Identification of short-chain α-hydroxyaldehydes produced by lipid peroxidation in bovine liver

Aldehydic lipid peroxidation products derived from linoleic acid

Lipid peroxidation (LPO) processes observed in diseases connected with inflammation involve mainly linoleic acid. Its primary LPO products, 9-hydroperoxy-10,12-octadecadienoic acid (9-HPODE) and 13-hydroperoxy-9,11-octadecadienoic acid (13-HPODE), decompose in multistep degradation reactions. These reactions were investigated in model studies: decomposition of either 9-HPODE or 13-HPODE by Fe(2+) catalyzed air oxidation generates (with the exception of corresponding hydroxy and oxo derivatives) identical products in often nearly equal amounts, pointing to a common intermediate. Pairs of carbonyl compounds were recognized by reacting the oxidation mixtures with pentafluorobenzylhydroxylamine. Even if a pure lipid hydroperoxide is subjected to decomposition a great variety of products is generated, since primary products suffer further transformations. Therefore pure primarily decomposition products of HPODEs were exposed to stirring in air with or without addition of iron ions. Thus we observed that primary products containing the structural element R-CH=CH-CH=CH-CH=O add water and then they are cleaved by retroaldol reactions. 2,4-Decadienal is degraded in the absence of iron ions to 2-butenal, hexanal and 5-oxodecanal. Small amounts of buten-1,4-dial were also detected. Addition of m-chloroperbenzoic acid transforms 2,4-decadienal to 4-hydroxy-2-nonenal. 4,5-Epoxy-2-decenal, synthetically available by treatment of 2,4-decadienal with dimethyldioxirane, is hydrolyzed to 4,5-dihydroxy-2-decenal.

Linoleic acid peroxidation--the dominant lipid peroxidation process in low density lipoprotein--and its relationship to chronic diseases

Modern separation and identification methods enable detailed insight in lipid peroxidation (LPO) processes. The following deductions can be made: (1) Cell injury activates enzymes: lipoxygenases generate lipid hydroperoxides (LOOHs), proteases liberate Fe ions--these two processes are prerequisites to produce radicals. (2) Radicals attack any activated CH2-group of polyunsaturated fatty acids (PUFAs) with about a similar probability. Since linoleic acid (LA) is the most abundant PUFA in mammals, its LPO products dominate. (3) LOOHs are easily reduced in biological surroundings to corresponding hydroxy acids (LOHs). LOHs derived from LA, hydroxyoctadecadienoic acids (HODEs), surmount other markers of LPO. HODEs are of high physiological relevance. (4) In some diseases characterized by inflammation or cell injury HODEs are present in low density lipoproteins (LDL) at 10-100 higher concentration, compared to LDL from healthy individuals.

Iron (II) ions induced oxidation of ascorbic acid and glucose

Lipid peroxidation (LPO) of polyunsaturated fatty acids (PUFAs) is suspected to be involved in the generation of chronic diseases. A model reaction for LPO is the air oxidation of PUFAs initiated by Fe2+ and ascorbic acid. In the course of such model reactions glycolaldehyde (GLA) was detected as main aldehydic product. Since it is difficult to explain the generat on of GLA by oxidation of PUFAs, it was suspected that GLA might be derived by oxidation of ascorbic acid. This assumption was verified by treatment of ascorbic acid with Fe2+. Produced aldehydic compounds were trapped by addition of pentafluorobenzylhydroxylamine hydrochloride (PFBHA-HCl), trimethylsilylated and finally identified by gas chromatography/mass spectronetry (GC/MS). Oxidation of ascorbic acid with O2 in presence of iron ions produced not only glycolaldehyde (GLA), but also glyceraldehyde (GA), dihydroxyacetone (DA) and formaldehyde. Glyoxal (GO) and malondialdehyde (MDA) were detected as trace compounds. The yield of the aldehydic compounds was increased by addition of lipid hydroperoxides (LOOH) or H2O2. The buffer influenced the reaction considerably: Iron ions react with Tris buffer by producing dihydroxyacetone (DA). Since ascorbic acid is present in biological systems and Fe2+ ions are obviously generated by cell damaging processes, the production of GLA and other aldehydic components might add to the damaging effects of LPO. Glucose suffers also oxidation to short-chain aldehydic compounds in aqueous solution, but this reaction requires addition of equimolar amounts of Fe2+ together with equimolar amounts of H2O2 or 13-hydroperoxy -9-cis-11-trans-octadecadienoic acid (13-HPODE). Therefore this reaction, also influenced by the buffer system, seems to be not of biological relevance.

Previously unknown aldehydic lipid peroxidation compounds of arachidonic acid

Arachidonic acid was oxidized by iron ascorbate. Samples were withdrawn in time intervals. The aldehydic oxidation products were trapped by preparation of pentafluorbenzyloximes. Their trimethylsilylated derivatives were subjected to analysis by GC/MS. The main aldehydic lipid peroxidation product was found to be the well-known 4-hydroxy-2-nonenal (HNE), but 2-hydroxy heptanal (HH) -- a previously unknown lipid peroxidation product of arachidonic acid -- was detected to be nearly equally abundant. Malondialdehyde (MDA), glyoxal and 2-hydroxy-4-decenal (HDE) were detected to be produced in up to 100 times lower amounts compared to HNE. The amounts of aldehydes increased steadily with time. In addition, n-l-hydroxy-n-oxo acids were detected. Similar aldehydes were obtained by iron ascorbate-induced oxidation of hydroxy acids derived by NaBH4-reduction of 13-hydroperoxy-9-cis-11-trans-octadecadienoic acid. Since this and analogous hydroxy acids (LOHs) are the main biological degradation products of hydroperoxides of unsaturated acids (LOOHs) their further peroxidation seems to be a main source of toxic aldehydes.

Gas chromatographic-electron impact mass spectrometric screening procedure for unknown hydroxyaldehydic lipid peroxidation products after pentafluorobenzyloxime derivatization

Aldehydic lipid peroxidation products can be detected after transformation to pentafluorobenzyloxime derivatives by GC-MS screening using characteristic ion traces. Thus the rather unstable unsaturated hydroxyaldehyde, 6-hydroxy-2,4-undecadienal, was identified as autoxidation product of linoleic acid. Its structure was unambiguously confirmed by comparison with an authentic sample. After Fe(2+)-ascorbate induced lipid peroxidation of oleic acid several 4-hydroxy-2-alkenals and 4-hydroxyalkanals were detected. These represent previously unknown secondary oxidation products of lipid peroxidation of oleic acid. Nevertheless oleic acid proved about 1000 times more stable against peroxidation than linoleic or higher unsaturated acids.

Detection of short-chain alpha-hydroxyaldehydic compounds as pentafluorbenzyloxime derivatives in bovine liver

Pentafluorbenzyloxime derivatization allows fast, gentle and unambiguous identification of alpha-hydroxyaldehydic lipid peroxidation products via GC/MS in biological material. Even 1.5 g of a bovine liver sample is sufficient to detect short-chain 2-hydroxyalkanales resulting from cleavage reactions of dioxygenated fatty acids. Quantification is achieved after secondary derivatization with N-methyl-N-t-butyldimethylsilyltrifluoracetamide (M-t-BSTFA) by mass spectrometry using characteristic ion traces of the derivatives. In addition, the corresponding (n-1)-hydroxy-n-oxo acids, previously unknown in biological material, could be detected.

Reinvestigation of lipid peroxidation of linolenic acid

Recently, we deduced a mechanism for lipid peroxidation of linoleic acid [1]. This mechanism was now applied to predict the occurrence of previously unknown lipid peroxidation products of linolenic acid. The proposed structures of peroxidation products allowed to search for these predicted compounds in reaction mixtures with the aid of 'ion trace' by mass spectrometry. Thus, a great number of previously unknown lipid peroxidation products was detected. It is assumed that these compounds also occur--at least as intermediates--in lipid peroxidation processes in mammalian tissue.

Generation of alpha-hydroxyaldehydic compounds in the course of lipid peroxidation

Based on 18O-labeling experiments a general scheme for the generation of hydroxy aldehydic compounds in the course of lipid peroxidation of linoleic acid is developed. Key intermediates are obviously dioxygenated fatty acids, since after reduction with either NaBH4 or Rh/H2 1.2 and 1.6 dihydroxy fatty acids can be identified. The postulated mechanism not only explains the formation of 2-hydroxyalkanals but also supports earlier hypothesis concerning the generation of 4-hydroxyalkenals. In addition it predicted the occurrence of (n - 1)-hydroxy-n-oxo fatty acids as additional oxidation products. A search for these previously unknown autoxidation products of linoleic acid was indeed successful.

alpha-Hydroxyaldehydes, products of lipid peroxidation

Besides the well known products of lipid peroxidation, 4-hydroxy-2-nonenal, saturated and unsaturated aldehydes, 2-hydroxy-heptanal was found to be a major aldehydic product of lipid peroxidation (LPO) of n-6 fatty acids (linoleic acid 18:2, arachidonic acid 20:4). 2-Hydroxyhexanal is produced also, but only in much lower yield. 2-Hydroxyalkanales with 8,9,10 and 11 C-atoms are derived from hydroperoxides of oleic acid. An oxidation product of all unsaturated fatty acids is glyoxal. Analyses were performed after aldehyde specific derivatization reactions by GC-MS.