Lipids have a key role within a variety of processes, either structural or signalling, within the plasma membrane, nuclear membrane, endoplasmic reticulum, Golgi apparatus and trafficking vesicles (Muro et al., 2014). As lipids are the predominant structural molecule of cells, lipid oxidation at an excess level can cause alterations to the physical properties of plasma membranes, as well as covalent modifications to nucleic acids and proteins (Gaschler and Stockwell, 2017).
Lipid oxidation is a general term which encompasses several types of reactions vital for physiological reactions within the body. The process of lipid oxidation occurs through different mechanisms: free radical-mediated oxidation; free radical independent non-enzymatic oxidation; and enzymatic oxidation (Shichiri et al., 2014).
A free radical chain mechanism drives free radical lipid oxidation, consisting of the processes of initiation, propagation and termination. In initiation, an unsaturated lipid loses a hydrogen radical, forming a lipid radical species, where the radical is located on the centre of the allylic carbon atom (Reis and Spickett, 2012). The alkyl radical of the unsaturated lipid, which contains a labile hydrogen, reacting with oxygen to form peroxyl radicals (Frankel, 2005). The process of propagation result in forming more carbon- or oxygen- centred radicals. The products within the propagation stage cause radical damage, as they form carbon-centred radicals on adjacent fatty acyl chains by transfer of hydrogen atoms from an unsaturated lipid for stabilisation, resulting in conversion to form a hydroperoxide (Reis and Spickett, 2012). In the final stage, termination, there are two different mechanisms, dependent on concentration levels. If there are high concentrations of radicals in the initiation phase, the peroxyl radicals accumulate and interact with each other to form non-radical products (Frankel, 2005). The alternative method, is where a lipophilic chain-breaking antioxidant intervenes, as they have fairly stable adjoined radical forms (Reis and Spickett, 2012).
Lipid peroxidation primarily occurs with polyunsaturated fatty acyl chains, producing a range of products: oxidized fatty acids or phospholipids which are full-chain length; short-chain oxidized phospholipids and fragmentation products due to chain scission reactions (Aldini et al., 2015). Fragmentation can occur by either enzymatic or non-enzymatic mechanisms, producing small aldehyde and carbonyl-containing compounds Sousa et al., 2017). The various products all have different chemical functions, properties, reactivity and toxicity. Therefore, these compounds have an array of biological implications, with associations to the incidence of various diseases (Gueraud et al., 2010). Primarily, aldehydes are formed by fragmentation of fatty acyl chains (Sousa et al., 2017).
Enzymatic oxidation of lipids is caused by numerous cellular enzymes, which produce physiological signalling compounds via lipid oxidation. Most research conducted into enzymatic lipid oxidation has focused on polyunsaturated fatty acids (Spickett and Forman). One cellular enzyme, lipoxygenase, catalyses molecular oxygen to be integrated at specific sites in the fatty acyl chain forming hydroperoxides of arachidonic acid or linoleic acid. Some lipoxygenases only work on free fatty acids, whereas other oxidize fatty acids esterified in phosphatidylcholine or phosphatidylethanolamine. Cyclooxygenases incorporate two oxygen molecules to free arachidonic acid to form prostaglandin G2, which is then converted to other prostanoids enzymatically via prostaglandin H2. Cytochrome P450 enzymes are mono-oxygenase with hydroxylase or epoxygenase activities, and therefore some form epoxylated products, which are hydrolysed to di-hydroxylated fatty acids (Roberts et al., 2015).
Non-enzymatic oxidation of lipids can occur by radical reactions or non-radical reactions. The typical radical attack is lipid peroxidation, which results in a chain reaction damaging unsaturated fatty acids. Esterified fatty acids are densely packed, allowing a cascade of reactions to occur, forming a high number of hydroperoxides from a single radical reaction event (Roberts et al., 2015). Non-radical reactions occur by oxidation of lipids by singlet oxygen and ozone, primarily via an ene-reaction generating a hydroperoxide with double bond migration. Minor side reactions also occur, such as 1,4-addition, generating carbonyl compounds (Niki, 2009).
Aldehydes formed from enzymatic and non-enzymatic lipid oxidation, can be classified by chemical structure into three groups: alkanals, alkenals and γ-substituted-alkenals. Alkanals are classified as saturated carbon chains which contain an aldehyde group; simple and relatively nonpolar aldehydes. Alkenals are aldehydes with a double bond in the hydrocarbon chain. Complex aldehydes include γ-substituted-alkenals, which contain additional functional groups, and therefore often have different reactivity and implications biologically (Sousa et al., 2017). These aldehydes include acrolein, malondialdehyde (MDA), pentane, 4-hydroxy-trans-2-hexenal (HHE), 4-hydroxy-trans-2-nonenal (HNE), 4-oxo-trans-2-nonenal (ONE), 4-hydroxy-trans-2-nonenoic acid (HNA), 4-hydroxydodecadienal (HDDE), 9,12-dioxo-10-E-dodecanoic acid (DODE) and 9-keto-12-oxo-dodecanoic acid (KODA).
Many lipid oxidation products react with nucleophilic groups in proteins (Aldini et al., 2015). Protein is the most sensitive cellular target of the aldehydes products, therefore numerous covalent adducts are formed through the process of protein lipoxidation. The mechanism can vary, due to the range of aldehyde structures, e.g. amino acid side chain variations (Spickett and Forman). The adduct formed is dependent on the aldehyde involved, and it can involve various protein modifications and additions of carbonyl groups to amino acid residues. During lipid oxidation, aldehydes react with lysine residues forming a Schiff base adduct or imine. The aldehydes generated are often mediators, in the presence of hydroperoxides, of cell damage as they can covalently modify nucleophilic biomolecules (Spickett and Forman).
Lipoxidation products function as damage-associated molecular patterns (DAMPs) which are self-derived host molecules which can be induced that indicate cellular infection, damage, stress, or transformation and that some innate receptors recognise proteins to mediate responses by innate immune cells. These DAMPs bind to Pattern recognition receptors (PRR) on cells, which recognise pathogen-associated molecular pattern (PAMP) on pathogens, and bind triggering signal-transduction pathways which turn on expression of genes with important functions in innate immunity (Krysko et al., 2011).
Products of lipid oxidation have been linked to a multitude of pathologies, which include metabolic diseases, cancers and neurodegenerative diseases due to their reactivity. This is since aldehyde products modulate cell processes: oxidative stress signalling, cell proliferation, transformation or cell death (Dalleau et al., 2013). MDA quantitatively, is the major product of lipid peroxidation, since it is formed from most fatty acids. Other lipid peroxidation products are dependent on polyunsaturated fatty acids, such as HNE. However, in relation to toxicity, the concentration is not necessarily an indicator, for example HNE has higher electrophilicity than MDA and more implications in pathological processes. Therefore, when looking at associations with disease, toxicity of the molecule needs to be accounted for alongside concentrations (Gueraud et al., 2010). HNE can be used as a marker of oxidative stress, indicating deregulation of mitochondrial function and energy metabolism in neurodegeneration (Perluigi et al., 2012). Increased lipid oxidation products have been reported in rheumatoid arthritis patients, as oxidative stress in rheumatoid arthritis patients also leads to the development of atherosclerosis (Isik et al., 2007). The role the aldehyde plays in disease can vary, but the effects are often caused by the aldehyde’s tendency to form adducts with the nucleophilic groups on proteins, DNA and specific phospholipid (Sousa et al., 2017).
By identifying the association between aldehyde levels within the body and metabolic diseases, these can be used as indicators of diseases, and potentially lead to further understanding of the pathology of the disease. The aim is to review concentrations of aldehydes in both diseased patients and control subjects for comparison, identifying the average levels for healthy individuals and patients of various diseases associated with lipid oxidation. These concentrations can be analysed to identify any relationship between aldehyde concentrations and disease, including the severity of the patient’s condition. It is anticipated that diseased patients will have increased concentrations of aldehyde in comparison to control subjects due the association between aldehydes and oxidative stress, and aldehydes ability to function as DAMPs. Aldehydes with increased toxicity, such as HNE, are more likely to associated with disease than aldehydes which are less likely to react. MDA is also expected to have a high association with disease, since it is the major product of lipid oxidation.