Modification of arginine residues in porcine pancreatic phospholipase A2

A New Group II Phospholipase A2 from Walterinnesia aegyptia Venom with Antimicrobial, Antifungal, and Cytotoxic Potential

Many venomous species, especially snakes, contain a variety of secreted phospholipases A2 that contribute to venom toxicity and prey digestion. We characterized a novel highly toxic phospholipase A2 of group II, WaPLA2-II, from the snake venom of Saudi Walterinnesia aegyptia (W. aegyptia). The enzyme was purified using a reverse phase C18 column. It is a monomeric protein with a molecular weight of approximately 14 kDa and an NH2-terminal amino acid sequence exhibiting similarity to the PLA2 group II enzymes. WaPLA2-II, which contains 2.5% (w/w) glycosylation, reached a maximal specific activity of 1250 U/mg at pH 9.5 and 55 °C in the presence of Ca2+ and bile salts. WaPLA2-II was also highly stable over a large pH and temperature range. A strong correlation between antimicrobial and indirect hemolytic activities of WaPLA2 was observed. Additionally, WaPLA2-II was found to be significantly cytotoxic only on cancerous cells. However, chemical modification with para-Bromophenacyl bromide (p-BPB) inhibited WaPLA2-II enzymatic activity without affecting its antitumor effect, suggesting the presence of a separate ‘pharmacological site’ in snake venom phospholipase A2 via its receptor binding affinity. This enzyme is a candidate for applications including the treatment of phospholipid-rich industrial effluents and for the food production industry. Furthermore, it may represent a new therapeutic lead molecule for treating cancer and microbial infections.

A Midgut Digestive Phospholipase A2 in Larval Mosquitoes, Aedes albopictus and Culex quinquefasciatus

Phospholipase A₂ (PLA₂) is a secretory digestive enzyme that hydrolyzes ester bond at sn-2 position of dietary phospholipids, creating free fatty acid and lysophospholipid. The free fatty acids (arachidonic acid) are absorbed into midgut cells. Aedes albopictus and Culex quinquefasciatus digestive PLA₂ was characterized using a microplate PLA₂ assay. The enzyme showed substantial activities at 6 and 8 μg/μl of protein concentration with optimal activity at 20 and 25 μg/μl of substrate concentration in Aedes albopictus and Culex quinquefasciatus, respectively. PLA₂ activity from both mosquitoes increased in a linear function up to 1 hour of the reaction time. Both enzymes were sensitive to pH and temperature. PLA₂ showed higher enzyme activities in pH 8.0 and pH 9.0 from Aedes albopictus and Culex quinquefasciatus, respectively, at 40°C of incubation. The PLA₂ activity decreased in the presence of 5 mM (Aedes albopictus) and 0.5 mM (Culex quinquefasciatus) site specific PLA₂ inhibitor, oleyloxyethylphosphorylcholine. Based on the migration pattern of the partially purified PLA₂ on SDS-PAGE, the protein mass of PLA₂ is approximately 20–25 kDa for both mosquitoes. The information on PLA₂ properties derived from this study may facilitate in devising mosquitoes control strategies especially in the development of inhibitors targeting the enzyme active site.

Renaturation and one step purification of the chicken GIIA secreted phospholipase A2 from inclusion bodies

The cDNA coding for a mature protein of 123 amino acids, containing all of the structural features of catalytically active group II sPLA2, has been amplified. The gene has been cloned into the bacterial expression vector pET-21a(+), which allows protein over-expression as inclusion bodies and enables about 3 mg per litre of pure refolded fully active enzyme to be obtained. Recombinant expression of chPLA2-IIA in Escherichia coli shows that the enzyme is Ca(2+) dependent, maximally active at pH 8-9, and hydrolyses phosphatidylglycerol versus phosphatidylcholine with a 15-fold preference. The ability to express reasonably large amounts of the sPLA2 Group IIA, compared to that obtained with the classical purification will provide a basis for future site directed mutagenesis studies of this important enzyme.

Effect of moisture, lipids, and select amino acid blocking agents on the formation and stability of metastable radicals in powdered soy proteins

Incremental increases in the moisture content of powdered soy protein products from 4.4% to 13.4% produced an inverse effect on the ability of soy proteins to maintain metastable free radicals. The corresponding reduction in electron paramagnetic resonance signal was not due to dielectric loss in the range of moisture contents examined. Subsequent evaluations of various treatments were conducted after drying soy proteins with molecular sieve to a water activity below 0.085 in order to minimize the influence from variations in moisture. Isolated soy protein (ISP) samples, prepared with "defatted flour" that had been further extracted with chloroform/methanol (2: 1), had a 96% reduction in total lipids compared to the control ISP samples. The initial rate of radical accumulation in the "reduced-lipid" ISP for the first 3 wk was not significantly different from the initial rate of radical increases in the control ISP. After 3 wk, radical accumulation in the "reduced-lipid" ISP continued to increase, but at a rate that was less than the control. These findings indicate that the initial reactions contributing to the formation of metastable radicals in the powdered ISP are not strongly dependent on associated lipids. Blocking sulfhydryl groups during ISP preparation with N-ethylmaleimide did not significantly slow the rate of radical accumulation compared to the control ISP. Blocking arginine residues in ISP samples with phenylglyoxal caused an increased rate of radical accumulation for the first 4 wk.

PRACTICAL APPLICATION

Levels of metastable radicals in powdered soy protein products typically range from 10 to 100 times greater than the free radicals in other food protein sources. This current research examines various compositional and treatment parameters that might be used to minimize the content of free radicals in foods containing soy proteins.

Purification and biochemical characterization of a secreted group IIA chicken intestinal phospholipase A₂

Background

Secretory phospholipase A2 group IIA (IIA PLA2) is a protein shown to be highly expressed in the intestine of mammals. However, no study was reported in birds.

Results

Chicken intestinal group IIA phospholipase A₂ (ChPLA₂-IIA) was obtained after an acidic treatment (pH.3.0), precipitation by ammonium sulphate, followed by sequential column chromatographies on Sephadex G-50 and mono-S ion exchanger. The enzyme was found to be a monomeric protein with a molecular mass of around 14 kDa. The purified enzyme showed a substrate preference for phosphatidylethanolamine and phosphatidylglycerol, and didn't hydrolyse phosphatidylcholine. Under optimal assay conditions, in the presence of 10 mM NaTDC and 10 mM CaCl_2, a specific activity of 160 U.mg^-1 for purified ChPLA₂-IIA was measured using egg yolk as substrate. The fifteen NH2-terminal amino acid residues of ChPLA₂-IIA were sequenced and showed a close homology with known intestinal secreted phospholipases A₂. The gene encoding the mature ChPLA₂-IIA was cloned and sequenced. To further investigate structure-activity relationship, a 3D model of ChPLA₂-IIA was built using the human intestinal phospholipase A₂ structure as template.

Conclusion

ChPLA2-IIA was purified to homogeneity using only two chromatographic colomns. Sequence analysis of the cloned cDNA indicates that the enzyme is highly basic with a pI of 9.0 and has a high degree of homology with mammalian intestinal PLA₂-IIA.

Characterization of the interaction between beta2-glycoprotein I and calmodulin, and identification of a binding sequence in beta2-glycoprotein I

beta2-Glycoprotein I was shown to bind reversibly to calmodulin in a Ca2+-dependent manner with a 1:1 stoichiometry, a Kd of 3 x 10(-9) M and a Hill coefficient of 1.4. A sequence in beta2-glycoprotein I (Lys-Pro-Gly-Tyr-Val-Ser-Arg-Gly-Gly-Met-Arg-Lys-Phe-Ile-) limited by Cys-32 and Cys-47 is suggested to be the calmodulin-binding region. This sequence was the only one in beta2-glycoprotein I theoretically having the ability to form a basic amphiphilic alpha-helix typical of a calmodulin binding sequence. The peptide corresponding to this sequence was synthesized and found to inhibit the interaction between beta2-glycoprotein I and calmodulin with an IC50 value of 0.38 x [beta2-glycoprotein I] and to displace the beta2-glycoprotein I from the beta2-glycoprotein I/calmodulin complex with an IC50 value of 0.90 x [beta2-glycoprotein I].

Polycations as prostaglandin synthesis inducers. Stimulation of arachidonic acid release and prostaglandin synthesis in cultured fibroblasts by poly(L-lysine) and other synthetic polycations

Poly(L-lysine) hydrobromide stimulates arachidonic acid release with concomitant synthesis and release of prostaglandins and lipoxygenase-mediated metabolites (hydroxyeicosatetraenoic acids) in cultures of 3T3 Swiss mouse fibroblasts biosynthetically labeled with [3H]arachidonic acid. The response is rapid, reversible with trypsin and persists for at least 50 min. An evaluation of the calcium dependence of the hydrolytic process was consistent with the rate-limiting step involving a cell-surface, calcium-dependent enzyme. The response involves stimulated hydrolysis of arachidonic acid-containing phospholipids, implying the activation of a phospholipase. Arachidonic acid release is stimulated only by poly(L-lysine) hydrobromide preparations with a molecular weight greater than 30 000, which corresponds to a polypeptide chain of more than 140 lysine hydrobromide residues. A variety of other polycations (Mr greater than 30 000), but not polyanions or neutral polymers, stimulated arachidonic acid release and prostaglandin synthesis. The results are consistent with an activation mechanism involving cross-linking of anionic sites on the cell surface. Poly(L-lysine) hydrobromide is also cytotoxic, but the cytotoxic response occurs at 10-fold higher concentrations than arachidonic acid release.

Modification of arginines in D-beta-hydroxybutyrate dehydrogenase

D-beta-Hydroxybutyrate dehydrogenase (D-3-hydroxybutyrate: NAD+ oxidoreductase, EC 1.1.1.30) is a lipid-dependent enzyme which has an absolute and specific requirement for phosphatidylcholine for function. Chemical derivatization studies using 1,2-cyclohexanedione, an arginine-specific reagent, have been carried out on the purified enzyme devoid of lipid as well as on the enzyme reactivated with phospholipid. Cyclohexanedione inactivated the active enzyme-phospholipid complex and the lipid-free enzyme was rendered inactivatable by phospholipid. From kinetic studies and by direct chemical derivatization studies with [1-14C]cyclohexanedione, we find that incorporation of a single cyclohexanedione molecule per enzyme monomer resulted in complete loss of enzymic activity. The presence of NADH or NAD+, cofactors for the enzyme, offered no protection for the rate of inactivation. The substrates beta-hydroxybutyrate and acetoacetate with or without coenzyme gave little or no protection. However, 2-methyl malonate, a competitive inhibitor for beta-hydroxybutyrate, strongly protected against inactivation. These studies indicate that: (1) a single arginine serves a vital role and is essential for function; (2) the arginine is located in the proximity of the substrate binding site.

Semisynthesis of phospholipase A2. The effect of substitution of amino-acid residues at positions 6 and 7 in bovine and porcine pancreatic phospholipases A2 on catalytic and substrate-binding properties

The N-terminal alpha-helical region of phospholipase A2 is an important part of the enzyme for catalytic activity and lipid binding. Porcine pancreatic phospholipase A2 has Arg-Ser at positions 6 and 7, whereas the bovine enzyme has Asn-Gly. To pursue further the effects of these variable residues on differences in enzymatic properties, we prepared and studied the following semisynthetic analogs of epsilon-amidinated phospholipase A2 (AMPA): porcine [Ala7]AMPA, [Gly7]AMPA, [Asn6]AMPA, [Asn6-Gly7]AMPA and bovine [Ser7]AMPA and [Arg6-Ser7]AMPA. As we had previously found for the Asn6 leads to Arg bovine substitution, an Asn6-Gly7 leads to Arg5-Ser7 bovine substitution similarly improves the catalytic activity, the affinity for neutral lipid-water interfaces and the capacity to penetrate lecithin monolayers, while just changing Gly7 leads to Ser produces almost no effect on these properties. Ser7 leads to Ala and Ser7 leads to Gly substitutions in porcine AMPA did not affect penetration or lipid binding, although they did diminish catalytic activity (which is true of all substitutions made in the porcine enzyme). Arg6 leads to Asn substitution in porcine AMPA decreases penetration of lecithin monolayers, but not as much as it was improved by the Asn6 leads to Arg substitution in bovine AMPA. In contrast to the dramatic increase in affinity for lipid-water interfaces of Asn6 leads to Arg substitution in bovine AMPA, no decrease in affinity was found for Arg6 leads to Asn substitution in porcine AMPA. This difference is most likely due to the fact that the porcine enzyme has positively charged Lys and His in place of the Lys10, Glu17 pair that lie very close to residue 6 in the bovine structure. It can thus be conclude that (with the exception of Gly7 leads to Ser in bovine AMPA) all the substitutions tried at positions 6 and 7 in bovine and porcine AMPAs have definite effects on the catalytic activity.

Modification of the phosphatidylcholine-transfer protein from bovine liver with butanedione and phenylglyoxal. Evidence for one essential arginine residue

1. Modification of arginine residues with 2,3-butanedione and phenylglyoxal completely inhibits the transfer activity of the phosphatidylcholine transfer protein from bovine liver. Removal of borate and butanedione leads to a slow reactivation of the protein. 2. Both alpha-dicarbonyl reagents modify three of the ten arginine residues present per protein molecule. The extent of modification is linearly related to the loss of activity. 3. Inactivation with butanedione is greatly diminished when the protein is bound to strongly negatively charged vesicles. Under these conditions a rapid modification of two arginine residues is observed. This suggests that the transfer protein contains one arginine residue essential for activity, probably as a binding site for the negatively charged phosphate group of the phosphatidylcholine molecule. 4. This study provides convincing evidence that arginine residues may play an essential role in phospholipidprotein interactions.