Isolation of an isozyme of soybean lipoxygenase

[Co-oxidation of beta-carotene and canthaxanthine by purified lipoxygenases from soya beans (author's transl)]

Isolation and purification of soya bean lipoxygenase (linoleate: O2 oxidoreductase, EC, 1.13.11.12) on Sephadex G-200, DEAE-cellulose and by isolectric focusing yields two isoenzymes of the L- 2 type (optimum pH 6.5) and two of the L-1 type (optimum pH9.0). Different crude extracts from soya beans as well as the purified L-2 isoenzymes exhibit the same capacity for co-oxidation of beta-carotene and canthaxanthine, when the comparison is based upon equal lipoxygenase activities. In contrast to L-2 the alkaline lipoxygenase L-1 is a poor "carotene oxidase".

[Carbonyl compounds from the reaction of barley isomerase with linoleic acid hydroperoxides. Their development from the 9- or the 13-hydroperoxide-isomer (author's transl)]

Hydroperoxid isomerase from barley was incubated with linoleic acid hydroperoxides (LHPO), containing nearly exclusively the 13-LHPO or the 9-LHPO isomer; the volatile reaction products were isolated, concentrated and investigated by means of gas and radio-gaschromatography. Thus it was possible to establish the precursors of the volatile compounds hexanal, 2-trans-heptenal and 2-trans-octenal, which develop during formerly described reactions of isomerase with substrates, containing 9- and 13-LHPO in equal amounts. 13-LHPO was found to be a precursor of hexanal and 2-tr-octenal, while the 9-LHPO isomer in the barley isomerase LHPO breakdown reaction obviously cannot be accepted as precursor of volatile components. The origin of 2-tr-heptenal could not be clarified; it occured neither in the experiments with predominating 9-LHPO nor in those with predominating 13-LHPO. Perhaps 2-tr-heptenal is only produced in the presence of a defined ratio of both isomeric hydroperoxides.

ABBREVIATIONS

13-LHPO = 13-hydroperoxy-9-cis, 11-trans-octadecadienoic acid. 9-LHPO = 9-hydroperoxy-10-trans, 12-cis-octadecadienoic acid.

Enzymatic oxydation of linoleic acid: formation of bittertasting fatty acids

Linoleic acid was oxidized with a protein fraction from soya beans (25 degrees C; 2h), in which lipoxygenase and peroxydase activities occurred. The fatty acids formed were isolated and, after emulsification with a sugar ester, were evaluated for bitter taste. The main components of the bitter-tasting fractions was a mixture of 9.12.13-trihydroxyoctadec-10- and 9.10.13-trihydroxyoctadec-11-enoic acids. The taste threshold lies in the range 0.6-0.9 mumol/ml. Two further trihydroxy-acids and two oxodihydroxy-acids were also identified in the bitter-tasting fraction.

Biochemistry of lipoxygenase in relation to food quality

A renewed interest in lipoxygenase has led to detailed studies of its isoenzymes, substrate specificity, and the nature of its reaction products. Lipoxygenase is highly specific for cis,cis-1,4-pentadiene systems such as linoleic, linolenic, and arachidonic acid (or ester) and catalyzes the formation of the corresponding hydroperoxides with a cis,-trans-conjugated diene system. The hydroperoxides can then undergo enzymic or spontaneous degradation, producing a range of carbonyl compounds. This review will discuss the biochemical properties of this enzyme and its contribution to the quality of raw and processed food products. An attempt has been made to discuss both the desirable and undesirable effects associated with the action of lipoxygenase, citing specific food examples where appropriate.

Affinity chromatography of lipoxygenases

A number of aminohexyl agarose derivatives of unsaturated fatty acids have been prepared and evaluated as materials for the affinity chromatography of soybean and pea lipoxygenases. A practical method for a one-stage purification of soybean lipoxygenase-1, with a purification factor of 16, is described, using either linolenate or docosa-4,7,10,13,16,19-hexaenoate as ligands. Results show that alleged competitive inhibitors do not cause sharp elution from the affinity column, and that there is an increasing specificity of binding and sharpness of elution as the proportion of unsaturation in the ligand is increased. These results are discussed in terms of the relative importance of the types of bonding involved in enzyme-substrate binding.

Steady-state kinetics of lipoxygenase oxygenation of unsaturated fatty acids

The oxygenation of linoleate and arachidonate catalyzed by soybean lipoxygenase is subject to competitive product inhibition. For normal conditions, there is an additional inhibition due to product that causes the reaction to cease before completion. This process is reversible upon addition of further substrate and is proposed to be a chemical (reversible) change of the enzyme. At very low enzyme concentrations, inactivation or adsorption of enzyme on the vessel surface (or "wall effect") is significant, leading to even lower rates and percent completions. In the very early stages of a typical catalyzed reaction, a lag, or induction period, occurs. It was previously known that this lag is eliminated by product hydroperoxide--and not by the corresponding alcohol. The hydroperoxide elimination of the lag is inhibited by the alcohol. It is proposed that this is a chemical activation of the enzyme to produce a catalytically functional form.

Conversion of 9-D- and 13-L-hydroperoxylinoleic acids by soybean lipoxygenase-1 under anaerobic conditions

Soybean lipoxygenase-1 reacts with both 9-D and 13-L-hydroperoxylinoleic acids under anaerobic conditions. Approximately 40% of the hydroperoxide is converted into oxodienes, absorbing at 285 nm. Concomitantly, more polar compounds are formed, tentatively identified as being mainly epoxy-hydroxy-octadecenoic acids. When oxygen is present, the reaction is strongly inhibited, until in a very slow reaction the oxygen has been depleted. This accounts for the occurrence of a lag period.

Conversion of linoleic acid hydroperoxide by soybean lipoxygenase in the presence of guaiacol: identification of the reaction products

Linoleic acid hydroperoxide formed by soybean lipoxygenase was metabolized by the same enzyme in the presence of guaiacol. The products of this reaction included, trihydroxyoctadecenoic acids, hydroperoxydihydroxyoctadecenoic acids, hydroxyepoxyoctadecenoic acids, dihydroxyoctadecenoic acids, hydroxyoctadecadienoic acids, and oxooctadecadienoic acids.

Lipoxygenase isozymes of peanut

Lipoxygenase was isolated and partially purified from peanut seed by ammonium sulfate precipitation, gel filtration, and ion exchange column chromatography. Three isozymes of lipoxygenase were identified. Two had pH optima of 6.2, and the other an optimum of 8.3. Molecular weight of each isozyme was 7.3 x 10(4), as determined by gel filtration. The alkaline optimum isozyme was not inhibited by NaCN and was inhibited by CaCl2 except at very low concentrations. The acid optimum isozymes were inhibited by NaCN and were stimulated by CaCl2 concentrations up to ca. 0.7 mM.

Demonstration by EPR spectroscopy of the functional role of iron in soybean lipoxygenase-1

1. The EPR spectrum at 15 degrees K of soybean lipoxygenase-1 in borate buffer pH 9.0 has been studied in relation to the presence of substrate (linoleic acid), product (13-L-hydroperoxylinoleic acid) and oxygen. 2. The addition of 13-L-hydroperoxylinoleic acid to lipoxygenase-1 at pH 9.0 gives rise to the appearance of EPR lines at g equals 7.5, 6.2, 5.9 and 2.0, and an increased signal at g equals 4.3. 3. In view of the effect of the end product on both the kinetic lag period of the aerobic reaction and the fluorescence of the enzyme, it is concluded that 13-L-hydroperoxylinoleic acid is required for the activation of soybean lipoxygenase-1. Thus it is proposed that the enzyme with iron in the ferric state is the active species. 4. A reaction scheme is presented in which the enzyme alternatingly exists in the ferric and ferrous states for both the aerobic and anaerobic reaction.

Dimorphotheca sinuata lipoxygenase: formation of 13-L-hydroperoxy-cis-9,trans- 11-octadecadienoic acid from linoleic acid

Lipoxygenase (EC 1.13.1.13) from the seed ofDimorphotheca sinuata oxidized linoleic acid to predominantly 13‐L‐hydroperoxy‐cis‐9,trans‐11‐octadecadienoic acid. When the reaction proceeded at pH 6.9, the 13‐hydroperoxide was the only isomer detected; but at pH 5.1, the 13‐isomer was 92% of the total, the remaining 8% being the 9‐hydroperoxide. At both pH's small amounts of hydroxyoctadecadienoic acid accumulated during the reaction. This acid from the pH 6.9 reaction was analyzed as 13‐hydroxy‐cis,trans‐octadecadienoic. The postulate advanced by many workers that dimorphecolic acid, 9‐D‐hydroxy‐trans‐10,trans‐12‐octadecadienoic acid, is biosynthesized via a lipoxygenase product was not proved. Although the product specificity ofD. sinuata lipoxygenase is like that of lipoxygenase type 1 from soybeans, its inactivity at pH 9 demonstrated that it is a novel enzyme.

An anaerobic reaction between lipoxygenase, linoleic acid and its hydroperoxides

In an anaerobic system soya-bean lipoxygenase together with linoleic acid induces a structural rearrangement of 13-hydroperoxyoctadeca-cis-9-trans-11-dienoic acid leading to the formation of 13-oxotrideca-cis(trans)-9-trans-11-dienoic acid and n-pentane as well as 13-oxo-octadeca-9,11-dienoic acid. It is proposed that the 13-peroxyoctadeca-cis-9-trans-11-dienoic acid radical formed through hydrogen radical abstraction by the linoleic acid radical is the key intermediate for these reactions.