ml of bovine serum albumin also blocked the disruptive action of N. n. atra PLA2, while 8 mg/ml was required to antagonize beta-bungarotoxin. A correlation between phospholipid hydrolysis and loss of membrane integrity was also observed. The generation of phospholipid hydrolytic products may be critical in the permeabilization of synaptic plasma membranes by these toxins and enzymes, however, they do not explain the presynaptic specificity and potency of beta-bungarotoxin and notexin.

Do chemical modifications dissociate between the enzymatic and pharmacological activities of beta bungarotoxin and notexin?

We have measured enzymatic, hemolytic and anticoagulant activities, lethal potencies and effects on contractions of the phrenic nerve-diaphragm preparation, by chemically modified derivatives of beta bungarotoxin (beta BuTX) and notexin, two presynaptically acting toxins which have PLA2 activity. The following chemical modifications of beta BuTX were tested: alkylation and methylation of histidine 48, alkylation of tryptophan 19, sulfonylation of tyrosine 68, oxidation of methionines 6 and 8, semicarbazide addition under varied conditions to carboxyl groups, varied extents of carbamylation or trinitrophenylation of lysines and guanidination of all lysines with or without trinitrophenylation of the N-terminal asparagine. Only the histidine, tryptophan and tyrosine residues were modified in notexin. The results obtained were compared with those previously obtained using chemically modified derivatives of Naja nigricollis and Naja naja atra PLA2 enzymes which do not have a specific presynaptic site of action. The results with oxidized methionine and lysine-modified derivatives of beta BuTX are supportive of the suggestions of others that the N-terminal region and basic residues away from the enzymatic active region contribute towards the beta type presynaptic neurotoxicity of the PLA2 toxins. Using modified derivatives of beta BuTX and notexin, the dissociations between enzymatic activities and pharmacological properties were not as marked as previously observed with N. nigricollis and N. n. atra PLA2; nevertheless, several dissociations were noted. We conclude that, just as with non-presynaptically acting PLA2 enzymes, some pharmacological actions of presynaptically acting PLA2 toxins may occur independently of phospholipid hydrolysis.

beta-Bungarotoxin-induced phospholipid hydrolysis in rat brain synaptosomes: effect of replacement of calcium by strontium

We tested whether, upon substitution of Ca2+ by Sr2+ in a medium containing beta-bungarotoxin, sufficient Ca2+ remained bound to the tissue to support phospholipid hydrolysis in rat brain synaptosomes. The phrenic nerve--diaphragm preparation could not be used, since replacement of Ca2+ by Sr2+ prolonged time to block of indirectly evoked contractions; however, no phospholipid hydrolysis could be detected (either in the presence of Ca2+ or Sr2+), due to the small amounts of presynaptic terminals. Following initial exposure of synaptosomes to a Ca2+ containing medium and then removal of Ca2+, incubation with beta-bungarotoxin (1 or 10 micrograms/ml) caused significant phospholipid hydrolysis whether or not Sr2+ was present. Therefore, conclusions as to whether phospholipase A2 activity is required for presynaptic actions of beta-bungarotoxin cannot be derived from studies in which Sr2+ is used to inhibit enzymatic activity.

Dantrolene and mepacrine antagonize the hemolysis of human red blood cells by halothane and bee venom phospholipase A2

Dantrolene is an effective antagonist of anesthesia-induced malignant hyperthermia due to a poorly understood action on skeletal muscle. The present study examines whether the red blood cell can be used as a model to investigate the mechanism of dantrolene action. Halothane (4.7 mM) caused 9% hemolysis of red blood cells. Phospholipase A2 (1 microM) alone caused less than 2% hemolysis, despite high levels (54%) of phosphatidylcholine hydrolysis. Incubation of red blood cells with halothane and phospholipase A2 caused 72% hemolysis. Halothane addition caused 100% hydrolysis of all diacylphosphoglycerides by phospholipase A2, suggesting a mutual potentiation. The major products of phospholipase A2 activity, arachidonic acid and lysophosphatidylcholine, when exogenously added, also greatly increased hemolysis induced by halothane, with arachidonic acid most closely resembling the synergism observed with phospholipase A2. Dantrolene (10 microM) and mepacrine (10 microM) significantly antagonized hemolysis induced by halothane and phospholipase A2 or halothane and exogenously added arachidonic acid and lysophosphatidylcholine. Dantrolene and mepacrine did not antagonize phospholipid hydrolysis or free fatty acid levels. Dantrolene and mepacrine antagonized the synergism between halothane and phospholipase A2 most likely by reducing the lytic action of halothane in the presence of arachidonic acid. The red blood cell is a useful model for studying the antagonism of halothane and phospholipase A2 toxicity by dantrolene and mepacrine.

Comparison of enzymatic and pharmacological activities of lysine-49 and aspartate-49 phospholipases A2 from Agkistrodon piscivorus piscivorus snake venom

The basic Lys-49 phospholipase A2 (PLA2) from Agkistrodon piscivorus piscivorus venom is homologous to the basic Asp-49 PLA2 from the same venom as well as other snake venom PLA2 enzymes. It differs, however, in several respects, most important being replacement of the previously invariant Asp-49 at the calcium binding site by Lys, resulting in a reversed order of addition of calcium and phospholipid, phospholipid binding first. Although the preferences for phospholipid substrates of the two enzymes are identical, the apparent Vmax of the Lys-49 PLA2 was only 1.4 to 3% that of the Asp-49 enzyme. Similarly, the Lys-49 PLA2, compared to the Asp-49 PLA2 had less than 3% of the intraventricular lethal potency and 4% of the anticoagulant activity. The intravenous lethal potency of the Lys-49 enzyme was 20% that of the Asp-49 PLA2 and both had little direct hemolytic activity. In contrast, both enzymes were approximately equipotent on the phrenic nerve-diaphragm preparation and on the isolated ventricle strip of the heart. On the cardiac and neuromuscular preparations, the effects of the Asp-49 PLA2 were accompanied by hydrolysis of phosphatidylcholine and phosphatidylethanolamine, whereas no phospholipid hydrolysis was observed with the Lys-49 PLA2. Evaluation of the present results, along with earlier findings using Asp-49 PLA2 enzymes from Naja nigricollis, Hemachatus haemachatus and Naja naja atra venoms, allows us to conclude that: The A. p. piscivorus Asp-49 PLA2 enzyme resembles the Asp-49 enzymes from N. n. atra and H. haemachatus. In contrast, the A. p. piscivorus Lys-49 PLA2 has much lower enzymatic and anticoagulant activities than the Asp-49 enzymes, but equal cardiotoxic and junctional effects. In contrast to some previous suggestions, basic PLA2 enzymes are not necessarily more toxic than neutral or acidic enzymes. Pharmacological effects upon the heart and phrenic nerve-diaphragm preparation correlate neither with in vitro measurements of PLA2 activity nor with actual levels of phospholipid hydrolysis in the heart or diaphragm. This suggests that PLA2 enzymes exert effects independent of phospholipid hydrolysis.

Lecithin: cholesterol acyltransferase and lysolecithin in coronary atherosclerosis

Of 100 arteriographically examined, hospitalized, male patients, those without myocardial infarctions were divided into the following categories: zero-, one-, two-, and three-vessel disease; patients diagnosed with myocardial infarction were classified separately. The fasting plasma samples from these patients were examined for concentrations of triglycerides and total cholesterol, lipoprotein profile, lecithin: cholesterol acyltransferase (LCAT) activity, and lysolecithin (LPC) concentration. Those parameters in this group which are commonly determined were consistent with the clinical classification of these patients. Of those remaining parameters, the LCAT activity was increased as the severity of coronary atherosclerosis increased and the changes in the activity of this enzyme were appropriately reflected by increases in LPC concentration and decreases in the proportion of the plasma cholesterol unesterified. The results of this study suggest that increased, rather than decreased, plasma LCAT activity and increased LPC concentrations are characteristic of coronary atherogenesis. The plasma concentrations of LPC observed in these atherosclerotic patients are more than sufficient to qualify this substance for its previously proposed roles of mediator of transmembrane diffusion of LDL and as an inhibitor of platelet aggregation.

Cardiotoxicity of Naja nigricollis phospholipase A2 is not due to alterations in prostaglandin synthesis

The basic phospholipase A2 from Naja nigricollis snake venom is cardiotoxic, causing decreased contractility and arrhythmias at concentrations which induce low levels of phospholipid hydrolysis. Cardiac tissue has a high content of arachidonic acid at the sn-2 position of the major membrane phospholipids, thus increased prostaglandin synthesis might contribute to the cardiotoxic effects of N. nigricollis phospholipase A2. Intracellular action potentials and cardiac contractility were monitored in the isolated right ventricular wall of the rat heart exposed for 1 hr to N. nigricollis phospholipase A2, with or without indomethacin, or to arachidonic acid. The tissues were homogenized, prostaglandins extracted and the 6-keto PGF1 alpha and PGE2 content of the hearts determined. The physiologic effects and prostaglandin content of hearts treated with N. nigricollis phospholipase A2 were not altered by indomethacin nor mimicked by concentrations of arachidonic acid comparable to that present in N. nigricollis phospholipase A2-treated tissue. These results support our previous suggestion that exogenously applied N. nigricollis phospholipase A2 causes cardiotoxic effects by a mechanism that is independent of phospholipid hydrolysis.

Effects of modification of tyrosines 3 and 62 (63) on enzymatic and toxicological properties of phospholipases A2 from Naja nigricollis and Naja naja atra snake venoms

Previously we selectively modified His (48), Arg, Lys, Asp, Glu and Trp residues in the basic phospholipase A2 from Naja nigricollis and the acidic phospholipase A2 from N. n. atra snake venoms. Evidence was obtained for the existence of separate but perhaps overlapping sites responsible, respectively, for their enzymatic and pharmacological properties. We have now modified one or two (Tyr 3, Tyr 62 [63], Tyr 3 + 62 [63]) out of the nine tyrosine residues in these enzymes using p-nitrobenzenesulfonyl fluoride. The derivatives were separated by HPLC, and modified residues determined by amino acid analysis. Enzymatic activity was tested on lecithin--Triton mixed micelles, egg yolk and heart and diaphragm homogenates. The N. nigricollis modified derivatives retained a greater percentage of their enzymatic activities than did the N. n. atra derivatives and also a greater percentage of their activity on natural substrates than on lecithin--Triton mixed micelles. The greatest loss in activity resulted when both tyrosines were modified and the least when tyrosine 3 was modified. Modification of tyrosine 62 of N. nigricollis caused a much greater loss of intraventricular lethal potency than of enzymatic activity, whereas modification of tyrosine 3 of N. n. atra increased lethal potency over six-fold while enzymatic activity decreased about 60%. Examples of dissociation between enzymatic and pharmacological potencies were also noted when hemolytic, anticoagulant and cardiotoxicity on isolated ventricular muscle were measured. The extents of phospholipid hydrolysis were relatively low in brain homogenates, synaptic plasma membranes and heart ventricular muscle. However, they were similar for the native enzymes and all of the tyrosine modified derivatives. These tyrosines do not appear to be part of the enzymatic active site, even though they are thought to be associated with substrate and calcium binding. These results strengthen our earlier conclusion that some pharmacological effects of phospholipase A2 are not due to enzymatic hydrolysis, and that there are separate but perhaps partly overlapping sites for enzymatic and pharmacological activities.

Cardiotoxic effects of Naja nigricollis venom phospholipase A2 are not due to phospholipid hydrolytic products

N. nigricollis phospholipase A2 (200 micrograms) has marked cardiotoxic actions on the perfused rat heart including the induction of arrhythmias, increases in ventricular thresholds, conduction and resting tension, and decreases in contractile tension. In contrast, perfusion with lysophosphatidyl choline and oleic acid, in concentrations comparable to those estimated to be formed during N. nigricollis treatment, has little effect on cardiac function. The less toxic N. n. atra phospholipase A2 also has little effect on cardiac function even though it causes approximately the same low percentage of phospholipid hydrolysis as produced by N. nigricollis phospholipase A2. Perfusion with albumin did not alter the phospholipase A2 induced changes in cardiac function. Lysophosphatidyl choline in concentrations higher than expected to be formed during N. nigricollis phospholipase A2 treatment, increased conduction time to a greater extent than ventricular threshold whereas the reverse was true for phospholipase A2. We conclude that the cardiotoxic effects of N. nigricollis phospholipase A2 are not due to the accumulation of phospholipid hydrolytic products, and on the basis of prior studies with chemically modified phospholipase A2 enzymes we suggest that N. nigricollis phospholipase A2 has a direct, non-enzymatic, cardiotoxic action.

Effect of carboxylate group modification on enzymatic and cardiotoxic properties of snake venom phospholipases A2

By treating Naja nigricollis and Naja naja atra phospholipase A2 with carbodiimide and semicarbazide, we obtained derivatives having varied numbers of modified carboxylate groups. When tested on artificial and natural substrates, derivatives of both enzymes with a modified carboxylate group at the active site (Asp-49) retained little enzymatic activity (1/41 to 10%). However, the derivatives of N. nigricollis also lost most of their lethal potency (5% of native), while those of N. n. atra retained considerable lethality (29%). Carboxyl modification with protection of Asp-49 in N. n. atra enzyme resulted in a derivative with lethal potency equal to or greater than the native enzyme and enzymatic activity which was low on all substrates (12-17% of native). Similar protection of Asp-49 at the active site in N. nigricollis enzyme produced a derivative with decreased enzymatic activity on artificial substrate (22% of native) and decreased lethality (17-33% of native), but with full enzymatic activity on natural substrates. When tested on electrical and mechanical properties of the isolated perfused heart and the isolated ventricle muscle wall, the derivatives of both enzymes retained considerably more of the cardiotoxic activity than would have been expected based on their residual enzymatic activity. The one exception occurred with the least modified N. nigricollis derivative which had an unaltered Asp-49, this enzyme retained both cardiotoxic activity and full enzymatic activity on natural substrates. The extent of phospholipid hydrolysis following treatment was measured in the isolated heart preparation and in hearts removed from mice following i.v. injection of the phospholipases. Very low levels of phospholipid hydrolysis were observed and no correlation could be made between the extent of hydrolysis and the pharmacological potencies of these enzymes. Modification of the enzymatic active site, whether of Asp-49 in this study of His-48 in prior studies, leads to a large decrease in both enzymatic activity and lethal potency. Asp and Glu residues outside of the enzymatic site contribute significantly to the lethal potency of the N. nigricollis enzyme and to the enzymatic activity of the N. n. atra enzyme. Based on these and previous data we conclude that changes in isoelectric points are not responsible for altered lethal potencies following chemical modification and that some pharmacological effects of snake venom phospholipases A2 are due to a non-enzymatic action, suggesting two distinct but perhaps overlapping active sites.

Ethoxyformylation and guanidination of snake venom phospholipases A2: effects on enzymatic activity, lethality and some pharmacological properties

Lysine residues in the basic and relatively toxic N. nigricollis phospholipase A2 and in the acidic and relatively nontoxic N. n. atra phospholipase A2 were modified by acylation with ethoxyformic anhydride (in the presence or absence of the substrate dihexanoyl lecithin) or guanidination with O-methylisourea. Ethoxyformylation gave rise to some protein fractions in which enzymatic activity was preserved to a greater degree than intraventricular lethality. Guanidination had little effect on the isoelectric point or catalytic activity of either enzyme or on the lethal potency of the N. n. atra enzyme. However, the intraventricular lethality of the N. nigricollis enzyme was decreased much more than was its intravenous lethality, direct hemolytic potency, anticoagulant activity or cardiotoxic action on rat atrium. These results are compared to those previously obtained when the lysines in these two enzymes were carbamylated with potassium cyanate, a procedure which markedly decreased the isoelectric point of the enzymes. It is concluded that charge alone does not account for differences in toxicity. The data also indicate that there are at least two distinct active sites in both enzymes, one being primarily responsible for enzymatic activity and the other(s) associated with lethal and pharmacological effects of the protein. Modification of lysines affects the latter site(s), while having little or no effect on enzymatic activity.

Phospholipid hydrolysis in serum lipoproteins by a basic phospholipase A2 from Naja nigricollis snake venom and an acidic phospholipase A2 from Naja naja atra snake venom

Apparent Km and Vmax values for PC and PE hydrolysis were determined following exposure of HDL, LDL, and VLDL to a basic phospholipase A2 from N. nigricollis snake venom and an acidic phospholipase A2 from N. nigricollis snake venom and an acidic phospholipase A2 from N. n. atra snake venom. Both enzymes hydrolyzed the lipoprotein phospholipids approximately as fast as they hydrolyzed pure phospholipids in mixed micelles, however, the N. nigricollis enzyme, which has a much stronger anticoagulant effect than the N. n. atra enzyme, had lower apparent Vmax values. These values were highest for phospholipids in VLDL and lowest for HDL, however, the differences between the lipoproteins were relatively small with the N. nigricollis enzyme while the differences were much larger with the N. n. atra enzyme. Fractions of the two enzymes in which varying numbers of lysines were carbamylated showed much larger differences in relative rates of phospholipid hydrolysis in HDL, LDL and VLDL. Triton X-100 eliminates these differences in rates of hydrolysis. These results are discussed in terms of the differences in the organized structure of the lipoprotein classes and in the penetration ability of the phospholipases.

Relationships between cholesterol and phospholipid concentrations in the serum of humans, calves, rabbits and chickens

In humans, calves and rabbits very strong correlations were found between the concentrations of serum total cholesterol and phospholipids; in these species more than 70% of the variance in the level of serum phospholipids is accounted for by the variance in the serum cholesterol concentration. The regression coefficients in the equations relating the concentration of serum cholesterol to that of phospholipids were almost identical in humans and rabbits, but differed from those observed in calves and chickens. It is suggested that the rabbit is a suitable model animal for studying the relation between cholesterol and phospholipids in human serum.

Isopycnic-zonal centrifugation of plasma membrane, sarcoplasmic reticular fragments, lysosomes, and cytoplasmic proteins from phasic skeletal muscle

Homogenates of the posterior latissimus dorsi muscle, a phasic muscle, were fractionated by a one-step zonal centrifugation technique into four major organelle populations and cytoplasmic constituents. These were: (1) Plasma membrane fragments with a modal equilibrium density of 1.10 and containing 5'-nucleotidase, alkaline phosphodiesterase, p-nitrophenylphosphatase and acid phosphatase (beta-glycerophosphate was used as the substrate). (2) Sarcoplasmic reticular fragments which could be further subdivided into calcium transport vesicles, with a model equilibrium density of 1.16, that exhibited calcium uptake; K+-ATPase; leucyl-bet-naphthylamidase; acid phosphodiesterase; acid phosphatase (using cytidine monophosphate as the substrate); and sarcoplasmic reticular lysosomes, with a model equilibrium density of 1.18, possessing dipeptidyl-aminopeptidase II, cathepsin D, alpha-glucosidase, N-acetyl-beta-glucosaminidase, and NADH oxidase activity. (3) Mitochondria with a modal equilibrium density of 1.21. (4) Catalase-containing vesicles with a modal equilibrium density of 1.22; and cytoplasmic constituents (modal density of 1.25) with phosphorylase, pyruvate kinase, myosin-ATPase, aldolase, and protein and RNA content. The purity of these organelles was equal to or better than previous efforts, with a 30-fold purification achieved for 5'-nucleotidase and alkaline phosphodiesterase. These results lend support to the hypothesis that the sarcoplasmic reticulum of phasic muscle, in addition to its specialized role in excitation-contraction coupling, represents a multifunctional membrane system, and that, similar to the smooth endoplasmic reticulum of other cells, it includes some membrane-bound lysosomal enzymes and NADH oxidase.

Solubilization of human red cell membranes by lysolecithins of various chain lengths

Red cells and membranes prepared from them were treated with graded amounts of palmitoyl, myristoyl and lauroyl lysolecithins. There was no release of phospholipids from the intact red cells in the absence of hemolysis. The solubilization pattern of lipid and protein from the red cell membranes by lysolecithins varies with their chain length.

Serum phospholipid fractionation after the use of long-acting progestational contraceptives

The effect of the two long-acting progestational contraceptive injection, depot-medroxyprogesterone acetate (DMPA) and norethisterone onenathate (NET-OEN), on blood phospholipid fractions were studied in eighteen women. The post-injection results showed a significant increase in total serum phospholipids, serum lecithin and serum cephalins. Serum lysolecithin was significantly decreased while serum sphingomyelin showed no significant change. The mechanisms responsible for these phospholipid changes remain somewhat unclear. However deranged liver function may share in its explanation.

Streptococcus mutans dextransucrase: stimulation of glucan formation by phosphoglycerides

Lysophosphatidylcholine (LPC) and other phosphoglycerides stimulated water-insoluble and water-soluble glucan production by the Streptococcus mutans 6715 dextransucrase (EC 2.4.1.5). LPC stimulated crude extracellular dextransucrase 1.7-fold, the water-insoluble glucan-producing alpha form of the enzyme 6.5-fold, the water-soluble glucan-producing beta form of the enzyme 2.1-fold, and the cell-associated dextransucrase 2.0-fold. Kinetic studies demonstrated that LPC did not change the K(m) for sucrose of alpha or beta but increased the maximum velocity of the enzymes. The K(m) for LPC of the alpha enzyme was 10(-5) M. LPC from various sources and synthetic preparations of lauroyl-LPC, myristoyl-LPC, and palmitoyl-LPC all stimulated glucan formation. Portions of phosphoglyceride molecules including fatty acids, phosphatidic acid, glycerophosphoric acid, glycerophos-phorylcholine, and choline, when tested individually or in combinations, did not enhance dextransucrase activity. The increased rates of glucan production caused by LPC and primer dextran were additive. Enzyme incubated with LPC before addition of sucrose was stimulated by dextran primer, and, conversely, enzyme treated with dextran was stimulated by addition of LPC with the sucrose substrate. Thus, dextransucrase can be activated by binding of intact phosphoglyceride molecules to a site on the enzyme that is distinct from either the glucosyl donor or glucosyl acceptor (primer) binding sites. Interactions between the S. mutans dextransucrase and amphipathic phosphoglycerides may explain properties of this enzyme which contribute to the cariogenicity of S. mutans.