Isolation of an isozyme of soybean lipoxygenase

Rabbit reticulocyte lipoxygenase catalyzes specific 12(S) and 15(S) oxygenation of arachidonoyl-phosphatidylcholine

The rabbit reticulocyte lipoxygenase is known to display an unusual facility for oxygenation of esterified polyunsaturated fatty acids, yet the precise structures of the products are not known. With free arachidonate as substrate the enzyme is known to catalyze 15S and 12S oxygenations, and demonstration of a facility for catalysis of these reactions on phospholipids would extend the potential scope of lipoxygenase reactions in cells. We elected to study in detail the reaction of the enzyme with a natural phospholipid, palmitoyl/arachidonoyl-phosphatidylcholine. We determined the nature of the products by initial isolation by RP-HPLC, followed by transesterification and identification of the oxygenated products by HPLC, uv, GC-MS, and steric analysis of hydroxyl configuration by HPLC. The major product was identified as a phosphatidylcholine in which the arachidonate component was converted to the 15(S)-hydroperoxy-eicosatetraenoate. A second oxygenated phospholipid was produced in smaller quantities (2-5% of the latter product) and identified as the 12(S)-oxygenated analog. These products were also identified after reaction of the reticulocyte lipoxygenase with human red cell membranes which were radiolabeled by preincubation with [3H]arachidonic acid. The finding of 12S oxygenation represents the first evidence that a lipoxygenase can control a reaction centered on the 10-carbon of an arachidonoyl phospholipid. This is an important precedent, because hydrogen abstraction from carbon-10 is a critical step in the lipoxygenase-catalyzed synthesis of 8- and 12-hydroperoxy-eicosatetraenoates (HPETEs) and for the conversion of 5- and 15-HPETEs to leukotrienes.

Soy flavor and its improvement

A highly characteristic and often undesirable flavor associated with soy protein materials largely explains the slower-than-expected progress over recent years in the development of high-protein foods based on soya. Apart from the inherent flavor of the bean, different flavors are produced on processing and on storage. Major problems are the absence of an attractive positive flavor, the presence of off-flavors of several kinds, the tenacious binding of such flavors to the soy protein molecules, and the difficulties of removing and/or masking these unacceptable qualities. This review provides a reappraisal of current literature evidence relating to each of these aspects and summarizes published patents of processes for soy flavor improvement.

Analysis of a specific oxygenation reaction of soybean lipoxygenase-1 with fatty acids esterified in phospholipids

Soybean lipoxygenase was reacted with phosphatidylcholine (at pH 9, with 10 mM deoxycholate), and the oxygenation products were analyzed by high-pressure liquid chromatography, UV, gas chromatography-mass spectrometry (GC-MS), and NMR. The structures of the intact glycerolipid products were established by GC-MS of diglycerides recovered by phospholipase C hydrolysis and by proton NMR of the intact phosphatidylcholine. These analyses, together with analyses of the transesterified fatty acids, indicated that arachidonyl and linoleoyl moieties in the phosphatidylcholine were converted exclusively to the 15(S)-hydroperoxy-5(Z),8(Z),11(Z),13(E)-eicosatetraenoate and 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoate analogues, respectively. Control experiments proved that the intact phospholipid (and not hydrolyzed/reesterified fatty acid) was the true substrate of the oxygenation reaction. Phosphatidylethanolamine and phosphatidylinositol lipids were also substrates for specific oxygenation by the soybean lipoxygenase. The results provide concrete evidence that fatty acids esterified in phospholipid can be subject to highly specific oxygenation by a lipoxygenase enzyme.

Hydroperoxide-dependent sulfoxidation catalyzed by soybean microsomes

The sulfoxidation of methiocarb, an aromatic-alkyl sulfide pesticide, catalyzed by soybean microsomes was found to be strongly stimulated in the presence of cumene and linoleic acid hydroperoxides. We have shown that this S-oxidation, which does not require cofactors such as NAD(P)H, is an hydroperoxide-dependent reaction: 18O2-labeling experiments demonstrated that the oxygen atom incorporated into the sulfoxide originated from hydroperoxides rather than from molecular oxygen. In the absence of exogenous hydroperoxides, soybean microsomes catalyzed methiocarb sulfoxide formation at a basal rate dependent on their endogenous hydroperoxides, especially those derived from free fatty acids. The nature of the sulfoxidase is discussed. Our results seem to rule out the participation of cytochrome P-450 in this oxidation, whereas the studied sulfoxidase presents some similarities to plant peroxygenase.

Two soybean seed lipoxygenase nulls accumulate reduced levels of lipoxygenase transcripts

Soybean (Glycine max L. [Merrill]) seed lipoxygenase cDNA clones were recovered from two cDNA libraries: a size-selected library in pBR322 and an expression library in pUC8. The pUC8 library was made with total poly(A)(+) embryo RNA and was screened with antiserum to lipoxygenase-1, one of 3 seed lipoxygenase isozymes. Three lipoxygenase antigen-producing clones were identified: two with identical cDNA inserts of 977 nucleotides representing an open-reading frame and a third truncated clone bearing a 3' end common to the longer clones. A long clone, pAL-134, was chosen for further study and was used to screen the size-selected cDNA library from which sixteen clones were identified. They fall into two homology classes represented by pLX-10 (ca. 1360 bp) and pLX-65 (2047 bp).The lipoxygenase expression clone pAL-134 hybridized much more strongly to pLX-65 than to pLX-10. pAL-134 and pLX-65 share 89% nucleotide homology and 75% deduced amino acid homology along their common sequence. Their deduced amino acid sequences each show 80% homology to sequences determined for isolated peptides of the lipoxygenase-1 isozyme.pAL-134 hybridizes poorly with a 3.8 kb RNA from LOX-1 null (lx1) embryos while pLX-65 hybridizes more strongly, but still to a lesser extent than its hybridization to standard embryo RNA or to RNA from embryos lacking lipoxygenase-2 (lx2) or lipoxygenase-3 (lx3) protein.The lx3 null lacks almost all embryo 3.8 kb RNA homologous to pLX-10. This hybridization pattern suggests that pLX-10 encodes LOX-3. Thus, the lx1 and lx3 genotypes accumulate little, if any, mRNA for the lipoxygenase-1 and lipoxygenase-3 isozymes, respectively.

Isolation of a cDNA clone for pea (Pisum sativum) seed lipoxygenase

A lipoxygenase cDNA clone, pCD45, was identified in a Pisum sativum L. (pea) seed mRNA cDNA library by hybrid-release/translation followed by immunoprecipitation with antiserum raised against lipoxygenase from Glycine max L. (soya bean). pCD45 hybrid-selected an mRNA encoding the larger of the two polypeptides of Mr approximately 95 000 that were immunoprecipitated from cell-free translation products of pea seed poly(A)-containing RNA by the G. max anti-lipoxygenase. 'Northern'-blot analysis showed the mRNA that hybridized to pCD45 to be approximately 3000 nucleotides in length. Three to five copies of the lipoxygenase gene corresponding to pCD45 were estimated to be present per haploid Pisum genome; hybridization of the cDNA insert from pCD45 to G. max DNA was also detected.

Immunological comparison of lipoxygenase isozymes-1 and -2 with soybean seedling lipoxygenases

An affinity-purified polyclonal antibody against soybean seed lipoxygenase-2 was prepared and used to characterize the immunological relatedness of lipoxygenase isozymes 1 and 2 and lipoxygenases from soybean seedling roots, hypocotyls, leaves, and cotyledons. All soybean lipoxygenases tested cross-reacted with the anti-lipoxygenase-2. Cross-reactivity of seed-derived lipoxygenases was evidenced by formation of a line of identity in double-diffusion tests, by positive results on an immunoblot, and by antibody precipitation of enzyme activity. Levels of anti-lipoxygenase-2, which inhibited lipoxygenase-2 activity, had no effect on lipoxygenase-1 activity. Root, hypocotyl, and leaf lipoxygenases did not form precipitation lines in double-diffusion tests but the anti-lipoxygenase-2 did inhibit and precipitate lipoxygenase activity from these sources as well as cross-react on immunoblots. All the cross-reactive lipoxygenases examined were found to have the same apparent molecular weight. Lipoxygenase activity found in soybean seedling roots, hypocotyls, leaves, and cotyledons is associated with proteins which are all immunologically related to the seed-derived enzymes.

Resolution of the isoenzymes of soybean lipoxygenase using isoelectric focusing and chromatofocusing

Isoelectric focusing in thin-layer polyacrylamide gels has been applied to the analysis of the enzymes involved in the formation and destruction of peroxides in soybeans [Glycine max (L.)], lipoxygenases and peroxidases, respectively. As a result of differences in pH optima for catalytic activity, lipoxygenases were selectively detected by adjusting the pH employed for activity-specific staining. Type-1 lipoxygenase was revealed not only by staining based on the conversion of linoleic acid to hydroperoxide but also by two stains based on the reduction of the hydroperoxide. These methods were found to be suitable for the analysis and characterization of isoenzyme patterns in different soybean cultivars. A substantial difference in the distribution of lipoxygenases maximally active near pH 7 was observed for cultivars Provar and Vickery. A similar degree of separation of the isoenzymes was achieved on a larger scale using chromato-focusing in the pH range 7.4-5.0.

Arachidonic acid-related elicitors of the hypersensitive response in potato and enhancement of their activities by glucans from Phytophthora infestans (Mont.) deBary

The dose response for elicitation of the hypersensitive reaction in potato tuber discs by arachidonic acid (AA) suggested saturation at higher concentrations. Glucans from Phytophthora infestans, inactive themselves as elicitors of the hypersensitive reaction, enhanced sesquiterpene accumulation and hypersensitive browning elicited by AA. Significant activity (seven times control values) was observed with 33 pmol AA/3.0-cm potato disc in the presence of glucans. Glucans did not affect accumulation of steroid glycoalkaloids, influence the timing or relative amounts of sesquiterpenes which accumulate, or affect recovery of AA added to potato discs. Glucans enhanced activity whether added to potato discs 18 h prior to AA, at the same time as AA, or 18 h after AA. Elicitor activity in the presence of glucans was evident with 20-carbon unsaturated fatty acids that had little or no elicitor activity in the absence of glucans. The position of double bonds had considerable influence on the specific activity of unsaturated fatty acids. The most active had a minimum of three double bonds in a methylene-interrupted series beginning with delta 5, e.g., delta 5,8,11. A delta 5 double bond conferred significant activity even if it was not part of a methylene-interrupted series. The 20-carbon chain length appeared optimal for elicitor activity. The 22-carbon chain acids had low activity, and 16- and 18-carbon acids were inactive. A free carboxyl group or easily transesterified group appeared necessary for activity. Arachidonyl alcohol had very low activity and arachidonyl cyanide was inactive. AA-containing phosphatidylcholine, lysophosphatidylcholine and monoacylglycerol were at least as active as free AA, AA-containing diacylglycerols were slightly less active than free AA, and triarachidonyl glycerol was inactive.

Determination of cis,cis-methylene interrupted polyunsaturated fatty acids in aqueous solutions by lipoxygenase chemiluminescence

The chemiluminescent reaction of luminol during lipoxygenase-catalyzed oxygenations was studied with the purpose of developing a specific luminometric assay for cis,cis-1,4-pentadiene fatty acids directly in aqueous solutions. The addition of picomole levels of either linoleic or arachidonic acids to reaction systems containing 0.04 mM luminol and 40 micrograms/ml of purified soybean lipoxygenase-1 gave light emission curves with a single sharp maximum. Under these conditions the peak heights were linearly dependent on the fatty acid concentration and the detection limit for both of the fatty acids was 2 pmol with a signal to noise ratio of 2. For maximum reproducibility of the assays a procedure for the proper quantitation of the enzyme was developed. The fact that the assay proved to be relatively interference-free was ascribed to the high molar enzyme/substrate ratio (above 1).

Inhibition of lipid peroxidation by casein. Evidence of molecular encapsulation of 1,4-pentadiene fatty acids

The capability of cyclodextrins to form molecular inclusion complexes with linoleate appeared in a lipoxygenase-linoleate model reaction as inhibition of oxygenation. The inhibited rates were established instantaneously upon addition of the complexant and maintained until linoleate was exhausted. Total cessation of the reaction was not obtained with cyclodextrins. All these features were reproduced also in casein-inhibited reaction mixtures. Both casein and cyclodextrins protected linoleate also against autoxidation although they did not change free radical generation by xanthine oxidase or Fe2+ reactions. Since neither of the inhibitors affected the enzyme directly, casein may also act by forming linoleate complexes which via a standing equilibrium reduce the oxidizable monomer fatty acids and cause substrate-limited reaction rates. Comparisons at acidic and alkaline pH, in the presence of increasing amounts of the complexants, detergent and hydroperoxides supported this view.

Affinity chromatography of antibodies directed against soybean lipoxygenase-1 and -2 and an enzyme-linked immunosorbent assay (ELISA) for antibodies and lipoxygenases

Crude immunoglobulin G (IgG) fractions of antisera directed against soybean lipoxygenase-1 and -2 were purified by being passed through an immunoadsorbent column containing lipoxygenase coupled to CNBr-activated Sepharose 4B. Bound immunoglobulin was desorbed with pulses of 2 M or 3 M ammonium thiocyanate or 0.1 M glycine-HCl buffer (pH 2.5). The total column recoveries of anti-lipoxygenase-1 IgG and anti-lipoxygenase-2 IgG were 45% and 58%, respectively. The affinity for lipoxygenase of immunospecific antibodies was determined in an enzyme-linked immunosorbent assay (ELISA). In a reaction with lipoxygenase-1, anti-lipoxygenase-1 IgG, which was eluted with glycine-HCl buffer (pH 2.5) with recovery of 24%, had a 6.5-times higher affinity than the whole IgG fraction of antiserum. The affinity of anti-lipoxygenase-2 IgG for lipoxygenase-2 increased 2.2-times after chromatography of IgG over an immunoadsorbent column using 2 M ammonium thiocyanate as eluent (recovery 21%).

Specificities of antisera directed against soybean lipoxygenases-1 and -2 and purification of lipoxygenase-2 by affinity chromatography

Antiserum directed against lipoxygenase-1 from soybean (Glycine Max (L). Merr. var. Williams) was developed in rabbits with electrophoretically pure lipoxygenase-1 as an antigen. To remove traces of lipoxygenase-1 from a lipoxygenase-2 preparation the latter was chromatographed over a column of Sepharose 4B to which antibodies directed against lipoxygenase-1 were coupled. During affinity chromatography of lipoxygenase-2 no protein and only a small amount of activity were lost. Affinity-purified enzyme was used for immunization of rabbits to produce lipoxygenase-2 antibodies. Results with double gel immunodiffusion tests with the two soybean lipoxygenases and antisera directed against them demonstrated that there is no immunological relationship between the isoenzymes. Lipoxygenases from different species of leguminosae crossreacted only with antiserum directed against soybean lipoxygenase-2, and not with anti soybean lipoxygenase-1 serum. No crossreaction could be detected between soybean lipoxygenase antisera and lipoxygenases from species of Gramineae, Linaceae and Solanaceae.

A study of oxygen isotope scrambling in the enzymic and non-enzymic oxidation of linoleic acid

Lipoxygenases-1 and -2 isolated from soybeans were incubated with linoleic acid in the presence of a mixture of 16O2 and 18O2. The formation of 16O/18O-molecules which is indicative for a head-to head reaction of peroxy radicals was determined and compared with that produced during autoxidation of a linoleic acid emulsion in the presence of ferric ions. Lipoxygenase-1 was much less active in scrambling than lipoxygenase-2 which was comparable to that found in the autoxidation reaction.

Oxidation of 3-oxygenated delta 4 and delta 5 - C27 steroids by soybean lipoxygenase and rat liver microsomes

The formation of dioxygenated metabolites of cholesterol, epicholesterol (5-cholesten-3 alpha-ol) 4-cholesten-3 beta-ol, 4-cholesten-3 alpha-one and 4-stigmasten-3-one was studied after incubations with soybean lipoxygenase and linoleic acid. From cholesterol and epicholesterol were formed the 7 alpha-hydroxy, 7 beta-hydroxy-, 7 beta-hydroperoxy-, 7-oxo and 5,6-epoxy-derivatives as well as 6 beta-hydroxy-4-cholesten-3-one. All delta 4-steroids were hydroxylated in the 6 alpha- and 6 beta-positions. The ratios between the yields of 6 beta- and 6 alpha-hydroxylated metabolites varied between 3:1 and 2:1. Incubations with 4-cholesten-3 alpha-ol and 4-cholesten-3 beta-ol also afforded the 4,5-epoxides of these steroids. The ratios between the yields of the 4 beta, 5 beta- and 4 alpha, 5 alpha-epoxides were ca. 4:1 for 4-cholesten 3 beta-ol and ca. 3:2 for 4-cholesten-3 alpha-ol. With iron-supplemented microsomes from rat liver, the compounds formed were qualitatively and quantitatively the same as with soybean lipoxygenase, whereas with 18,000 X g rat liver supernatant fractions the yields of all products formed--except 7 alpha-hydroxycholesterol and 6 beta-hydroxy-4-cholesten-3-one--were markedly decreased. The results indicate the presence of a rat liver microsomal 6 beta-hydroxylase which can use 4-cholesten-3-one as a substrate and extend previous findings of similarities between soybean lipoxygenase and a nonspecific lipoxygenase in rat liver microsomes.

Some properties of a basic L-amino-acid oxidase from Anacystis nidulans

An L-amino acid oxidase (L-amino-acid oxygen oxidoreductase (deaminating), EC 1.4.3.2) from the blue-green alga Anacystis nidulans has been purified to homogeneity with an overall yield of about 10%. Purification included ammonium sulfate fractionation and CM-Sephadex, DEAE-Sephadex, and hydroxyapatite chromatography. The purified enzyme has an absorption spectrum which is characteristic of a flavoprotein, and contains 1 mol FAD per mol enzyme. The native enzyme has a molecular weight of 98 000 as determined by gel exclusion chromatography. Electrophoresis in SDS-polyacrylamide gels gives a single protein band corresponding to a molecular weight of 49 000, which suggests that the native enzyme is composed of 2 subunits of equal molecular weight. As previously demonstrated, the enzyme catalyzes the oxidative deamination of the basic amino acids: L-arginine, L-lysine, L-ornithine and L-histidine. In the presence of catalase and of any of these amino acids, 0.5 mol O2 is consumed, and 1 mol ammonia is formed for each mol amino acid oxidized. HCN is formed from L-histidine when the L-amino acid oxidase is supplemented with peroxidase. In addition to the unusual substrate specificity of this L-amino acid ozidase, it also has an unusual set of inhibitors including o-phenanthroline as well as divalent cations of which Cu2+, Zn2+, and Cd2+ are the most effective ones, but Mg2+ and Ca2+ also inhibit. This inhibition can be reversed by chelating agents such as EDTA. ATP and ADP, but not AMP, can also overcome the inhibition caused by Mg2+, for example. The inhibitory effect of cations can be demonstrated in vivo.

Product specificity of rice germ lipoxygenase

Incubation of linoleic acid with partially purified lipoxygenase from rice germ yielded a ratio of 9- to 13-hydroperoxides of linoleic acid of 97:3 as measured by high performance liquid chromatography. Under similar conditions, hematin gave the 9- to 13-hydroperoxides at a ratio of 51:49, and soybean lipoxygenase-a at 9:91. Infrared spectral analysis revealed cis-trans configuration to predominate in the reaction products with the rice germ enzyme as was with the soybean enzyme.

9-LR-linoleyl hydroperoxide, a novel product from the oxygenation of linoleic acid by type-2 lipoxygenases from soybeans and peas

Type-2 lipoxygenases from soybeans and peas, which have a pH optimum of 6--7 were examined for oxygenation activity at pH 9.0. The reaction velocity was found to be strongly dependent on substrate concentration. At higher substrate concentrations an inhibitory effect was observed, which is connected with the occurrence of a kinetic lag phase. On incubation of linoleic acid at pH 9.0 with either of these enzymes predominantly 9-LR-hydroperoxy-10-trans,12-cis-octadecadienoic acid is formed. The similarity of the product specificity with that of prostaglandin synthetase is discussed in view of the formation of prostaglandin-like substances by soybean lipoxygenase-2 (Bild, G.S., Bhat, S.G., Ramadoss, C.S. and Axelrod, B. (1978) J. Biol. Chem, 253, 21--23).

Co-oxidation of carotenes requires one soybean lipoxygenase isoenzyme

The type II lipoxygenase (optimum pH 6.5) from soybeans was purified and separated into two fractions either by chromatography on DEAE-Sephadex or by isoelectric focusing. In the presence of linoleic acid and oxygen both fractions co-oxidise canthaxanthine or beta-carotene as effectively as a combination of these fractions. Oxygenation of linoleic acid and co-oxidation of canthaxanthine by type II lipoxygenase is stimulated by 13-hydroperoxy-cis-9,trans-11-octadecadienoic acid but not by 13-hydroxy-cis-9,trans-11-octadecadienoic acid or 9-hydroperoxy-trans-10,cis-12-octadecadienoic acid.

Cyclic peroxides from a soya lipoxygenase-catalysed oxygenation of methyl linolenate

Incubation of methyl linolenate with an aqueous extract of soyabean flour at neutral pH gives the hydroperoxyendoperoxides 16-hydroxyperoxy-13,15-endoperoxylinolenate and 9-hydroxyperoxy-10,12-endoperoxylinolenate as the principal oxygenation products in addition to monohydroperoxides. Lipoxygenase I (EC 1.13.11.12) does not catalyse the oxygenation under this condition. The enzyme contributing most to the formation of the hydroperoxyendoperoxides is assumed to be a high molecular mass lipoxygenase aggregate.

Purification and characterization of two isoenzymes of lipoxygenase from soybeans

A chromatographic procedure for the purification of two lipoxygenase isoenzymes (linoleate: O2 oxidoreductase, EC 1.13.11.12.) from soybean is described. The procedure for the purification of isoenzyme L-1 includes optimalized extraction, ammonium sulfate fractionation, heat treatment and gradient elution from a CM-Sephadex C-50 column. The purification of L-2 includes ammonium sulfate fractionation, gelfiltration on Sephadex G-150 and gradient elution from a DEAE-cellulose column. Both isoenzymes L-1 and L-2 appear homogeneous after Disc-PAGE. The isoelectric points are 5.6 for L-1 and 5.8 for L-2. Molecular weights are estimated as 100,000 for L-1 as well as L-2 applying three different methods. Both isoenzymes contain 0.9 mol iron per mol protien. The estimated turn over numbers are 8,200 mol linoleate per mol enzyme and min for L-1 and 3,100 for L-2. Amino acid compositions determined after acid hydrolysis show marked differences between L-1 and L-2, particularly with respect to the amino acids Lys, Phe, Ser, Gly and Leu. L-1 posesses a total of 9 cysteine molecules, 6 of which are present as disulfide bonds. L-2 posesses a total of 8 cysteine molecules with only one disulfide bond.